LM20242

SNVS534D – OCTOBER 2007 – REVISED MAY 2008

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OPERATION DESCRIPTION

GENERAL

The LM20242 switching regulator features all of the functions necessary to implement an efficient low voltage

buck regulator using a minimum number of external components. This easy to use regulator features two

integrated switches and is capable of supplying up to 2A of continuous output current. The regulator utilizes peak

current mode control with nonlinear slope compensation to optimize stability and transient response over the

entire output voltage range. Peak current mode control also provides inherent line feed-forward, cycle-by-cycle

current limiting and easy loop compensation. The switching frequency can be varied from 100 kHz to 1 MHz with

an external resistor to ground. The device can operate at high switching frequency allowing use of a small

inductor while still achieving efficiencies as high as 93%. The precision internal voltage reference allows the

output to be set as low as 0.8V. Fault protection features include: current limiting, thermal shutdown, over voltage

protection, and shutdown capability. The device is available in the HTSSOP package featuring an exposed pad

to aid thermal dissipation. The typical application circuit for the LM20242 is shown in Figure 9 in the design

guide.

PRECISION ENABLE

The enable (EN) pin allows the output of the device to be enabled or disabled with an external control signal.

This pin is a precision analog input that enables the device when the voltage exceeds 1.25V (typical). The EN pin

has 50 mV of hysteresis and will disable the output when the enable voltage falls below 1.2V (typical). If the EN

pin is not used, it should be disconnected so the internal 2 µA pull-up will default this function to the enabled

condition. Since the enable pin has a precise turn-on threshold it can be used along with an external resistor

circuitry will remain active even when the device is disabled.

PEAK CURRENT MODE CONTROL

In most cases, the peak current mode control architecture used in the LM20242 only requires two external

components to achieve a stable design. The compensation can be selected to accommodate any capacitor type

or value. The external compensation also allows the user to set the crossover frequency and optimize the

transient performance of the device.

For duty cycles above 50% all current mode control buck converters require the addition of an artificial ramp to

avoid sub-harmonic oscillation. This artificial linear ramp is commonly referred to as slope compensation. What

makes the LM20242 unique is the amount of slope compensation will change depending on the output voltage.

When operating at high output voltages the device will have more slope compensation than when operating at

lower output voltages. This is accomplished in the LM20242 by using a non-linear parabolic ramp for the slope

compensation. The parabolic slope compensation of the LM20242 is much better than the traditional linear slope

compensation because it optimizes the stability of the device over the entire output voltage range.

CURRENT LIMIT

The precise current limit enables the device to operate with smaller inductors that have lower saturation currents.

When the peak inductor current reaches the current limit threshold, an over current event is triggered and the

internal high-side FET turns off and the low-side FET turns on, allowing the inductor current to ramp down until

the next switching cycle. For each sequential over-current event, the reference voltage is decremented and PWM

pulses are skipped resulting in a current limit that does not aggressively fold back for brief over-current events,

while at the same time providing frequency and voltage foldback protection during hard short circuit conditions.

SOFT-START AND VOLTAGE TRACKING

The SS/TRK pin is a dual function pin that can be used to set the startup time or track an external voltage

source. The startup or soft-start time can be adjusted by connecting a capacitor from the SS/TRK pin to ground.

The soft-start feature allows the regulator output to gradually reach the steady state operating point, thus

reducing stresses on the input supply and controlling startup current. If no soft-start capacitor is used the device

defaults to the internal soft-start circuitry resulting in a startup time of approximately 1 ms. For applications that

require a monotonic startup or utilize the PGOOD pin, an external soft-start capacitor is recommended. The

SS/TRK pin can also be set to track an external voltage source. The tracking behavior can be adjusted by two

external resistors connected to the SS/TRK pin as shown in Figure 14 in the design guide.

10

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