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General Description
The MAX1896 step-up DC-DC converter incorporates
high-performance current-mode, fixed-frequency,
pulse-width modulation (PWM) circuitry and an internal
0.7ΩN-channel MOSFET to provide a highly efficient
regulator with fast response.
High switching frequency (1.4MHz) allows fast loop
response and easy filtering with small components. The
MAX1896 can produce an output voltage as high as
13V from an input as low as 2.6V. Soft-start is program-
mable with an external capacitor, which sets the input
current ramp rate. In shutdown mode, current con-
sumption is reduced to 0.01µA.
The MAX1896 is available in a space-saving 6-pin
SOT23 package. The ultra-small package and high
switching frequency allow cost and space-efficient
implementations.
Applications
Notebook Computers
LCD Displays
PCMCIA Cards
Portable Applications
Hand-Held Devices
Features
♦>90% Efficiency
♦Adjustable Output Up to 13V
♦Guaranteed 12V/120mA Output from 5V Input
♦2.6V to 5.5V Input Range
♦LT1613 Pin Compatible
♦0.01µA Shutdown Current
♦Programmable Soft-Start
♦Space-Saving 6-Pin SOT23 Package
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
________________________________________________________________ Maxim Integrated Products 1
GND
SHDNFB
16IN
5SS
LX
MAX1896
SOT23
TOP VIEW
2
34
Pin Configuration
ON
OFF
MAX1896
INPUT
2.6V TO 5.5V
OUTPUT
UP TO 13V
UP TO 600mA
R1
R2
IN LX
FBSHDN
SS GND
Typical Operating Circuit
PART TEMP RANGE
PIN-PACKAGE
MAX1896EUT-T -40°C to +85°C6 SOT23-6
19-2221; Rev 1; 3/04
Ordering Information
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 3V, FB = GND, SS = open, TA= 0°C to +85°C, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
LX to GND ..............................................................-0.3V to +14V
IN, SHDN, FB to GND...............................................-0.3V to +6V
SS to GND ...................................................-0.3V to (VIN + 0.3V)
RMS LX Pin Current ..............................................................0.6A
Continuous Power Dissipation (TA= +70°C) (Note 1)
6-Pin SOT23 (derate 9.1mW/°C above +70°C)...........727mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Input Supply Range VIN 2.6 5.5 V
Output Voltage Adjust Range VOUT Circuit of Figure 1 13 V
VIN Undervoltage Lockout UVLO VIN rising, 50mV hysteresis
2.25
2.4
2.55
V
VFB = 1.3V, not switching 0.2 0.4
Quiescent Current IIN VFB = 1.0V, switching 1 5
mA
V SHDN = 0, TA = +25°C
0.01
0.5
Shutdown Supply Current V SHDN = 0
0.01
10 µA
ERROR AMPLIFIER
Feedback Regulation Set Point VFB 1.2
1.24 1.25
V
FB Input Bias Current IFB VFB = 1.24V 21 80 nA
Line Regulation 2.6V < VIN < 5.5V
0.05 0.20
%/V
OSCILLATOR
Frequency fOSC
1000 1400 1800
kHz
Maximum Duty Cycle DC 82 86 %
POWER SWITCH
Current Limit (Note 2) ILIM VFB = 1V, duty cycle = 50%
0.55
0.8 A
On-Resistance RON 0.7 1 Ω
VLX = 12V, TA = +25°C 0.1 1
Leakage Current
ILXOFF
VLX = 12V 10 µA
SOFT-START
Reset Switch Resistance 100 Ω
Charge Current VSS = 1.2V 1.5 4 7.0 µA
CONTROL INPUT
Input Low Voltage VIL V SHDN, VIN = 2.6V to 5.5V 0.3 V
Input High Voltage VIH V SHDN, VIN = 2.6V to 5.5V 1.0 V
V SHDN = 3V 25 50
SHDN Input Current ISHDN V SHDN = 0
0.01
0.1 µA
Note 1: Thermal properties are specified with product mounted on PC board with one square-inch of copper area and still air.
