source. This can be done, but the voltage on the output pin

of the LM2770 must not be brought above the input voltage.

The output pin will draw a small amount of current when

driven externally due the internal feedback resistor divider

connected between V

SOFT START

The LM2770 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 volt-

age. This occurs in 200µs (typ.). Soft-start is engaged when

the part is enabled, including situations where voltage is

established simultaneously on the V

THERMAL SHUTDOWN

Protection from overheating-related damage is achieved

with a thermal shutdown feature. When the junction tem-

down mode. The LM2770 disengages thermal shutdown

when the junction temperature of the part is reduced to

thermal shutdown and/or thermal cycling should not be en-

countered 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 sufficiently dissipated out of the LLP pack-

age.

SHORT-CIRCUIT AND CURRENT LIMIT PROTECTION

The LM2770 charge pump contains circuitry that protects the

device from destructive failure in the event of a direct short to

ground on the output. This short-circuit protection circuit

limits the output current to 400mA (typ.) when the output

voltage is below 165mV (typ.). The sleep-mode LDO con-

tains a 60mA (typ.) current limit circuit.

Application Information

RECOMMENDED CAPACITOR TYPES

The LM2770 requires 4 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 alumi-

num electrolytic capacitors generally are not recommended

for use with the LM2770 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

LM2770. These capacitors have tight capacitance tolerance

Capacitors with a Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM2770. These

types of capacitors typically have wide capacitance toler-

ance (+80%, -20%) and vary significantly over temperature

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 capaci-

tance requirements of the LM2770.

Net capacitance of a ceramic capacitor decreases with in-

creased DC bias. This degradation can result in lower ca-

pacitance than expected on the input and/or output, resulting

in higher ripple voltages and currents. Using capacitors at

DC bias voltages significantly below the capacitor voltage

rating will usually minimize DC bias effects. Consult capaci-

tor manufacturers for information on capacitor DC bias char-

acteristics.

Capacitance characteristics can vary quite dramatically with

different application conditions, capacitor types, and capaci-

tor manufacturers. It is strongly recommended that the

LM2770 circuit be thoroughly evaluated early in the

design-in process with the mass-production capacitors of

choice. This will help to ensure that any such variability in

capacitance does not negatively impact circuit performance.

The table below lists some leading ceramic capacitor manu-

facturers.

Manufacturer

Contact Information

AVX

www.avx.com

Murata

www.murata.com

Taiyo-Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Vishay-Vitramon

www.vishay.com

OUTPUT CAPACITOR AND OUTPUT VOTAGE RIPPLE

The output capacitor in the LM2770 circuit (C

impacts the magnitude of output voltage ripple. Other promi-

nent factors also affecting output voltage ripple include input

voltage, output current and flying capacitance. Due to the

complexity of multi-gain and PFM switching, providing equa-

tions or models to approximate the magnitude of the ripple

can not be easily accomplished. But one important generali-

zation can be made: increasing (decreasing) the output ca-

pacitance will result in a proportional decrease (increase) in

output voltage ripple. This can be observed in the output

voltage ripple waveforms in the Typical Performance Char-

acteristics section.

In typical high-current applications, a 10µF low-ESR ceramic

output capacitor is recommended. Different output capaci-

tance values can be used to reduce ripple, shrink the solu-

tion size, and/or cut the cost of the solution. But changing the

output capacitor may also require changing the flying capaci-

tors and/or input capacitor to maintain good overall circuit

performance. Performance of the LM2770 with different ca-

pacitor setups in discussed in the section Recommended

Capacitor Configurations.

High ESR in the output capacitor increases output voltage

ripple. If a ceramic capacitor is used at the output, this is

usually not a concern because the ESR of a ceramic capaci-

tor is typically vey low and has only a minimal impact on

ripple magnitudes. If a different capacitor type with higher

ESR is used (tantalum, for example), the ESR could result in

high ripple. To eliminate this effect, the net output ESR can

be significantly reduced by placing a low-ESR ceramic ca-

pacitor in parallel with the primary output capacitor. The low

ESR of the ceramic capacitor will be in parallel with the

higher ESR, resulting in a low net ESR based on the prin-

ciples of parallel resistance reduction.

Due to the PFM nature of the LM2770, output voltage ripple

is highest at light loads. To eliminate this ripple, consider

running the LM2770 in sleep mode when load currents are

20mA or less. Sleep mode disables the charge pump and

enables the internal low-noise bypass linear regulator (LDO).

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