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 (VFB*) as shown in Figure 18. Assuming that
Functional Description
1. Start-up: In previous generations of Fairchild Power
Switches (FPS™), the VCC 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 (Cvcc) connected to the
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 (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
VCC pin, as illustrated in Figure 17. When VCC 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 VCC goes below the stop voltage of 8V.
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 Rsense 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 (tLEB) after the SenseFET is turned on.
VDC
CVcc
Vref
VCC
Idelay
IFB
VCC
Vstr
VFB
VO
SenseFET
3
6
OSC
4
H11A817A
D1
D2
CB
2.5R
R
Istart
+
Vfb
Gate
driver
*
Vref
KA431
-
8V/12V
VCC good
OLP
Rsense
Internal
Bias
VSD
FSDM0565RE Rev: 00
FSDM0565RE Rev: 00
Figure 18. Pulse-Width-Modulation (PWM) Circuit
Figure 17. Internal Start-up 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 VCC to fall. When
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 Rsense 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.
VCC reaches the UVLO stop voltage of 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 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).
© 2006 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FSDM0465RE, FSDM0565RE, FSDM07652RE Rev. 1.0.1
12