Rev.2.1_01
STEP-DOWN, BUILT-IN FET, SYNCHRONOUS
RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series
Seiko Instruments Inc. 1
The S-8550/8551 Series is a CMOS synchronous rectification step-down
switching regulator which mainly consists of a reference voltage circuit, an
oscillator, an error amplifier, a phase compensation circuit, a PWM controller, an
under voltage lockout circuit (UVLO), a current limit circuit, and a power MOS
FET. The oscillation frequency is high at 1.2 MHz, so a high efficiency, large
output current, step-down switching regulator can be achieved by using small
external parts.
The built-in synchronous rectification circuit makes achieving high efficiency
easier compared with conventional step-down switching regulators. A ceramic
capacitor can be used as an output capacitor. High-density mounting is
supported by adopting a small SOT-23-5 package.
The S-8550 and S-8551 Series are offered according to different pin configuration.
Features
• Oscillation frequency : 1.2 MHz
• Input voltage range : 2.0 to 5.5 V
• Output voltage range : Arbitrarily settable by external output voltage setting resistor
• Output current : 600 mA
• Reference voltage : 0.6 V ±2.0%
• Efficiency : 92%
• Soft-start function : 1 ms typ.
• Shutdown function : Shutdown current consumption : 1.0 µA max.
• Built-in current limit circuit
• Pch power MOS FET on-resistance : 0.4 Ω typ.
• Nch power MOS FET on-resistance : 0.3 Ω typ.
• Constant continuous mode operation (no light load mode)
• Small package : SOT-23-5
• Lead-free products
Applications
• Mobile devices, such as mobile phones, Bluetooth devices, wireless devices, digital audio players, digital still
cameras, portable DVD players, and portable CD players
Package
Drawing Code
Package Name Package Tape Reel
SOT-23-5 MP005-A MP005-A MP005-A
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
2 Seiko Instruments Inc.
Block Diagram
L
ON/OFF
V
IN
+
−
+
−
C
IN
FB
VIN
CONT
VSS
C
FB
R
FB1
R
FB2
V
OUT
C
OUT
*1
*1
Reference
voltage PWM comparator
PWM control
circuit
UVLO
circuit
ON/OFF
circuit
IC internal
power supply
Current limit
circuit
Error amplifier
T
r
i
angu
l
ar wave
generation
circuit
*1. Parasitic diode
Figure 1
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 3
Product Name Structure
Product name
S-855 x A A - M5T1
G
Package name (abbreviation) and packing specification*1
M5T1 : SOT-23-5, tape
Oscillation frequency
A : 1.2 MHz
Pin configuration setting*2
0 : Pin configuration 1
1 : Pin configuration 2
*1. Refer to the taping specifications at the end of this book.
*2. Refer to Table 1 and Table 2 of “ Pin Configurations”.
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
4 Seiko Instruments Inc.
Pin Configurations
Table 1 S-8550 Series (Pin Configuration 1)
Pin No. Symbol Description
1 VIN IC power supply pin
2 VSS GND pin
3
ON/OFF
Shutdown pin
“H” : Power on (normal operation)
“L” : Power off (standby)
4 FB Output voltage feedback pin
5 CONT External inductor connection pin
SOT-23-5
Top view
5 4
3 2 1
Figure 2
Table 2 S-8551 Series (Pin Configuration 2)
Pin No. Symbol Description
1
ON/OFF
Shutdown pin
“H” : Power on (normal operation)
“L” : Power off (standby)
2 VSS GND pin
3 CONT External inductor connection pin
4 VIN IC power supply pin
5 FB Output voltage feedback pin
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 5
Absolute Maximum Ratings
Table 3 Absolute Maximum Ratings
(Unless otherwise specified: Ta = 25°C, VSS = 0 V)
Parameter Symbol Absolute Maximum Rating Unit
VIN pin voltage VIN V
SS − 0.3 to VSS + 6.0 V
FB pin voltage VFB V
SS − 0.3 to VIN + 0.3 V
CONT pin voltage VCONT V
SS − 0.3 to VIN + 0.3 V
ON/OFF pin voltage VON/OFF V
SS − 0.3 to VIN + 0.3 V
CONT pin current ICONT 1300 mA
Power dissipation PD 600*1 mW
Operating temperature Topr −40 to +85 °C
Storage temperature Tstg −40 to +125 °C
*1. When mounted on printed circuit board
[Mounted board]
(1) Board size: 114.3 mm × 76.2 mm × t1.6 mm
(2) Board name: JEDEC STANDARD51-7
Caution 1. The absolute maximum ratings are rated values exceeding which the product could suffer
physical damage. These values must therefore not be exceeded under any conditions.
2. Since this IC has a built-in power MOS FET, make sure that dissipation of the power MOS FET
does not exceed the allowable power dissipation of the package. (Refer to Figure 3.)
Generally, dissipation of a switching regulator can be calculated by the following equation.
Dissipation = (100 (%) − efficiency (%)) / efficiency (%) × output voltage × load current
The greater part of dissipation depends on the built-in power MOS FET, however, dissipation
of the inductor is also included.
In addition, since power dissipation of the package also changes according to a mounting
board or a mounting state, fully check them using an actually mounted mode.
