Data Sheet

FSDM0465RE, FSDM0565RE, FSDM07652RE — Green Mode Power Switch
Functional Description
1. Start-up: In previous generations of
Power Switches the V
CC
pin had an external start-
up resistor to the DC input voltage line. In this
generation, the start-up resistor is replaced by an
internal high-voltage current source. At start-up, the
internal high-voltage current source supplies the
internal bias and charges the external capacitor
(C
vcc
) connected to the V
CC
pin, as illustrated
in Figure 17. When V
CC
reaches 12V, the
FSDM0x65RE begins switching and the internal high-
voltage current source is disabled. The
FSDM0x65RE continues normal switching operation and
the power is supplied from the auxiliary transformer
winding unless V
CC
goes below the stop voltage of 8V.
Figure 17. Internal Start-up Circuit
2. Feedback Control: FSDM0x65RE employs current-
mode control, as shown in Figure 18. An opto-coupler
(such as the H11A817A) and shunt regulator (such as
the KA431) are typically used to implement the feedback
network. Comparing the feedback voltage with the
voltage across the R
sense
resistor, plus an offset voltage,
makes it possible to control the switching duty cycle.
When the reference pin voltage of the shunt regulator
exceeds the internal reference voltage of 2.5V, the opto-
coupler LED current increases, pulling down the
feedback voltage and reducing the duty cycle. This event
typically occurs when the input voltage is increased or
the output load is decreased.
2.1 Pulse-by-Pulse Current Limit: Because current-
mode control is employed, the peak current through the
SenseFET is limited by the inverting input of PWM
comparator (V
FB
*) as shown in Figure 18. Assuming that
the 0.9mA current source flows only through the internal
resistor (2.5R + R = 2.8kΩ), the cathode voltage of diode
D2 is about 2.5V. Since D1 is blocked when the feedback
voltage (V
FB
) exceeds 2.5V, the maximum voltage of the
cathode of D2 is clamped at this voltage, thus clamping
V
FB
*. Therefore, the peak value of the current through
the SenseFET is limited.
2.2 Leading Edge Blanking (LEB): At the instant the
internal SenseFET is turned on, a high-current spike
occurs through the SenseFET, caused by primary-side
capacitance and secondary-side rectifier reverse
recovery. Excessive voltage across the R
sense
resistor
would lead to incorrect feedback operation in the current
mode PWM control. To counter this effect, the
FSDM0x65RE employs a leading-edge blanking (LEB)
circuit. This circuit inhibits the PWM comparator for a
short time (t
LEB
) after the SenseFET is turned on.
Figure 18. Pulse-Width-Modulation (PWM) Circuit
3. Protection Circuit: The FSDM0x65RE has several
self-protective functions, such as overload protection
(OLP), over-voltage protection (OVP), and thermal
shutdown (TSD). Because these protection circuits are
fully integrated into the IC without external components,
the reliability is improved without increasing cost. Once a
fault condition occurs, switching is terminated and the
SenseFET remains off, which causes V
CC
to fall. When
V
CC
reaches the UVLO stop voltage of 8V, the protection
is reset and the internal high-voltage current source
charges the V
CC
capacitor via the V
str
pin. When V
CC
reaches the UVLO start voltage of 12V, the
FSDM0x65RE resumes normal operation. In this
manner, the auto-restart can alternately enable and
disable the switching of the power SenseFET until the
fault condition is eliminated (see Figure 19).
8V/12V
3
Vref
Internal
Bias
V
CC
6
V
str
I
start
V
CC
good
V
DC
C
Vcc
FSDM0565RE Rev: 00
4
OSC
V
CC
V
ref
I
delay
I
FB
V
SD
R
2.5R
Gate
driver
OLP
D1 D2
+
V
fb
*
-
V
FB
KA431
C
B
V
O
H11A817A
R
sense
SenseFET
FSDM0565RE Rev: 00
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