LP3218-04

Jun.-2015

www.lowpowersemi.com

Page 5 of 8

Preliminary Datasheet

LP3218

Functional Description

The LP3218 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

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 (2.2μH 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

the peak inductor current. 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

next clock cycle.

Control Loop

The LP3218 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

LP3218 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 150°C with 15°C of hysteresis.

Once an over-temperature or over-current fault conditions is

removed, the output voltage automatically recovers.