0
400
0
Power dissipation (PD) [mW]
Ambient temperature (Ta) [°C]
200
500
300
100
600
700
50 100 150
Figure 3 Power Dissipation of Package (Mounted on Board)
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
6 Seiko Instruments Inc.
Electrical Characteristics
Table 4 Electrical Characteristics
(Unless otherwise specified: VIN = 3.6 V, VOUT = 1.8 V (the conditions in Table 5), Ta = 25°C)
Parameter Symbol Conditions Min. Typ. Max. Unit
Test
Circuit
Operating input voltage VIN 2.0 5.5 V 2
Output voltage range*1 VOUT V
IN = VOUT(S) + 0.4 V to 5.5 V 1.1 4.0 V 2
FB voltage VFB V
IN = VOUT(S) + 0.4 V to 5.5 V 0.588 0.6 0.612 V 2
FB voltage temperature
coefficient
∆VFB
∆Ta
Ta = −40°C to +85°C ±100 ppm/°C 2
FB pin input current IFB V
IN = 2.0 V to 5.5 V, FB pin −0.1 +0.1 µA 1
Current consumption
during shutdown ISSS VIN = 2.0 V to 5.5 V,
VON/OFF = 0 V 1.0 µA 1
Current consumption 1 ISS1 fosc = 1.2 MHz, no external parts,
VFB = VFB(S) × 1.1 V 200 400 µA 1
RPFET I
CONT = 100 mA 0.4 0.6 Power MOS FET
on-resistance RNFET I
CONT = −100 mA 0.3 0.5 Ω 1
Power MOS FET
leakage current ILSW VIN = 2.0 V to 5.5 V,
VON/OFF = 0 V, VCONT = 0 or 3.6 V ±0.01 ±0.5 µA 1
Limit current ILIM 800 1000 1200 mA 1
Oscillation frequency fosc 1.02 1.2 1.38 MHz 2
Soft-start time tSS Time required to reach 90% of
VOUT(S) 0.7 1.0 1.3 ms 2
High level input voltage VSH V
IN = 2.0 V to 5.5 V, ON/OFF pin 0.9 V 2
Low level input voltage VSL V
IN = 2.0 V to 5.5 V, ON/OFF pin 0.3 V 2
High level input current ISH V
IN = 2.0 V to 5.5 V, ON/OFF pin −0.1 0.1 µA 1
Low level input current ISL V
IN = 2.0 V to 5.5 V, ON/OFF pin −0.1 0.1 µA 1
UVLO detection voltage
VUVLO 1.4 1.6 1.78 V 2
*1. V
OUT(S) is the output voltage set value, and VOUT is the typ. value of the actual output voltage.
V
OUT(S) can be set depending on the ratio between the VFB value and output voltage set resistors (RFB1, RFB2).
For details, refer to “ External Parts Selection”.
External Parts When Measuring Electrical Characteristics
Table 5 External Parts
Element Name Symbol Constant Manufacturer Part Number
Inductor L 3.3 µH Taiyo Yuden Co., Ltd. NR4018T3R3M
Input capacitor CIN 4.7 µF TDK Corporation C3216X7R1E475K
Output capacitor COUT 10 µF TDK Corporation C3216X7R1C106K
Output voltage set resistor 1 RFB1 36 kΩ Rohm Co., Ltd. MCR03 Series 3602
Output voltage set resistor 2 RFB2 18 kΩ Rohm Co., Ltd. MCR03 Series 1802
Phase compensation capacitor CFB 68 pF Murata Manufacturing Co., Ltd. GRM1882C1H680J
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 7
Test Circuits
1.
S-855xA
ON/OFF
A
↓
C
IN
CONT
FB
VSS
VIN
Figure 4
2.
L
S-855xA
ON/OFF
C
IN
CONT
FB
VSS
VIN
V
V↓I
OUT
V
OUT
C
FB
C
OUT
R
FB1
R
FB2
Figure 5
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
8 Seiko Instruments Inc.
Operation
1. Synchronous rectification PWM control step-down switching regulator
1.1 Synchronous rectification
The synchronous rectification method lowers voltage drop to greatly reduce power dissipation since an Nch
power MOS FET, having resistance much lower than conventional switching regulators, is used.
In conventional switching regulators, current flows in the diode connected between the GND and CONT pins
when the Pch power MOS FET is off. The forward drop voltage (Vf) of such diodes is large, between 0.3 to
0.7 V, so the power dissipation used to be very large. Synchronous rectification ultra-low resistance Nch
transistors repeat on and off, in synchronization with the operation of the Pch driver, in the reverse cycle of
the Pch driver. Moreover, the built-in P and N through prevention circuit helps much reduction of power
consumption during operation.
1.2 PWM control
The S-8550/8551 Series is a switching regulator using a pulse width modulation method (PWM) and features
low current consumption.
In conventional PWM control switching regulators, pulses are skipped when the output load current is low,
causing a fluctuation in the ripple frequency of the output voltage, resulting in an increase in the ripple
voltage.
In the S-8550/8551 Series, the switching frequency does not change, although the pulse width changes from
0 to 100% corresponding to each load current. The ripple voltage generated from switching can thus be
removed easily using a filter because the switching frequency is constant.
2. Soft-start function
The soft-start circuit built in the S-8550/8551 Series controls the rush current and the overshoot of the output
voltage when powering on, the ON/OFF pin is switched from the “L” level to the “H” level, or the UVLO operation
is released. A reference voltage adjustment method is adopted as the soft-start method.
