Preliminary Datasheet

LP6342

LP6342-03

Version 1.3 Datasheet

Oct.-2011

www.lowpowersemi.com

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Functional Description

The LP6342 is a high output current monolithic

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

operates at a fixed 1.5MHz 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 optimizes efficiency at the heavy load range. The slope

compensation 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

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 LP6342 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 trans conductance 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 LP6342 into a low-power, non-switching state. The total

input current during shutdown is less than 1μA.

Current Limit and Over-Temperature Protection

For overload conditions, the peak input current is limited to

3.5A. To minimize power dissipation and stresses under

current limit and short-circuit conditions, switching is

terminated after entering current limit for a series of pulses.

The termination lasts for seven consecutive clock cycles

after a current limit has been sensed during a series of four

consecutive clock cycles.

Thermal protection completely disables switching when

internal dissipation becomes excessive. The junction

over-temperature threshold is 170°C with 10°C of

hysteresis. Once an over-temperature or over-current fault

conditions is removed, the output voltage automatically

recovers.

Dropout Operation

When the battery input voltage decreases near the value of

the output voltage, the LP6342 allows the main switch to

remain on for more than one switching cycle and increases

the duty cycle until it reaches 100%. The duty cycle D of

a step-down converter is defined as:

oscillator frequency. The output voltage then is the input

voltage minus the voltage drop across the main switch and

the P-channel MOSFET increases, and the efficiency of the

converter decreases. Caution must be exercised to ensure

the heat dissipated does not exceed the maxi-mum junction

temperature of the IC.

Maximum Load Current

The LP6342 will operate with an input supply voltage as

low as 2.5V, however, the maximum load current decreases

at lower input voltages due to a large IR drop on the main

switch and synchronous rectifier. The slope compensation

signal reduces the peak inductor current as a function of the

duty cycle to prevent sub-harmonic oscillations at duty

cycles greater than 50%. Conversely, the current limit

increases as the duty cycle decreases.

Setting the Output Voltage

Figure 1 shows the basic application circuit for the LP6342.

The LP6342 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