Datasheet

PWM
FB
1.26V
5k
COMP
VDD
1.3V
V
OUT
LM5015
R
FB
R
1
R
2
C
2
C
1
Hz0 =
22
CR2
1
xxS
=
EA
F
POLE_DC
ZERO
F
POLE
F
=
¸
¸
¹
·
¨

x
21
21
CC
CC
1
¨
©
§
xxS
2
R2
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.
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