3. Shutdown pin
This pin stops or starts step-up operations.
Switching the shutdown pin to the “L” level stops operation of all the internal circuits and reduces the current
consumption significantly. DO NOT use the shutdown pin in a floating state because it is not pulled up or
pulled down internally. DO NOT apply voltage of between 0.3 and 0.9 V to the shutdown pin because applying
such a voltage increases the current consumption. If the shutdown pin is not used, connect it to the VIN pin.
Table 6
Shutdown Pin CR Oscillation Circuit Output Voltage
“H” Operates Set value
“L” Stops Hi-Z
VIN
ON/OFF
VSS
Figure 6
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 9
4. Current limit circuit
A current limit circuit is built in the S-8550/8551 Series.
The current limit circuit monitors the current that flows in the Pch power MOS FET and limits current in order to
prevent thermal destruction of the IC due to an overload or magnetic saturation of the inductor.
When a current exceeding the current limit detection value flows in the Pch power MOS FET, the current limit
circuit operates and turns off the Pch power MOS FET since the current limit detection until one clock of the
oscillator ends. The Pch power MOS FET is turned on in the next clock and the current limit circuit resumes
current detection operation. If the value of the current that flows in the Pch power MOS FET remains the
current limit detection value or more, the current limit circuit functions again and the same operation is repeated.
Once the value of the current that flows in the Pch power MOS FET is lowered up to the specified value, the
normal operation status restores. A slight overshoot is generated in the output voltage when the current limit is
released.
The current limit detection value is fixed to 1 A (typ.) in the IC. If the time taken for the current limit to be
detected is shorter than the time required for the current limit circuit in the IC to detect, the current value that is
actually limited increases. Generally, the voltage difference between the VIN and VOUT pins is large, the
current limit detection status is reached faster and the current value increases.
5. 100% duty cycle
The S-8550/8551 Series operates up to the maximum duty cycle at 100%. Even when the input voltage is
lowered up to the output voltage value set using the external output voltage setting resistor, the Pch power MOS
FET is kept on and current can be supplied to the load. The output voltage at this time is the input voltage from
which the voltage drop due to the direct resistance of the inductor and the on-resistance of the Pch power MOS
FET are subtracted.
6. UVLO function
The S-8550/8551 Series includes a UVLO (under-voltage lockout) circuit to prevent the IC from malfunctioning
due to a transient status when power is applied or a momentary drop of the supply voltage. When UVLO is in
the detection state, the Pch and Nch power MOS FETs stop switching operation, and the CONT pin become
Hi-Z. Once the S-8550/8551 is in the UVLO detection status, the soft-start function is reset, but the soft-start
operates by the releasing operation of UVLO after that.
Note that the other internal circuits operate normally and that the status is different from the power-off status.
The hysteresis width is set for the UVLO circuit to prevent a malfunction due to a noise that is generated in the
input voltage. A voltage about 150 mV (typ.) higher than the UVLO detection voltage is the release voltage.
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
10 Seiko Instruments Inc.
Operation Principle
The S-8550/8551 Series is a step-down synchronous rectification switching regulator based on constant PWM
control. Figure 7 shows the basic circuit diagram.
A step-down switching regulator starts current supply by the input voltage (VIN) when the Pch power MOS FET is
turned on and holds energy in the inductor at the same time. When the Pch power MOS FET is turned off, the
current held in the inductor is released. The released current flows in the smoothing circuit, with the energy loss
held minimum, supplies the output voltage (VOUT) lower than VIN. VOUT is kept constant by controlling the
switching frequency (fosc) and ON time (ton). With the PWM control method, VOUT is made constant by controlling
the ON time with fOSC unchanged.
L
Control
circuit
Pch power MOS FET
Nch power MOS FET
V
IN
V
OUT
C
OUT
I
1
I
2
Figure 7 Basic Circuit Drawing of Step-down Switching Regulator
1. Continuous mode
The following explains how the current flows to the inductor when the step-down operation is constant and
stable.
When the Pch power MOS FET is turned on, current I1 flows in the direction shown by the arrow in Figure 7,
and energy is stored in the inductor (L). When the output capacitor (COUT) is charged, supply of the output
current (IOUT) is started at the same time. The inductor current (IL) gradually increases in proportion to the ON
time (tON) of the Pch power MOS FET as shown in Figure 8 (changes from IL min. to IL max.). When the Pch
power MOS FET is turned off, the Nch power MOS FET is turned on and IL tries to hold IL max. Consequently,
current I2 flows in the direction shown by the arrow in Figure 7. As a result, IL gradually decreases and
reaches IL min. when the OFF time (tOFF) has elapsed. When tOFF has elapsed, the Nch power MOS FET is
turned off and the next cycle is entered. The above sequence is repeated.
As explained in the above, the continuous mode refers to the operation in the current cycle in which IL linearly
changes from IL min. to IL max. Even if IL min. is less than 0 A, IL min. keeps flowing (backflow current flows).
ton toff
T = 1/fOSC
IL
IL max.