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 3V, FB = GND, SS = open, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)
Note 2: Current limit varies with duty cycle due to slope compensation. See the Output Current Capability section.
Note 3: Specifications to -40°C are guaranteed by design and not production tested.
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Input Supply Range VIN 2.6 5.5 V
Output Voltage Adjust Range VOUT Circuit of Figure 1 13 V
VIN Undervoltage Lockout
UVLO
VIN rising, 50mV hysteresis.
2.25
2.55 V
VFB = 1.3V, not switching 0.4
Quiescent Current IIN VFB = 1.0V, switching 5 mA
Shutdown Supply Current V SHDN = 0 10 µA
ERROR AMPLIFIER
Feedback Regulation Set Point VFB 1.2 1.25 V
FB Input Bias Current IFB VFB = 1.24V 80 nA
Line Regulation 2.6V < VIN < 5.5V 0.20
%/V
OSCILLATOR
Frequency fOSC
1000 1800
kHz
Maximum Duty Cycle DC 82 %
POWER SWITCH
Current Limit (Note 2) ILIM VFB = 1V, duty cycle = 50%
0.55
A
On-Resistance RON 1Ω
Leakage Current
ILXOFF
VLX = 12V 10 µA
SOFT-START
Reset Switch Resistance 100 Ω
Charge Current VSS = 1.2V
1.25
7.50 µA
CONTROL INPUT
Input Low Voltage VIL V SHDN = VIN = 2.6V to 5.5V 0.3 V
Input High Voltage VIH V SHDN = VIN = 2.6V to 5.5V 1.0 V
V SHDN = 3V 50
SHDN Input Current
I SHDN
V SHDN = 0 0.1 µA
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
4_______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 3.3V, TA= +25°C, unless otherwise noted.)
100
50 110100 1000
EFFICIENCY vs. OUTPUT CURRENT
60
MAX1896 toc01
OUTPUT CURRENT (mA)
EFFICIENCY (%)
70
80
90
VIN = 3.3V,
VOUT = 5V,
CIRCUIT OF FIGURE 1
100
50 110100 1000
EFFICIENCY vs. OUTPUT CURRENT
60
MAX1896 toc02
OUTPUT CURRENT (mA)
EFFICIENCY (%)
70
80
90
VIN = 3.3V,
VOUT = 13V,
CIRCUIT OF FIGURE 3
100
50 110100 1000
EFFICIENCY vs. OUTPUT CURRENT
60
MAX1896 toc03
OUTPUT CURRENT (mA)
EFFICIENCY (%)
70
80
90
VIN = 5V,
VOUT = 13V,
CIRCUIT OF FIGURE 3
2.0
1.5
1.0
0.5
02.5 4.03.0 3.5 4.5 5.0 5.5
NO LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1896 toc04
INPUT VOLTAGE (V)
NO LOAD SUPPLY CURRENT (mA)
VOUT = 13V,
CIRCUIT OF FIGURE 3
12.90
12.95
13.00
13.05
13.10
OUTPUT VOLTAGE vs. OUTPUT CURRENT
MAX1896 toc05
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
0 10050 150 200
CIRCUIT OF FIGURE 3
TA = -40°CTA = +85°C
TA = +25°C
LOAD
CURRENT
200mA/div
OUTPUT
VOLTAGE
AC-COUPLED
200mV/div
INDUCTOR
CURRENT
500mA/div
400µs/div
COUT = 0.1µF CERAMIC + 22µF TANTALUM
LOAD TRANSIENT (VOUT = 5V)
MAX1896 toc07
SHDN
5V/div
OUTPUT
VOLTAGE
5V/div
INDUCTOR
CURRENT
500mA/div
100µs/div
VIN = 3.3V, COUT = 0.1µF CERAMIC + 3.3µF TANTALUM
CIRCUIT OF FIGURE 3
STARTUP WAVEFORM
WITHOUT SOFT-START
MAX1896 toc08
SHDN
5V/div
OUTPUT
VOLTAGE
5V/div
IOUT = 10mA
INDUCTOR
CURRENT
500mA/div
2ms/div
VIN = 3.3V, CSS = 33nF,
COUT = 3.3µF TANTALUM + 0.1µF CERAMIC
CIRCUIT OF FIGURE 3
STARTUP WAVEFORM
WITH SOFT-START
MAX1896 toc09
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
_______________________________________________________________________________________ 5
Pin Description
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.3V, TA= +25°C, unless otherwise noted.)
