Datasheet

LM20333
SNVS558D –MAY 2008–REVISED APRIL 2013 www.ti.com
OPERATION DESCRIPTION
GENERAL
The LM20333 switching regulator features all of the functions necessary to implement an efficient 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 3A 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 synchronized to an external oscillator over the
range of 250 kHz to 1.5 MHz. The SYNC function allows the device to operate at high switching frequencies
minimizing the size of the inductor while still achieving efficiencies as high as 94%. 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 LM20333 is shown in
Figure 25 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
divider network from V
IN
to configure the device to turn-on at a precise input voltage. The precision enable
circuitry will remain active even when the device is disabled.
FREQUENCY SYNCHRONIZATION
The frequency sychronization pin(SYNC) allows the switching frequency of the device to be controlled with an
external clock signal. This feature allows the user to sychronize multiple converters, avoiding undesirable
frequency bands of operation.
The switching frequency of the device will synchronize to the rising edge of the clock source that is driving the
SYNC pin. The logic low level for the input clock must be below 0.8V and the logic high level must exceed 2.0V
to ensure proper operation. The device will synchronize to frequencies from 250 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 200 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 LM20333 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 peak 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 LM20333 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 LM20333 by using a non-linear parabolic ramp for
the slope compensation. The parabolic slope compensation of the LM20333 is an improvement over the
traditional linear slope compensation because it optimizes the stability of the device over the entire output voltage
range.
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