IL min.
t
Figure 8 Continuous Mode (Current Cycle of Inductor Current (IL))
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 11
2. Backflow current
The S-8550/8551 Series performs PWM synchronous rectification even if IL min. is less than 0 A, so a backflow
current is generated in VIN and the backflow current becomes maximum when no load is applied (see Figure 9).
Use the following equation to calculate the maximum backflow current value, which should be taken into
consideration when designing.
Duty (IOUT = 0) = VOUT / VIN
Example : VIN = 3.6 V, VOUT = 1.8 V …… Duty = 50%
∆IL = ∆V / L × ton = (VIN − VOUT) × Duty / (L × fOSC)
Example : VIN = 3.6 V, VOUT = 1.8 V, fOSC = 1.2 MHz, L = 3.3 µH …… ∆IL = 227 mA
I
L max. = ∆IL / 2 = 113.5 mA, IL min. = −∆IL / 2 = −113.5 mA
The current value waveform of the inductor is a triangular wave, of which the maximum value is IL max. and the
minimum value is IL min. (negative value), and the negative value (the portion marked by diagonal lines in
Figure 9) backflows when no load is applied (see Figure 9).
If about 113.5 mA of IOUT flows in the above conditions, the minimum value (IL min.) of the triangular wave is
made 0 mA and no backflow current flows.
When an input capacitor (CIN) is connected, the backflow current is absorbed by CIN, thus reducing the backflow
current to flow in the power supply. Be sure to connect an input capacitor to reduce backflow current to the
power supply (see Figure 10).
The above presents the conditions required to prevent backflow current from flowing, which is only a guideline.
Perform sufficient confirmation using an actual application.
Inductor current with no load Inductor current when load is a
current of 113.5 mA
Backflow
current
−113.5 mA
113.5 mA
IL min.
IL max.
∆IL
IL
0 mA
Backflow current = 0 mA
0 mA
I
OUT
113.5 mA
I
OUT
227 mA
IL min.
IL max.
∆IL
IL
Figure 9 Example of Conditions to Prevent Backflow Current from Flowing
VOUT
VIN
CONT
Backflow current
Inductor
current IL
CIN
VIN
Figure 10 Backflow Current
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
12 Seiko Instruments Inc.
External Parts Selection
1. Inductor
The inductance (L value) has a strong influence on the maximum output current (IOUT) and efficiency (η).
The peak current (IPK) increases by decreasing L and the stability of the circuit improves and IOUT increases. If
L is decreased further, the current drive capability of the external transistor is insufficient and IOUT decreases.
If the L value is increased, the loss due to IPK of the power MOS FET decreases and the efficiency becomes
maximum at a certain L value. Further increasing L decreases the efficiency due to the increased loss of the
DC resistance of the inductor.
The recommended L value for the S-8550/8551 Series is 3.3 µH.
When selecting an inductor, note the allowable current of the inductor. If a current exceeding this allowable
current flows through the inductor, magnetic saturation occurs, substantially lowering the efficiency.
Therefore, select an inductor so that IPK does not exceed the allowable current. IPK is expressed by the
following equations in the discontinuous mode and continuous mode.
2 × f
OSC
× L × V
IN
I
PK
= I
OUT
+ V
OUT
× (V
IN
− V
OUT
)
fOSC = Oscillation frequency
Table 7 Typical Inductors
Manufacturer Part Number L Value DC
Resistance Rated Current Dimensions
(L × W × H) [mm]
NR4018T3R3M 3.3 µH 0.07 Ω max. 1.23 A max. 4.0 × 4.0 × 1.8
Taiyo Yuden Co., Ltd.
NR3012T3R3M 3.3 µH 0.1 Ω max. 0.91 A max. 3.0 × 3.0 × 1.2
CDRH3D16/HP-3R3 3.3 µH 0.085 Ω max. 1.40 A max. 4.0 × 4.0 × 1.8
Sumida Corporation
CDRH2D11/HP-3R3 3.3 µH 0.173 Ω max. 0.9 A max. 3.2 × 3.2 × 1.2
VLF4012AT-3R3M 3.3 µH 0.12 Ω max. 1.3 A max. 3.7 × 3.5 × 1.2
TDK Corporation
VLF3010AT-3R3M 3.3 µH 0.17 Ω max. 0.87 A max. 2.6 × 2.8 × 1.0
MIP3226D3R3M 3.3 µH 0.104 Ω max. 1.2 A max. 3.2 × 2.6 × 1.0
FDK Corporation
MIPS2520D3R3M 3.3 µH 0.156 Ω max. 1.0 A max. 2.5 × 2.0 × 1.0
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 13
2. Capacitors (CIN, COUT)
A ceramic capacitor can be used for the input (CIN) and output (COUT) sides. CIN lowers the power supply
impedance and averages the input current to improve efficiency. Select CIN according to the impedance of the
power supply to be used. The recommended capacitance is 4.7 µF for the S-8550/8551 Series when a general
lithium ion rechargeable battery is used.
Select as COUT a capacitor with large capacitance and small ESR for smoothing the ripple voltage. The
optimum capacitor selection depends on the L value, capacitance value, wiring, and application (output load).
Select COUT after sufficient evaluation under actual use conditions.
3. Output voltage setting resistors (RFB1, RFB2), capacitor for phase compensation (CFB)
With the S-8550/8551 Series, VOUT can be set to any value by external divider resistors. Connect the divider
resistors across the VOUT and VSS pins. Because VFB = 0.6 V typ., VOUT can be calculated by this equation.
