LM20133, LM20133Q

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SNVS526F – OCTOBER 2007 – REVISED MARCH 2013

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 1.18V (typical). The EN pin

has 66 mV of hysteresis and will disable the output when the enable voltage falls below 1.11V (typical). If the EN

pin is not used, it should be connected to VIN. Since the enable pin has a precise turn on threshold it can be

voltage. The precision enable circuitry will remain active even when the device is disabled.

FREQUENCY SYNCHRONIZATION

The frequency synchronization pin (SYNC) allows the switching frequency of the device to be controlled with an

external clock signal. This feature allows the user to synchronize multiple converters, avoiding undesirable

frequency bands of operation. When used with the SYNCOUT of the LM20154, multiple devices can be

configured to switch out of phase reducing input capacitor requirements and EMI of the power supply system.

The turn on of the high side switch will lock-on to the rising edge of the SYNC pin input. The logic low level for

the input clock must be below 0.8V and the logic high level must exceed 2.0V for proper operation. The device

will synchronize to frequencies from 500 kHz to 1.5 MHz. If the synchronization clock is removed or not present

during startup, the oscillator of the device will run at approximately 410 kHz. If the SYNC pin is not used it should

be connected to ground.

PEAK CURRENT MODE CONTROL

In most cases, the peak current mode control architecture used in the LM20133 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 LM20133 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 LM20133 by using a non-linear parabolic ramp for the slope

compensation. The parabolic slope compensation of the LM20133 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 of the LM20133 is set at the factory to be within 10% over the entire operating

temperature range. This 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 start up time or track an external voltage

source. The start up 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 start up current. If no Soft-Start capacitor is used the device

defaults to the internal Soft-Start circuitry resulting in a start up time of approximately 1 ms. For applications that

require a monotonic start up 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 35 in the design guide.

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Product Folder Links: LM20133 LM20133Q