Operation Description

OVERVIEW

The LM2760 is a regulated switched capacitor converter that

efficiently provides a regulated 3.3V output capable of sup-

plying a load up to 20mA. The part has two regulation

modes: a doubler mode and a pass mode. The part config-

ures itself for doubler mode (2x) when the input voltage is

At higher input voltages, voltage boost is not needed, so the

part configures itself for pass mode (1x) to optimize effi-

ciency. Efficiency is optimized even further, especially when

output current is low, through the implementation of pulse-

frequency-modulated (PFM) regulation. The descriptions in

the sections to follow refer to the block diagram on the

previous page.

PULSE FREQUENCY MODULATED (PFM) REGULATION

The LM2760 uses pulse frequency modulation (PFM) to

provide highly efficient output voltage regulation. PFM is a

complicated-sounding term that simply means the part only

pumps when it needs to. If the output voltage is at or above

3.3V, the pump is turned off and supply current is minimized.

While in this rest state, output current is supplied by the

charge on the output capacitor. As charge is pulled off the

output capacitor, the output voltage will steadily fall until it

droops below 3.3V, at which time the pump reactivates and

output voltage is boosted. With PFM, the LM2760 requires

very little supply current "overhead": the input current is

generally only slightly above what is required to deliver

power to the output. This mode of operation allows the

LM2760 to maximize total solution efficiency.

DOUBLER MODE

When in doubler mode, the LM2760 operates as a regulated

switched capacitor voltage doubler. A two-phase non-

overlapping clock generated internally controls the operation

of the doubler. During the charge phase (ø1), the flying

capacitor (C

through internal pass-transistor switches (S1 and S2) and is

charged to the input voltage. In the pump phase that follows

(ø2), the flying capacitor is connected between the input and

output through similar switches (S3 and S4). Stacked atop

the input, the charge of the flying capacitor boosts the output

voltage and supplies the load current.

PASS MODE

Pass mode of the LM2760 is implemented to improve effi-

boost is not required. In this mode of operation, switches S1

and S2 are on continuously, and switch S4 is always off.

Resistance of switch S1 is increased in this mode to optimize

performance. Output voltage regulation is achieved by ap-

plying a PFM clock to switch S3.

N0-LOAD OPERATION

The minimal supply-current overhead of the LM2760 is most

apparent, and beneficial, when the LM2670 is operating

unloaded. With zero output current, supply current of the

LM2760 is only 10µA (typ). There are two components to this

10µA input current (listed as I

istics): quiescent current (I

rent. Quiescent current is the steady-state DC current con-

sumed by the LM2760 internal circuitry, and is typically 6µA

when the part is unloaded. This is best observed by mea-

suring the input current of the part while forcing the voltage

on the output slightly above 3.3V (ensures the pump will not

activate: see below).

Even when output current of the LM2760 is zero, the pump

must occasionally be activated to keep the output voltage

from drooping. There is a resistor divider internal to the

LM2760 that is part of the regulation feedback loop, and is

connected to V

pulls charge from the output capacitor when the LM2760 is in

its rest state. The LM2760 must occasionally activate the

pump to restore charge to the output capacitor, thus keeping

the output voltage at the desired level. With a high-quality

10µF ceramic output capacitor (ensures capacitor current

leakage is minimal), the pump is only activated typically once

every 2 seconds. When the input current of these bursts are

averaged over time, they contribute only 4µA to the total

unloaded supply current of the LM2760.

The very low input current when the part is virtually unloaded

makes the LM2760 an excellent converter for generating

low-power keep-alive voltage rails.

STARTUP

When voltage is first connected to the input of the LM2760,

the output voltage is typically significantly less than the input

voltage, placing the part in pass mode. Pass mode, with the

increased resistance of switch S1, provides a controlled

start-up that limits input inrush current. Turn-on time of the

part, the time it takes for the output voltage to establish, is

typically 1ms when V

THERMAL SHUTDOWN

To protect itself from damage caused by overheating, the

LM2760 implements a thermal shutdown mechanism. When

switches itself into shutdown mode. The LM2760 releases

thermal shutdown when the junction temperature of the part

often triggered by self-heating, which occurs when there is

excessive power dissipation in the device and/or insufficient

thermal dissipation. When self-heating causes thermal shut-

down, thermal cycling usually occurs. Thermal cycling is the

repeating process where the part self-heats, enters thermal

shutdown (where internal power dissipation is practically

zero), cools, turns-on, and then heats up again to the ther-

mal shutdown threshold. Thermal cycling is recognized by a

pulsing output voltage and can be stopped by reducing the

internal power dissipation (reduce output current or input

voltage) or the ambient temperature.

SHORT CIRCUIT PROTECTION

The LM2760 contains short circuit protection circuitry that

protects the device from damage in the event of output

current and/or output shorts to ground. Current is limited to

170mA (typ.) when the output is shorted directly to ground

(V

dissipation in the device is likely to be quite high, especially

when higher input voltages are present. In this event, ther-

mal cycling should be expected (see "Thermal Shutdown"

section).

Application Information

OUTPUT VOLTAGE RIPPLE

The magnitude of the voltage ripple on the output of the

LM2760 is highly dependent on application conditions: out-

put current and output capacitor properties, specifically. This

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