SHDN
5V/div
OUTPUT
VOLTAGE
5V/div
IOUT = 100mA
INDUCTOR
CURRENT
500mA/div
2ms/div
VIN = 3.3V, CSS = 33nF,
COUT = 3.3µF TANTALUM + 0.1µF CERAMIC
CIRCUIT OF FIGURE 3
STARTUP WAVEFORM
WITH SOFT-START
MAX1896 toc10
LX VOLTAGE
5V/div
OUTPUT
VOLTAGE
AC-COUPLED
200mV/div
INDUCTOR
CURRENT
500mA/div
400ns/div
VIN = 5V,
COUT = 0.1µF CERAMIC + 2.2µF CERAMIC
SWITCHING WAVEFORM
MAX1896 toc11
IOUT = 150mA
600
500
VOUT = 5V VOUT = 12V
300
400
100
200
02.5 4.03.0 3.5 4.5 5.0 5.5
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX1896 toc12
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
MAXIMUM OUTPUT
CURRENT DEFINED AT
90% OF NO LOAD
OUTPUT VOLTAGE
PIN NAME FUNCTION
1LX
Power Switching Connection. Connect LX to the inductor and output rectifier. Connect components
as close to LX as possible.
2GND Ground
3FB
Feedback Input. Connect a resistive voltage-divider from the output to FB to set the output voltage.
See the Setting the Output Voltage section.
4SHDN
Shutdown Input. Drive SHDN low to turn off the converter. To automatically start the converter,
connect SHDN to IN. Drive SHDN with a slew rate of 0.1V/µs or greater. Do not leave SHDN
unconnected. SHDN draws up to 50µA.
5SS
Soft-Start Input. Connect a soft-start capacitor from SS to GND to soft-start the converter. Leave SS
open to disable the soft-start function. See the Soft-Start section.
6IN
Internal Bias Voltage Input. Connect IN to the input voltage source. Bypass IN to GND with a
1µF or greater capacitor as close to IN as possible.
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
6_______________________________________________________________________________________
Detailed Description
The MAX1896 is a highly efficient power supply that
employs a current-mode, fixed-frequency pulse-width
modulation (PWM) architecture for fast-transient response
and low-noise operation. The functional diagram is shown
in Figure 2. As the load varies, the error amplifier sets the
inductor peak current necessary to supply the load and
regulate the output voltage. To maintain stability at high
duty cycle, a slope-compensation signal is internally
summed with the current-sense signal.
At light loads, this architecture allows the MAX1896 to
skip cycles to prevent overcharging the output voltage.
In this region of operation, the inductor ramps up to a
peak value of about 100mA, discharges to the output
and waits until another pulse is needed again.
Output-Current Capability
The output-current capability of the MAX1896 is a func-
tion of current limit, input voltage, and inductor value.
Because of the slope compensation used to stabilize
the feedback loop, the duty cycle affects the current
limit. The output-current capability is governed by the
following equation:
where:
ILIM = current limit specified at 50% (see Electrical
Characteristics)
VDIODE = catch diode forward drop at ILIM, (V)
fOSC = oscillator frequency, (Hz)
L = inductor value, (H)
η= conversion efficiency, 0.85 nominal
VIN = input voltage, (V)
VOUT = output voltage, (V)
Soft-Start
The MAX1896 can be programmed for soft-start upon
power-up with an external capacitor. When the
MAX1896 is turned on, the soft-start capacitor (CSS) is
charged at a constant current of 4µA, ramping up to
0.5V. During this time, the SS voltage directly controls
the peak-inductor current, allowing 0A at VSS = 0.5V to
the full current limit at VSS = 1.5V. The maximum load
current is available after the soft-start cycle is complet-
ed. When the MAX1896 is turned off, the soft-start
capacitor is internally discharged to ground.