R
FB2
V
OUT
= (R
FB1
+ R
FB2
) × 0.6
Connect divider resistors RFB1 and RFB2 as close to the IC to minimize effects from of noise. If noise does have
an effect, adjust the values of RFB1 and RFB2 so that RFB1 + RFB2 < 100 kΩ.
CFB connected in parallel with RFB1 is a capacitor for phase compensation.
By setting the zero point (the phase feedback) by adding capacitor CFB to output voltage setting resistor RFB1 in
parallel, the feedback loop gains the phase margin. As a result, the stability can be obtained. In principle, to use
the portion how much the phase has feed back by the zero point effectively, define CFB referring to the following
equation.
kHz70xRxx2
1
C
1FB
FB π
≅
This equation is the reference.
The followings are explanation regarding the proper setting.
To use the portion how much the phase has feed back by the zero point effectively, set RFB1 and CFB so that the
zero point goes into the higher frequency than the pole frequency of L and COUT. The following equations are the
pole frequency of L and COUT and the zero point frequency by CFB and RFB1.
OUTCxLxx2
1
fpole π
≅
FB1FB
zero CxRxx2
1
fπ
≅
The transient response can be improved by setting the zero point frequency in the range of lower frequency.
However, since the gain becomes higher in the range of high frequency, the total phase of feedback loop delays
180° or more by setting the zero point frequency in the significantly lower range. As a result, the gain cannot be
0 dB or lower in the frequency range thus the operation might be unstable. Determine the proper value after the
sufficient evaluation under the actual condition.
The typical constants by our evaluation are in Table 8.
Table 8 Constant for External Parts
VOUT(s) [V] RFB1 [kΩ] RFB2 [kΩ] CFB [pF] L [µH]*1 C
OUT [µF]*1
1.1 36 43 56 3.3 10
1.8 36 18 68 3.3 10
3.3 36 8 120 3.3 10
4.0 51 9 100 3.3 10
*1. The recommended parts in Table 5
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
14 Seiko Instruments Inc.
Standard Circuit
L
ON/OFF
V
IN
+
−
+
−
C
IN
FB
VIN
CONT
VSS
C
FB
R
FB1
R
FB2
V
OUT
C
OUT
*1
*1
4.7
µ
F
3.3
µ
H
36 k
Ω
18 k
Ω
10
µ
F
68 pF
1.0
µ
F
Reference
voltage
PWM comparator
PWM control
circuit
UVLO circuit
ON/OFF
circuit
IC internal
power supply
Current limit
circuit
Error amplifier
Ground point
Triangular wave
generation
circuit
*1. Parasitic diode
Figure 11
Caution The above connection diagram and constant will not guarantee successful operation. Perform
thorough evaluation using an actual application to set the constants.
Precaution
• Mount external capacitors, diodes, and inductor as close as possible to the IC.
• Characteristics ripple voltage and spike noise occur in IC containing switching regulators. Moreover rush current
flows at the time of a power supply injection. Because these largely depend on the inductor, the capacitor and
impedance of power supply used, fully check them using an actually mounted model.
• The 1.0 µF capacitance connected between the VIN and VSS pins is a bypass capacitor. It stabilizes the power
supply in the IC when application is used with a heavy load, and thus effectively works for stable switching
regulator operation. Allocate the bypass capacitor as close to the IC as possible, prioritized over other parts.
• Although the IC contains a static electricity protection circuit, static electricity or voltage that exceeds the limit of
the protection circuit should not be applied.
• The power dissipation of the IC greatly varies depending on the size and material of the board to be connected.
Perform sufficient evaluation using an actual application before designing.
• Seiko Instruments Inc. assumes no responsibility for the way in which this IC is used on products created using
this IC or for the specifications of that product, nor does Seiko Instruments Inc. assume any responsibility for any
infringement of patents or copyrights by products that include this IC either in Japan or in other countries.
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 15
Characteristics (Typical Data)
1. Example of Major Power Supply Dependence Characteristics (Ta = 25°C)
1. 1 Current consumption 1 (I
SS1
) vs. Input voltage (V
IN
)
1. 2 Current consumption during shutdown (I
SSS
) vs. Input voltage (V
IN
)
2.0 4.0 5.0 5.5
I
SS1
[µA]
500
400
300
200
100
0
V
IN
[V]
2.5 3.0 3.5 4.5
2.0 4.0 5.0 5.5
I
SSS
[µA]
1.0
0.8
0.6
0.4
0.2
0
V
IN
[V]
2.5 3.0 3.5 4.5
1. 3 Oscillation frequency (f
osc
) vs. Input voltage (V
IN
)
1. 4 Soft-start time (tSS) vs. Input voltage (VIN)
2.0 4.0 5.0 5.5
f
OSC
[MHz]
1.38
1.30
1.18
1.10
1.02
V
IN
[V]
2.5 3.0 3.5 4.5
1.06
1.14
1.22
1.26
1.34
2.0 4.0 5.0 5.5
t
SS
[ms]
1.3
1.1
1.0
0.9
0.7
V
IN
[V]
2.5 3.0 3.5 4.5
0.8
1.2
1. 5 Power MOS FET on-resistance (R
FET
) vs. Input voltage (V
IN
)
1. 6 Power MOS FET leakage current (I
LSW
) vs. Input voltage (V
IN
)
2.0 4.0 5.0 5.5
R
FET
[Ω]
0.8
0.6
0.5
0.4
0.2
V
IN
[V]
2.5 3.0 3.5 4.5
0.3
0.7
Nch
Pch
2.0 4.0 5.0 5.5
I
LSW
[µA]
0.5
0.1
0
−0.1
−0.5
V
IN
[V]
2.5 3.0 3.5 4.5
−0.4
0.4
Nch
Pch
0.2
0.3
−0.2
−0.3
1. 7 ON/OFF pin input voltage“H” (V
SH
) vs. Input voltage (V
IN
)
1. 8 ON/OFF pin input voltage“L” (V
SL
) vs. Input voltage (V
IN
)
2.0 4.0 5.0 5.5
V
SH
[V]
0.9
0.6
0.3
V
IN
[V]
2.5 3.0 3.5 4.5
0.7
0.8
0.5
0.4
2.0 4.0 5.0 5.5
V
SL
[V]
0.9
0.6
0.3
V
IN
[V]
