Datasheet

LM20125
SNVS525E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
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
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.
PEAK CURRENT MODE CONTROL
In most cases, the peak current mode control architecture used in the LM20125 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 LM20125 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 LM20125 by using a non-linear parabolic ramp for the slope
compensation. The parabolic slope compensation of the LM20125 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 LM20125 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 30 in the design guide.
PRE-BIAS START UP CAPABILITY
The LM20125 is in a pre-biased state when the device starts up with an output voltage greater than zero. This
often occurs in many multi-rail applications such as when powering an FPGA, ASIC, or DSP. In these
applications the output can be pre-biased through parasitic conduction paths from one supply rail to another.
Even though the LM20125 is a synchronous converter it will not pull the output low when a prebias condition
exists. During start-up LM20125 will not sink current until the Soft-Start voltage exceeds the voltage on the FB
pin. Since the device can not sink current it protects the load from damage that might otherwise occur if current
is conducted through the parasitic paths of the load.
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