ERROR

AMP

SS

FB

40k

100k

0.75V

60k

R1

R2

R3

C2

C1

PWM

COMP

CV

No Lead

Network

Required

CS

Current

Sense Amp

1.27V

CV

CO

Gm = 15 mA/V

Negative Current

Comparator

1V

Current

Limit Amp

2V

PWM

Comparator

CRMIX

to PWM

Latch

Low Side

Enable

= 16

AV

Vbias

LM25115

www.ti.com

SNVS418A – NOVEMBER 2005 – REVISED APRIL 2013

Figure 15. Current Sensing and Limiting

Figure 16. Voltage Sensing and Feedback

Current Limiting (CS, CO and VOUT)

Current limiting is implemented through the current sense amplifier as illustrated in Figure 15. The current sense

amplifier monitors the inductor current that flows through a sense resistor connected between CS and VOUT.

The voltage gain of the current sense amplifier is nominally equal to 16. The output of current sense amplifier is

level shifted by 1.27V to produce the internal CV reference signal. The CV signal drives a current limit amplifier

with nominal transconductance of 16mA/V. The current limit amplifier has an open drain (sink only) output stage

and its output pin, CO is typically connected to the COMP pin. During normal operation, the voltage error

amplifier controls the COMP pin voltage which adjusts the PWM duty cycle by varying the internal CRMIX level

down on COMP through the CO pin. Pulling COMP low reduces the CRMIX signal below the CV signal level.

When CRMIX does not exceed the CV signal, the PWM comparator inhibits output pulses until the CRMIX signal

increases to a normal operating level.

A current limit fold-back feature is provided by the LM25115 to reduce the peak output current delivered to a

shorted load. When the common mode input voltage to the current sense amplifier (CS and VOUT pins) falls

below 2V, the current limit threshold is reduced from the normal level. At common mode voltages > 2V, the

current limit threshold is nominally 45mV. When VOUT is reduced to 0V the current limit threshold drops to 36mV

to reduce stress on the inductor and power MOSFETs.

Copyright © 2005–2013, Texas Instruments Incorporated

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