2.5 3.0 3.5 4.5
0.7
0.8
0.5
0.4
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
16 Seiko Instruments Inc.
1. 9 FB voltage (VFB) vs. Input voltage (VIN)
2.0 4.0 5.0 5.5
V
FB
[mV]
612
600
588
V
IN
[V]
2.5 3.0 3.5 4.5
604
608
596
592
2. Example of Major Temperature Characteristics (Ta = −40 to 85°C)
2. 1 Current consumption 1 (I
SS1
) vs. Temperature (Ta)
2. 2 Current consumption during shutdown (I
SSS
) vs. Temperature (Ta)
−40 75 85
I
SS1
[
µA
]
500
0
−25 0 25 50
300
400
200
100
Ta [ °C]
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
−40 75 85
I
SSS
[
µA
]
1.0
0
−25 0 25 50
0.6
0.8
0.4
0.2
Ta [ °C]
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
2. 3 Oscillation frequency (f
osc
) vs. Temperature (Ta)
2. 4 Soft-start time (tSS) vs. Temperature (Ta)
−40 75 85
fOSC
[MHz]
1.32
1.08
−25 0 25 50
1.28
1.20
1.16
Ta [ °C]
V
IN
= 5.5 V
V
IN
= 3.6 V
V
IN
= 2.0 V
1.12
1.24
−40 75 85
tSS
[ms]
1.3
0.7
−25 0 25 50
1.1
1.0
0.9
Ta [ °C]
V
IN
= 5.5 V
V
IN
= 3.6 V
V
IN
= 2.0 V
0.8
1.2
2. 5 Power MOS FET on-resistance (R
FET
) vs. Temperature (Ta)
2. 6 Power MOS FET leakage current (I
LSW
) vs. Temperature (Ta)
−40 75 85
R
FET
[Ω]
0.8
0.2
−25 0 25 50
Ta [ °C]
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
Pch Nch
0.6
0.5
0.4
0.3
0.7
−40 75 85
I
LSW
[µA]
0.5
−0.5
−25 0 25 50
Ta [ °C]
VIN = 5.5 V
VIN = 5.5 V
Pch
Nch
0.1
0
−0.1
−0.4
0.4
0.2
0.3
−0.2
−0.3
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 17
2. 7 ON/OFF pin input voltage“H” (V
SH
) vs. Temperature (Ta)
2. 8 ON/OFF pin input voltage“L” (V
SL
) vs. Temperature (Ta)
−40 75 85
VSH
[V]
0.9
0.3
−25 0 25 50
Ta [ °C]
0.6
0.7
0.8
0.5
0.4
V
IN
= 3.6 V
V
IN
= 5.5 V
V
IN
= 2.0 V
−40 75 85
VSL
[V]
0.9
0.3
−25 0 25 50
Ta [ °C]
0.6
0.7
0.8
0.5
0.4
V
IN
= 5.5 V
V
IN
= 3.6 V
V
IN
= 2.0 V
2. 9 UVLO detection voltage (V
UVLO
) vs. Temperature (Ta)
2. 10 FB voltage (VFB) vs. Temperature (Ta)
−40 75 85
V
UVLO
[V]
1.80
1.40
−25 0 25 50
Ta [ °C]
1.60
1.70
1.75
1.55
1.45
1.50
1.65
−40 75 85
V
FB
[mV]
612
588
−25 0 25 50
Ta [ °C]
600
604
608
596
592
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
3. Examples of Transient Response Characteristics
(Unless otherwise specified, the used parts are ones shown in
External Parts When Measuring Electrical Characteristics
.)
