Preliminary Datasheet

LP6343

LP6343 –00 Version 1.0 Datasheet

Feb.-2010

www.lowpowersemi.com

Page 7 of 12

Application Information

The LP6343 is a high output current monolithic

switch-mode step-down DC-DC converter. The device

operates at a fixed 1.2MHz switching frequency, and uses a

slope compensated current mode architecture. This

step-down DC-DC converter can supply up to 2A output

current at VIN = 3V and has an input voltage range from

2.5V to 5.5V. It minimizes external component size and

opti-mizes efficiency at the heavy load range. The slope

com-pensation allows the device to remain stable over a

wider range of inductor values so that smaller values (1μH

to 4.7μH) with lower DCR can be used to achieve higher

efficiency. Apart from the small bypass input capacitor,

only a small L-C filter is required at the output. The device

can be programmed with external feedback to any voltage,

ranging from 0.6V to near the input volt-age. It uses

internal MOSFETs to achieve high efficiency and can

generate very low output voltages by using an internal

reference of 0.6V. At dropout, the converter duty cycle

increases to 100% and the output voltage tracks the input

voltage minus the low RDS(ON) drop of the P-channel

high-side MOSFET and the inductor DCR. The internal

error amplifier and compensation provides excellent

transient response, load and line regulation. Internal soft

start eliminates any output voltage over-shoot when the

enable or the input voltage is applied.

Current Mode PWM Control

Slope compensated current mode PWM control provides

stable switching and cycle-by-cycle current limit for

excellent load and line response with protection of the

internal

main

switch

(P-channel

MOSFET)

and

synchronous rectifier (N-channel MOSFET). During

normal operation, the internal P-channel MOSFET is turned

on for a specified time to ramp the inductor current at each

rising edge of the internal oscillator, and switched off when

the peak inductor current is above the error volt-age. The

When the main switch is off, the synchronous rectifier turns

on immediately and stays on until either the inductor

current starts to reverse, as indicated by the current reversal

Control Loop

The LP6343 is a peak current mode step-down converter.

The current through the P-channel MOSFET (high side) is

sensed for current loop control, as well as short circuit and

overload protection. A slope compensation signal is added

to the sensed current to maintain stability for duty cycles

greater than 50%. The peak current mode loop appears as a

voltage-programmed current source in parallel with the

output capacitor. The output of the voltage error amplifier

programs the current mode loop for the necessary peak

switch current to force a constant output voltage for all load

and line conditions. Internal loop compensation terminates

the transconductance voltage error amplifier output. The

error amplifier reference is fixed at 0.6V.

Soft Start / Enable

Soft start limits the current surge seen at the input and

eliminates output voltage overshoot. The enable pin is

active high. When pulled low, the enable input (EN) forces

the LP6343 into a low-power, non-switching state. The total

input current during shutdown is less than 1μA.

Setting the Output Voltage

Figure 1 shows the basic application circuit for the LP6343.

The LP6343 can be externally programmed. Resistors R1

and R2 in Figure 1 program the output to regulate at a

voltage higher than 0.6V. To limit the bias current required

for the external feedback resistor string while maintaining

good noise immunity, the minimum suggested value for R2

is 59kΩ. Although a larger value will further reduce

quiescent current, it will also increase the impedance of the

feedback node, making it more sensitive to external noise

and interference. Table 1 summarizes the resistor values for

various output volt-ages with R2 set to either 59kΩ for

good noise immunity or 316kΩ for reduced no load input

current. The LP6343, combined with an external feed

forward capacitor (C3 in Figure 1), delivers enhanced

transient response for extreme pulsed load applications. The

addition of the feed forward capacitor typically requires a

larger output capacitor C2 for stability. The external resistor

sets the output voltage according to the following equation: