RT1 =

0.8 x RT2

VOUT1 - 0.8

LM25115A

SNVS501B – FEBRUARY 2007 – REVISED APRIL 2013

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Error Amplifier and Soft-Start (FB, CO, COMP & TRK/SS)

An internal wide bandwidth error amplifier is provided within the LM25115A for voltage feedback to the PWM

controller. The amplifier’s inverting input is connected to the FB pin. The output of the auxiliary converter is

regulated by connecting a voltage setting resistor divider between the output and the FB pin. Loop compensation

networks are connected between the FB pin and the error amplifier output (COMP). The amplifier has two non-

inverting inputs. The first non-inverting input connects to a 0.75V bandgap reference while the second non-

inverting input connects to the TRK/SS pin and it has 15 µA pull-up current source. The TRK/SS pin can be tied

to an external resistor divider from the master output for tracking, or it can be tied to a capacitor for soft-start .

TRK/SS is the reference input to the amplifier when the voltage applied to the pin is < 0.75V. For higher inputs,

the internal reference controls the amplifier. When the VCC voltage is below the UVLO threshold, the TRK/SS

pin is discharged to ground. When VCC rises and exceeds the positive going UVLO threshold (4.25V), the

TRK/SS pin is released and allowed to rise. If an external capacitor is connected to the TRK/SS pin, it will be

charged by the internal 15uA pull-up current source to gradually increase the non-inverting input of the error

amplifier to 0.75V. During start-up, the output of the LM25115A converter will follow the following equation:

VOUT(t) = VOUT(final) x15 µA x t /(.75 Vx Css )

where

•

Css = external Soft-Start capacitor

•

VOUT(final) = regulator output set point

Pull-up current for the error amplifier output is provided by an internal 300µA current source. The PWM threshold

signal at the COMP pin can be controlled by either the open drain error amplifier or the open drain current

amplifier connected through the CO pin to COMP. Since the internal error amplifier is configured as an open

drain output it can be disabled by connecting FB to ground. The current sense amplifier and current limiting

function will be described in a later section.

Power-Up/Power-Down Tracking

The LM25115A can track the output of a master power supply during soft start by connecting a resistor divider to

the TRACK pin (Figure 14). Therefore, the output voltage slew rate of the LM25115A will be controlled by the

master supply for loads that require precise sequencing. In order to track properly the output voltage of the

LM25115A must be lower than the output voltage of the master supply.

One way to use the tracking feature is to design the tracking resistor divider so that the master supply output

voltage (VOUT1) and the LM25115A output voltage (VOUT2) both rise together and reach their target values at

the same time. For this case, the equation governing the values of the tracking divider resistors RT1 and RT2 is:

A value of 10k

Ω (1%) is recommended for RT2 as a good compromise between high precision and low quiescent

current through the divider. If the master supply voltage was 3.3V and the LM25115A output voltage was 2.5 V,

then the value of RT1 needed to give the two supplies identical soft start times would be 2.94 k

Ω (1%). The

timing diagram and waveforms for the equal soft start time configuration are shown in Figure 15 and Figure 16.

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