3. 1 Powering ON (VOUT = 1.8 V, VIN = 0 V → 3.6 V, Ta = 25°C)
(1) IOUT = 1 mA (2) IOUT = 600 mA
−0.2 0.6 1.0 1.6
V
IN
, V
OUT
[V]
4
3
1
0
−1
t [ms]
I
L
[A]
0.6
0.2
0.4
−0.2
0
0 0.2 0.4 0.8
V
OUT
V
IN
I
L
2
1.2 1.4
−0.2 0.6 1.0 1.6
VIN, VOUT [V]
4
3
1
0
−1
t [ms]
IL [A]
1.5
0.5
1.0
−0.5
0
0 0.2 0.4 0.8
2
1.2 1.4
V
OUT
V
IN
I
L
3. 2 Shutdown pin response (VOUT = 1.8 V, VIN = 3.6 V, VON/OFF = 0 V → 3.6 V, Ta = 25°C)
(1) IOUT = 1 mA (2) IOUT = 600 mA
−0.2 0.6 1.0 1.6
V
ON/OFF
, V
OUT
[V]
4
3
1
0
−1
t [ms]
I
L
[A]
0.6
0.2
0.4
−0.2
0
0 0.2 0.4 0.8
2
1.2 1.4
V
OUT
V
ON/OFF
I
L
−0.2 0.6 1.0 1.6
VON/OFF, VOUT [V]
4
3
1
0
−1
t [ms]
IL [A]
1.5
0.5
1.0
−0.5
0
0 0.2 0.4 0.8
2
1.2 1.4
V
OUT
V
ON/OFF
I
L
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
18 Seiko Instruments Inc.
3. 3 Power supply fluctuations (VOUT = 1.8 V, Ta = 25°C)
(1) IOUT = 1 mA, VIN = 2.6 V → 3.6 V → 2.6 V (2) IOUT = 600 mA, VIN = 2.6 V → 3.6 V → 2.6 V
−0.1 0.3 0.5 0.7
V
OUT
[V]
2.2
2.0
1.8
1.6
1.4
t [ms]
V
IN
[V]
3.5
2.5
1.5
0.5
4.5
0 0.1 0.2 0.4 0.6
V
OUT
V
IN
−0.1 0.3 0.5 0.7
VOUT [V]
2.2
2.0
1.8
1.6
1.4
t [ms]
VIN [V]
3.5
2.5
1.5
0.5
4.5
0 0.1 0.2 0.4 0.6
V
OUT
V
IN
3. 4 Load fluctuations (VOUT = 1.8 V, VIN = 3.6 V, Ta = 25°C)
(1) IOUT = 0.1 mA → 100 mA → 0.1 mA (2) IOUT = 0.1 mA → 300 mA → 0.1 mA
−0.1 0.3 0.5 0.7
V
OUT
[V]
1.90
1.85
1.80
1.75
1.70
t [ms]
I
OUT
[mA]
200
100
0
−100
400
0 0.1 0.2 0.4 0.6
V
OUT
I
OUT
300
−0.1 0.3 0.5 0.7
VOUT [V]
1.90
1.85
1.80
1.75
1.70
t [ms]
IOUT [mA]
200
100
0
−100
400
0 0.1 0.2 0.4 0.6
300
V
OUT
I
OUT
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 19
Reference Data
1. Reference data for external parts
Properties of External Parts
Element Name Product Name Manufacture Characteristics
Inductor NR4018T3R3M
Taiyo Yuden Co., Ltd 3.3 µH, DCRMAX = 0.07 Ω, IMAX = 1.23 A
Input capacitor C3216X7R1E475K TDK Corporation 4.7 µF
Output capacitor C3216X7R1C106K TDK Corporation 10 µF
Caution The values of the external parts are based on the materials provided by each manufacturer. However,
consider the characteristics of the original materials when using the above products.
2. Output current (I
OUT
) vs. Efficiency (
η
) Characteristics and Output current (I
OUT
) vs. Output voltage (V
OUT
) Characteristics
2. 1 VOUT = 1.1 V (RFB1 = 36 kΩ, RFB2 = 43 kΩ)
(1) Output current (IOUT) vs. Efficiency (η) (2) Output current (IOUT) vs. Output voltage (VOUT)
0 1000
η [%]
100
0
1 10 100
I
OUT
[mA]
50
70
80
30
10
20
40
60
90 VIN = 2.0 V
VIN = 3.6 V
VIN = 5.5 V
0 1000
VOUT [V]
1.3
0.9
1 10 100
I
OUT
[mA]
1.1
1.2
1.0
VIN = 5.5 V
VIN = 3.6 V
VIN = 2.0 V
2. 2 VOUT = 1.8 V (RFB1 = 36 kΩ, RFB2 = 18 kΩ)
(1) Output current (IOUT) vs. Efficiency (η) (2) Output current (IOUT) vs. Output voltage (VOUT)
0 1000
η [%]
100
0
1 10 100
IOUT [mA]
50
70
80
30
10
20
40
60
90 V
IN
= 2.2 V
V
IN
= 3.6 V
V
IN
= 5.5 V
0 1000
V
OUT
[V]
2.0
1.6
1 10 100
IOUT [mA]