Shutdown
The MAX1896 shuts down to reduce the supply current
to 0.01µA when SHDN is low. In this mode, the internal
reference, error amplifier, comparators, biasing circuit,
and N-channel MOSFET are turned off. The step-up
converter’s output is still connected to IN via the exter-
nal inductor and output rectifier.
Applications Information
The MAX1896 operates well with a variety of external
components. The components in Figure 1 are suitable
for most applications. See the following sections to opti-
mize external components for a particular application.
Inductor Selection
Inductor selection depends on input voltage, output volt-
age, maximum current, size, and availability of inductor
values. Other factors can include efficiency and ripple
voltage. Inductors are specified by their inductance (L),
peak current (IPK), and resistance (RL). The following
step-up circuit equations are useful in choosing the
inductor values based on the application. They allow the
trading of peak current and inductor value while consid-
ering component availability and cost.
The equation used here assumes a constant LIR, which
is the ratio of the inductor peak-to-peak AC current to
average DC inductor current. A good compromise
between the size of the inductor versus loss and output
ripple is to choose an LIR of 0.3 to 0.5. The peak induc-
tor current is then given by:
where:
IOUT(MAX) = maximum output current, (A)
VIN(MIN) = minimum input voltage, (V)
IIxV
xV xLIR
PK OUT MAX OUT
IN MIN
()
()
=
+
η12
DUTY DUTY CYCLE
VVV
VIxRV
OUT IN DIODE
OUT LIM ON DIODE
==
+
+
−
−
I
IDuty x V
f
xx
V
V
OUT MAX
LIM xxDuty IN
OSC x L
IN
OUT
()
( . . ))
( .
=
×
−−145 09 05
η
The inductance (H) value is then given by:
Diode Selection
The output diode should be rated to handle the output
voltage and the peak switch current. Make sure the
diode’s peak current rating is at least IPK and that its
breakdown voltage exceeds VOUT. Schottky diodes are
recommended. If a junction rectifier is used, it must be
an ultra-fast type (trr < 50ns) to prevent excessive loss
in the rectifier.
Input and Output Capacitor Selection
The MAX1896 operates with both tantalum and ceramic
output capacitors. When using tantalum capacitors, the
zero caused by the ESR of the tantalum is used to
ensure stability. When using ceramic capacitors, the
zero due to the ESR will be at too high a frequency to
be useful in stabilizing the control loop. When using
ceramic capacitors, use a feedforward capacitor to
increase the phase margin, improving the control-loop
stability. Figure 3 shows the circuit with ceramic capac-
itors and the feedforward capacitor, CFF. Use the fol-
lowing equation to determine the value of the
feedforward capacitor:
where:
R1 = see Figure 3, (Ω)
COUT = total output capacitance including any bypass
capacitor on the output bus, (Farads). See Figure 3.
VOUT = output voltage, (V)
VIN = input voltage, (V).
Setting the Output Voltage
The MAX1896 operates with an adjustable output from
VIN to 13V. Connect a resistive voltage-divider from the
output to FB (see Typical Operating Circuit). Choose a
value for R2 between 10kΩand 50kΩ. Calculate R1
using the equation:
where VFB, the step-up regulator feedback set point, is
1.24V. Connect the resistive-divider as close to the IC
as possible.
