FSDM07652R Fairchild Semiconductor, FSDM07652R Datasheet - Page 11

• Manufacturer Part Number: FSDM07652R
• Description: SMPS Power Switch For LCD Monitors Flyback, (Green)The FSDM07652R is an Integrated Pulse Width Modulator (PWM) And Sense Fet Specifically Designed For High Performance Offline Switch Mode Power Supplies (SMPS) With Minimal External Components. This D
• Manufacturer: Fairchild Semiconductor

Functional Description
1. Startup : In previous generations of Fairchild Power
Switches (FPS
tor to the DC input voltage line. In this generation the startup
resistor is replaced by an internal high voltage current
source. At startup, an internal high voltage current source
supplies the internal bias and charges the external capacitor
(C
4. When Vcc reaches 12V, the FPS
the internal high voltage current source is disabled. Then, the
FPS
power is supplied from the auxiliary transformer winding
unless Vcc goes below the stop voltage of 8V.
2. Feedback Control : FSDM07652R employs current
mode control, as shown in figure 5. An opto-coupler (such as
the H11A817A) and shunt regulator (such as the KA431) are
typically used to implement the feedback network. Compar-
ing the feedback voltage with the voltage across the Rsense
resistor plus an offset voltage makes it possible to control the
switching duty cycle. When the reference pin voltage of the
KA431 exceeds the internal reference voltage of 2.5V, the
H11A817A LED current increases, thus pulling down the
feedback voltage and reducing the duty cycle. This event
typically happens when the input voltage is increased or the
output load is decreased.
vcc
TM
) that is connected to the Vcc pin as illustrated in figure
8V/12V
continues its normal switching operation and the
Figure 4. Internal startup circuit
TM
3
) the Vcc pin had an external start-up resis-
Vcc
C
Vcc
Vcc good
TM
begins switching and
I
start
Internal
Vref
Bias
6
Vstr
V
DC
2.1 Pulse-by-pulse current limit: Because current mode
control is employed, the peak current through the Sense FET
is limited by the inverting input of PWM comparator (Vfb*)
as shown in figure 5. Assuming that the 0.9mA current
source flows only through the internal resistor (2.5R +R= 2.8
kΩ), the cathode voltage of diode D2 is about 2.5V. Since D1
is blocked when the feedback voltage (Vfb) exceeds 2.5V,
the maximum voltage of the cathode of D2 is clamped at this
voltage, thus clamping Vfb*. Therefore, the peak value of
the current through the Sense FET is limited.
2.2 Leading edge blanking (LEB) : At the instant the inter-
nal Sense FET is turned on, there usually exists a high cur-
rent spike through the Sense FET, caused by primary-side
capacitance and secondary-side rectifier reverse recovery.
Excessive voltage across the Rsense resistor would lead to
incorrect feedback operation in the current mode PWM con-
trol. To counter this effect, the FPS
edge blanking (LEB) circuit. This circuit inhibits the PWM
comparator for a short time (T
turned on.
3. Protection Circuit : The FSDM07652R has several self
protective functions such as over load protection (OLP),
abnormal over current protection (AOCP), over voltage pro-
tection (OVP) and thermal shutdown (TSD). Because these
protection circuits are fully integrated into the IC without
external components, the reliability can be improved without
increasing cost. Once the fault condition occurs, switching is
terminated and the Sense FET remains off. This causes Vcc
to fall. When Vcc reaches the UVLO stop voltage, 8V, the
protection is reset and the internal high voltage current
source charges the Vcc capacitor via the Vstr pin. When Vcc
reaches the UVLO start voltage,12V, the FPS
normal operation. In this manner, the auto-restart can alter-
nately enable and disable the switching of the power Sense
FET until the fault condition is eliminated (see figure 6).
Vo
Figure 5. Pulse width modulation (PWM) circuit
H11A817A
KA431
Vfb
C
B
4
I
delay
V
Vcc
SD
D1
Vref
LEB
I
V
FB
D2
+
fb
-
*
) after the Sense FET is
2.5R
TM
R
OSC
employs a leading
OLP
TM
driver
Gate
FSDM07652R
resumes its
R
SenseFET
sense
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