1.8
1.9
1.7
V
IN
= 5.5 V
V
IN
= 3.6 V
V
IN
= 2.0 V
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
20 Seiko Instruments Inc.
2. 3 VOUT = 3.3 V (RFB1 = 36 kΩ, RFB2 = 8 kΩ)
(1) Output current (IOUT) vs. Efficiency (η) (2) Output current (IOUT) vs. Output voltage (VOUT)
0 1000
η [%]
100
0
1 10 100
IOUT [mA]
50
70
80
30
10
20
40
60
90 V
IN
= 3.7 V
V
IN
= 5.5 V
0 1000
V
OUT
[V]
3.5
3.1
1 10 100
IOUT [mA]
3.3
3.4
3.2
V
IN
= 5.5 V
V
IN
= 3.7 V
2. 4 VOUT = 4.0 V (RFB1 = 51 kΩ, RFB2 = 9 kΩ)
(1) Output current (IOUT) vs. Efficiency (η) (2) Output current (IOUT) vs. Output voltage (VOUT)
0 1000
η [%]
100
0
1 10 100
IOUT [mA]
50
70
80
30
10
20
40
60
90 V
IN
= 4.4 V
V
IN
= 5.5 V
0 1000
V
OUT
[V]
4.2
3.8
1 10 100
IOUT [mA]
4.0
4.1
3.9
V
IN
= 5.5 V
V
IN
= 4.4 V
3. Output current (IOUT) vs. Ripple voltage (Vr) Characteristics
3. 1 VOUT = 1.1 V (RFB1 = 36 kΩ, RFB2 = 43 kΩ)
(1) VIN = 3.6 V (2) VIN = 5.5 V
0 1000
Vr [mV]
50
0
1 10 100
I
OUT
[mA]
30
40
10
20
0 1000
Vr [mV]
50
0
1 10 100
I
OUT
[mA]
30
40
10
20
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01 S-8550/8551 Series
Seiko Instruments Inc. 21
3. 2 VOUT = 1.8 V (RFB1 = 36 kΩ, RFB2 = 18 kΩ)
(1) VIN = 3.6 V (2) VIN = 5.5 V
0 1000
V
r
[mV]
50
0
1 10 100
IOUT [mA]
30
40
10
20
0 1000
V
r
[mV]
50
0
1 10 100
IOUT [mA]
30
40
10
20
3. 3 VOUT = 3.3 V (RFB1 = 36 kΩ, RFB2 = 8 kΩ)
(1) VIN = 3.6 V (2) VIN = 5.5 V
0 1000
Vr [mV]
50
0
1 10 100
I
OUT
[mA]
30
40
10
20
0 1000
Vr [mV]
50
0
1 10 100
I
OUT
[mA]
30
40
10
20
3. 4 VOUT = 4.0 V (RFB1 = 51 kΩ, RFB2 = 9 kΩ)
(1) VIN = 5.5 V
0 1000
V
r
[mV]
50
0
1 10 100
IOUT [mA]
30
40
10
20
STEP-DOWN
,
BUILT-IN FET
,
SYNCHRONOUS RECTIFICATION
,
PWM CONTROL SWITCHING REGULATORS
S-8550/8551 Series Rev.2.1_01
22 Seiko Instruments Inc.
Marking Specification
(1) SOT-23-5
(1) to (3) : Product code (Refer to Product name vs. Product code.)
(4) : Lot number
5 4
1 3
2
(1) (2) (3) (4)
SOT-23-5
To
p
vie
w
Product name vs. Product code
(a) S-8550 Series (b) S-8551 Series
Product Code Product Code
Product Name (1) (2) (3) Product Name (1) (2) (3)
S-8550AA-M5T1G R 5 A S-8551AA-M5T1G R 5 C
No.
TITLE
SCALE
UNIT mm
Seiko Instruments Inc.
2.9±0.2
1.9±0.2
0.95±0.1
0.4±0.1
0.16 +0.1
-0.06
123
4
5
No. MP005-A-P-SD-1.2
MP005-A-P-SD-1.2
SOT235-A-PKG Dimensions
No.
TITLE
SCALE
UNIT mm
Seiko Instruments Inc.
ø1.5 +0.1
-0 2.0±0.05
ø1.0 +0.2
-0 4.0±0.1
1.4±0.2
0.25±0.1
3.2±0.2
123
45
No. MP005-A-C-SD-2.1
MP005-A-C-SD-2.1
SOT235-A-Carrier Tape
Feed direction
4.0±0.1(10 pitches:40.0±0.2)
No.
TITLE
SCALE
UNIT mm
Seiko Instruments Inc.
12.5max.
9.0±0.3
ø13±0.2
(60°) (60°)
QTY. 3,000
No. MP005-A-R-SD-1.1
MP005-A-R-SD-1.1
SOT235-A-Reel
Enlarged drawing in the central part
•
The information described herein is subject to change without notice.
• Seiko Instruments Inc. is not responsible for any problems caused by circuits or diagrams described herein
whose related industrial properties, patents, or other rights belong to third parties. The application circuit
examples explain typical applications of the products, and do not guarantee the success of any specific
mass-production design.
• When the products described herein are regulated products subject to the Wassenaar Arrangement or other
agreements, they may not be exported without authorization from the appropriate governmental authority.
• Use of the information described herein for other purposes and/or reproduction or copying without the
express permission of Seiko Instruments Inc. is strictly prohibited.
• The products described herein cannot be used as part of any device or equipment affecting the human
body, such as exercise equipment, medical equipment, security systems, gas equipment, or any apparatus
installed in airplanes and other vehicles, without prior written permission of Seiko Instruments Inc.
• Although Seiko Instruments Inc. exerts the greatest possible effort to ensure high quality and reliability, the
failure or malfunction of semiconductor products may occur. The user of these products should therefore
give thorough consideration to safety design, including redundancy, fire-prevention measures, and
malfunction prevention, to prevent any accidents, fires, or community damage that may ensue.
