LP6288 datasheet(15/24 Pages) POWER

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Preliminary Datasheet

LP6288

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LP6288

Version 0.1 JAN.-2018

Application Information(Continued)

Buck3 Converter (VHAVDD)

The synchronous current mode buck3 converter

operates

with

PWM

architecture

with

750kHz

operation frequency and fast transient response. The

converter drivers an internal N-MOS, it need connect

a 100nF ceramic capacitor between the BST3 and

BKLX3 to provide gate driver voltage. The converter

drivers an internal High site and Low site N-MOS. The

output voltage can be adjustable by internal register.

The converter has an internal soft start 20ms.

Buck3 Over Current Protection

The Buck3 offers cycle-to-cycle and peak ±2A(typ.)

current limiting for internal MOS. The current limit is

relatively constant regardless of duty cycles.

Buck3 Over Voltage Protection

The buck converter has an over voltage protection.

When the BKO3 voltage rises above 120%, the Buck3

converter turn off the BKLX3. Until the voltage sense

falls below the over voltage threshold, the converter

will resume operation.

Buck3 Under Voltage Protection

When the output voltage

drop below 80% of its

nominal output, and the

status continues for 50ms,

LP6288 will be shut down until VIN power restart.

Buck3 Short Circuit Protection

The LP6288 provides short circuit protection function

to prevent the device damage from short condition.

When the BKO3 sense is lower than 40% of the

setting voltage level, the LP6288 will be shut down

immediately until VIN power cycled.

Buck Input Capacitor Selection

The use of the input capacitor is filtering the switch

spike voltage and the input voltage ripple.

For better

input bypassing, low-ESR ceramic capacitors are

recommended. The input capacitor ripple current

RMS value is calculated as:

IIN(RMS)=IOUT×

�VOUT

VIN

�1-VOUT

VIN

�

This formula has a maximum at VIN=2*VOUT, where

IIN(RMS)=IOUT/2. For the input capacitor, a 2

0μF

low-ESR ceramic capacitor is recommended.

Buck Output Capacitor Selection

The output capacitor is used to keep the output

voltage and source the load transient current.

When

operating in constant current mode, the output ripple

can analysis by two components:

VRIPPLE=VRIPPLE(ESR)+VRIPPLE(C)

The following equation show the form of the ripple

contributions.

VRIPPLE(ESR)=

VOUT

FOSC×L

�1-VOUT

VIN

�×ESR

VRIPPLE(C)=

VOUT

8×FOSC

2×L×C

OUT

�1-VOUT

VIN

�

Where FOSC is the main frequency, L is the inductance

value, VIN is the input voltage, VOUT is the output

voltage, ESR is the equivalent series resistance value

of the output capacitor, and the COUT is the output

capacitor.

Buck Inductor Selection

The output inductor is used for storing energy and

filtering output ripple current.

A nice compromise

value between size and efficiency is to set the

peak-to-

peak ripple current ΔI

L between 20%~40% of

the maximum current.

Then the inductance can be

calculated with the following equation:

∆I

L=0.3×IOUT(MAX)

VOUT

FOSC

×∆I

�1-VOUT

VIN

�

To guarantee enough output current, peak inductor

current must be lower than the high-side MOSFET

current limit.

LP6288 Datasheet(PDF) 15 Page - Lowpower Semiconductor inc

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