PWM

FB

1.26V

5k

COMP

VDD

1.3V

VOUT

LM5015

RFB

R1

R2

C2

C1

Hz

0

=

2

2

C

R

2

1

x

x

S

=

EA

F

POLE

_

DC

ZERO

F

POLE

F

=

¸¸

¹

·

¨

x

2

1

2

1

C

C

C

C

1

¨

©

§

x

x

S

2

R

2

LM5015

SNVS538C – NOVEMBER 2007 – REVISED APRIL 2013

www.ti.com

Figure 15. Type II Compensator

When isolation between primary and secondary circuits is required, the Error Amplifier is usually disabled by

connecting the FB pin to AGND. This allows the COMP pin to be driven directly by the collector of an opto-

coupler. In isolated designs the error amplifier is located on the secondary circuit and drives the opto-coupler

LED. The compensation network is connected to the secondary side error amplifier. An example of an isolated

regulator with an opto-coupler is shown in Figure 25.

Current Sense Amplifier

The LM5015 employs peak current mode control which also provides a cycle-by-cycle over current protection

feature. An internal 42 milli-Ohm current sense resistor measures the current in the low side power MOSFET

source. The sense resistor voltage is amplified 30 times to provide a 1.25V/A signal into the current limit

comparator. Current limiting is initiated if the internal current limit comparator input exceeds the 1.5V threshold,

corresponding to 1.2A. When the current limit comparator is triggered, the HO and LO output pins immediately

switches to the high impedance state.

The current sense signal provides the PWM comparator with a control signal that reaches 1.5V when the

MOSFET current is 1.2A. To prevent erratic operation at low duty cycle, a leading edge blanking circuit

attenuates the current sense signal for 100 ns when the power MOSFET is turned on. When the MOSFET is

initially turned on, current spikes from the power MOSFET drain-source and gate-source capacitances flow

through the current sense resistor. These transient currents normally cease within 50 ns with proper selection of

rectifier diodes and proper PC board layout.

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the IC in the event the maximum junction temperature

is exceeded. When the 165°C junction temperature threshold is reached, the regulator is forced into a low power

standby state, disabling all functions except the VCC regulator. Thermal hysteresis allows the IC to cool down by

25°C before it is re-enabled.

12

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