CS

VOUT

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

VCL

Vbias

Main

PWM

Auxilary

PWM

Transformer

Current

Trailing Edge

Modulation

Leading Edge

Modulation

Peak Current

Threshold

Peak Current

Threshold

Main

PWM

Auxilary

PWM

Transformer

Current

LM5115

SNVS343E – MARCH 2005 – REVISED MARCH 2013

www.ti.com

Leading edge modulation of the auxiliary PWM controller is required if the main converter is implemented with

peak current mode control. If trailing edge modulation were used, the additional load on the transformer

secondary from the auxiliary channel would be drawn only during the first portion of the phase signal pulse.

Referring to Figure 16, the turn off the high side MOSFET of the auxiliary regulator would create a non-

monotonic negative step in the transformer current. This negative current step would produce instability in a peak

current mode controller. With leading edge modulation, the additional load presented by the auxiliary regulator on

the transformer secondary will be present during the latter portion of the phase signal. This positive step in the

phase signal current can be accommodated by a peak current mode controller without instability.

Figure 16. Leading versus Trailing Edge Modulation

Voltage Mode Control with Current Injection

The LM5115 controller uniquely combines elements and benefits of current mode control in a voltage mode

PWM controller. The current sense amplifier shown in Figure 17 monitors the inductor current as it flows through

a sense resistor connected between CS and VOUT. The voltage gain of the sense amplifier is nominally equal to

16. The current sense output signal is shifted by 1.27V to produce the internal CV reference signal. The CV

signal is applied to the negative input of the PWM comparator and compared to CRMIX as illustrated in

Figure 15. Thus the PWM threshold of the voltage mode controller (CV) varies with the instantaneous inductor

current. Insure that the Vbias voltage is at least 3V above the regulated output voltage (VOUT).

Figure 17. Current Sensing and Limiting

12

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