Operation Description
OVERVIEW
The LM2772 is a switched capacitor converter that produces
a regulated, low voltage output. The core of the part is a highly
efficient charge pump that utilizes fixed frequency pre-regu-
lation and Pulse Frequency Modulation to minimize ripple and
power losses over wide input voltage and output current
ranges. A description of the principal operational character-
istics of the LM2772 is detailed in the Circuit Description,
and Efficiency Performance sections. These sections refer
to details in the Block Diagram.
CIRCUIT DESCRIPTION
The core of the LM2772 is a two-phase charge pump con-
trolled by an internally generated non-overlapping clock. The
charge pump operates by using external flying capacitors
C1, C2, and C3 to transfer charge from the input to the output.
At input voltages below 3.5V (typ.) the LM2772 operates in a
1/2x Gain, with the input current being equal to 1/2 of the load
current. At input voltages between 3.5V to 4.6V(typ.) the part
utilizes a gain of 2/5x, resulting in an input current equal to 2/5
times the load current. At input voltages above 4.6V (typ.), the
part is in a gain of 1/3, with the input current being 1/3 of the
load current.
The two phases of the switched capacitor switching cycle will
be referred to as the "charge phase" and the "discharge
phase". During the charge phase, the flying capacitor is
charged by the input supply. After half of the switching cycle
[ t = 1/(2×FSW) ], the LM2772 switches to the discharge phase.
In this configuration, the charge that was stored on the flying
capacitors in the charge phase is transferred to the output.
The LM2772 uses fixed frequency pre-regulation to regulate
the output voltage to 1.2V during moderate to high load cur-
rents. The input and output connections of the flying capaci-
tors are made with internal MOS switches. Pre-regulation
limits the gate drive of the MOS switch connected between
the voltage input and the flying capacitors. Controlling the on
resistance of this switch limits the amount of charge trans-
ferred into and out of each flying capacitor during the charge
and discharge phases, and in turn helps to keep the output
ripple very low.
When output currents are low (<40mA typ.), the LM2772 au-
tomatically switches to a low-ripple Pulse Frequency Modu-
lation (PFM) form of regulation. In PFM mode, the flying
capacitors stay in the discharge phase until the output voltage
drops below a predetermined trip point. When this occurs, the
flying capacitors switch back to the charge phase. After being
charged, the flying capacitors repeat the process of staying
in the discharge phase and switching to the charge phase
when necessary.
EFFICIENCY PERFORMANCE
Charge-pump efficiency is derived in the following two ideal
equations (supply current and other losses are neglected for
simplicity):
IIN = G × IOUT
E = (VOUT × IOUT) ÷ (VIN × IIN) = VOUT ÷ (G × VIN)
In the equations, G represents the charge pump gain. Effi-
ciency is at its highest as G×VIN approaches VOUT. Refer to
the efficiency graph in the Typical Performance Character-
istics section for detailed efficiency data. The transition be-
tween gains of 1/2, 2/5, and 1/3 are clearly distinguished by
the sharp discontinuity in the efficiency curve.
SHUTDOWN
The LM2772 is in shutdown mode when the voltage on the
enable pin (EN) is logic-low. In shutdown, the LM2772 draws
virtually no supply current. When in shutdown, the output of
the LM2772 is completely disconnected from the input. Inter-
nal feedback resistors pull the output voltage down to 0V
during shutdown.
SOFT START
The LM2772 employs soft start circuitry to prevent excessive
input inrush currents during startup. At startup, the output
voltage gradually rises from 0V to the nominal output voltage.
This occurs in 150µs (typ.). Soft-start is engaged when the
part is enabled, including situations where voltage is estab-
lished simultaneously on the VIN and EN pins.
THERMAL SHUTDOWN
Protection from damage related to overheating is achieved
with a thermal shutdown feature. When the junction temper-
ature rises to 150ºC (typ.), the part switches into shutdown
mode. The LM2772 disengages thermal shutdown when the
junction temperature of the part is reduced to 140ºC (typ.).
Due to the high efficiency of the LM2772, thermal shutdown
and/or thermal cycling should not be encountered when the
part is operated within specified input voltage, output current,
and ambient temperature operating ratings. If thermal cycling
is seen under these conditions, the most likely cause is an
inadequate PCB layout that does not allow heat to be suffi-
ciently dissipated out of the LLP package.
CURRENT LIMIT PROTECTION
The LM2772 charge pump contains current limit protection
circuitry that protects the device during VOUT fault conditions
where excessive current is drawn. Output current is limited to
500mA (typ).
Application Information
RECOMMENDED CAPACITOR TYPES
The LM2772 requires 5 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR, ≤ 15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alu-
minum electrolytic capacitors generally are not recommended
for use with the LM2772 due to their high ESR, as compared
to ceramic capacitors.
For most applications, ceramic capacitors with an X7R or X5R
temperature characteristic are preferred for use with the
LM2772. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55ºC to 125ºC; X5R: ±15% over -55ºC to
85ºC).
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2772. These
types of capacitors typically have wide capacitance tolerance
(+80%, -20%) and vary significantly over temperature (Y5V:
+22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56%
over +10ºC to +85ºC range). Under some conditions, a 1µF-
rated Y5V or Z5U capacitor could have a capacitance as low
as 0.1µF. Such detrimental deviation is likely to cause Y5V
and Z5U capacitors to fail to meet the minimum capacitance
requirements of the LM2772.
Net capacitance of a ceramic capacitor decreases with in-
creased DC bias. This degradation can result in lower capac-
itance than expected on the input and/or output, resulting in
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LM2772