Soft-Start Capacitor
The soft-start capacitor should be large enough that the
current limit does not reach final value before the output
has reached regulation. Calculate CSS to be:
where:
k2= 21 x 10-6, (S)
VOUT = maximum output voltage, (V)
IINRUSH = peak inrush current allowed, (A)
IOUT = maximum output current during power-up stage, (A)
VIN = minimum input voltage, (V)
The soft-start duration (tSS) is the time it takes the cur-
rent limit to reach its final value. The soft-start duration
can be calculated by the equation:
tss = k3✕CSS
where:
k3= 6.67 ✕105Ω
CkxCx
VVxV
VxI I xV
SS OUT
OUT IN OUT
IN INRUSH OUT OUT
>
−
−
2
2
RRx
V
V
OUT
FB
12 1 =
−
kxwith units of xF
A
171410 405
.
.
=
−Ω
Ck
RxCxV
V
FF OUT OUT
IN
.
=
1
1
205
LVxxVV
VxLIR x I x f
IN MIN OUT IN MIN
OUT OUT MAX OSC
[ ( )]
() ()
()
=−
2
2
η
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
_______________________________________________________________________________________ 7
MAX1896
Application Circuits
1-Cell to 3.3V SEPIC Power Supply
Figure 4 shows the MAX1896 in a single-ended primary
inductance converter (SEPIC) topology. This topology
is useful when the input voltage can be either higher or
lower than the output voltage, such as when converting
a single lithium-ion (Li+) cell to a 3.3V output. L1 and
L2 are two windings on a single inductor or two sepa-
rate inductors. The coupling capacitor between these
two windings must be a low-ESR type to achieve maxi-
mum efficiency, and must also be able to handle high
ripple currents. Ceramic capacitors are best for this
application.
Layout Procedure
Good PC board layout and routing are required in high-
frequency switching power supplies to achieve good
regulation, and stability. It is strongly recommended
that the evaluation kit PC board layouts be followed as
closely as possible. Refer to the MAX1896 EV kit for a
good layout. Place power components as close togeth-
er as possible, keeping their traces short, direct, and
wide. Avoid interconnecting the ground pins of the
power components using vias through an internal
ground plane. Instead, keep the power components
close together and route them in a star ground configu-
ration using component side copper, then connect the
star ground to internal ground using multiple vias.
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
8_______________________________________________________________________________________
Figure 1. Typical Application Circuit
LX
IN
VIN
2.6V TO 4.5V
GND
VOUT
5V
SHDN
FB
MAX1896
ON/OFF
SS
CSS
33nF
CIN
C1
10µF
10V
C2
0.1µF
L
10µH
SUMIDA
CD43-100
0.1µFCOUT
22µF
16V
R1
36kΩ
R2
12kΩ
NIHON
EC10QSO2L
Chip Information
TRANSISTOR COUNT: 970
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
_______________________________________________________________________________________ 9
Figure 2. Functional Diagram
GND
LX
IN
FB
4µA
N
ERROR
COMPARATOR
TRANSCONDUCTANCE
ERROR AMPLIFIER
ENABLE
COMPARATOR
SS
CLOCK
ENABLE
BIAS
SHDN
MAX1896
ΣCURRENT
SENSE
CONTROL
AND DRIVER
LOGIC
SOFT-
START
SLOPE
COMPEN-
SATION
OSCILLATOR
1.24V
Figure 3. MAX1896 with Ceramic Output Capacitor and Feed-
forward Capacitor
LX
IN
VIN
2.6V TO 5.5V
CSS
33nF
COUT
10µF
CERAMIC
10µF
CERAMIC
0.1µF
CERAMIC
D1
NIHON
EC10QSO2L
CFF
100pF
L
10µH
CD43-100
R2
12kΩ
VOUT
13V
R1
115kΩ
R3
10kΩ
GND
SHDN
FB
MAX1896
SS
ON/OFF
Figure 4. MAX1896 in an SEPIC Configuration
LX
IN
VIN
2.6V TO 5.5V
GND
VOUT
SHDN
ON/OFF
FB
MAX1896
SS
R2
R1
COUT
L1
C1
L2
C2
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
10 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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