LP3906 datasheet(36/47 Pages) TI

36 / 47 page

where

Iripple =

VIN - VOUT

VOUT

VIN ¹

Isat > Ioutmax + Iripple

LP3906

SNVS456M – AUGUST 2006 – REVISED MAY 2013

www.ti.com

APPLICATION NOTES

SYSTEM CLOCK INPUT (SYNC) PIN

Pin 23 of the chip allows for a system clock input in order to synchronize the buck converters in PWM mode. This

is useful if the user wishes to force the bucks to work synchronously with the system. Otherwise, the user should

tie the pin to GND and the bucks will operate on an internal 2 MHz clock.

The signal applied to the SYNC pin must be 13 MHz as per application processor specifications, but we can be

contacted to modify that specification if so desired. Upon inputting the 13 MHz clock signal, the bucks will scale it

down and continue to run at 2 MHz based off the 13 MHz clock.

ANALOG POWER SIGNAL ROUTING

All power inputs should be tied to the main VDD source (i.e. battery), unless the user wishes to power it from

another source. (i.e. external LDO output).

The analog VDD inputs power the internal bias and error amplifiers, so they should be tied to the main VDD. The

analog VDD inputs must have an input voltage between 2.7 and 5.5 V, as specified on pg. 6 of the datasheet.

The other VINs (VINLDO1, VINLDO2, VIN1, VIN2) can actually have inputs lower than 2.7V, as long as it's higher

than the programmed output (+0.3V, to be safe).

The analog and digital grounds should be tied together outside of the chip to reduce noise coupling.

COMPONENT SELECTION

Inductors for SW1 and SW2

There are two main considerations when choosing an inductor; the inductor should not saturate and the inductor

current ripple is small enough to achieve the desired output voltage ripple. Care should be taken when reviewing

the different saturation current ratings that are specified by different manufacturers. Saturation current ratings are

typically specified at 25ºC, so ratings at maximum ambient temperature of the application should be requested

from the manufacturer.

There are two methods to choose the inductor saturation current rating:

Method 1

The saturation current is greater than the sum of the maximum load current and the worst case average to peak

inductor current. This can be written as follows:

(5)

IRIPPLE: Average to peak inductor current

IOUTMAX: Maximum load current

VIN:

Maximum input voltage to the buck

Min inductor value including worse case tolerances (30% drop can be considered for method 1)

Minimum switching frequency (1.6 MHz)

VOUT: Buck Output voltage

Method 2

A more conservative and recommended approach is to choose an inductor that has saturation current rating

greater than the maximum current limit of 2375 mA.

Given a peak-to-peak current ripple (IPP) the inductor needs to be at least:

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