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LM20343 Arkusz danych(PDF) 11 Page - Texas Instruments

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Numer części LM20343
Szczegółowy opis  36V, 3A Adjustable Frequency Synchronous Buck Regulator
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Strona internetowa  http://www.ti.com
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LM20343 Arkusz danych(HTML) 11 Page - Texas Instruments

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LM20343
www.ti.com
SNVS559A – MAY 2008 – REVISED AUGUST 2008
OPERATION DESCRIPTION
GENERAL
The LM20343 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 varied from 250 kHz to 1 MHz with an external
resistor to ground. 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 LM20343 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 VIN 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.
PEAK CURRENT MODE CONTROL
In most cases, the peak current mode control architecture used in the LM20343 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 LM20343 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 LM20343 by using a non-linear parabolic ramp for
the slope compensation. The parabolic slope compensation of the LM20343 is an improvement over 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 30 in the design guide.
Copyright © 2008, Texas Instruments Incorporated
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