DM0465RS View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
DM0465RS Datasheet PDF : 19 Pages
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Functional Description
1. Startup: In previous generations of Fairchild Power
Switches (FPS™) the VCC pin had an external startup
resistor 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 (Cvcc) that is connected to
the VCC pin shown in Figure 17. When VCC reaches 12V,
the FSDM0465RS begins switching and the internal
high-voltage current source is disabled. The
FSDM0465RS continues its normal switching operation
and the power is supplied from the auxiliary transformer
winding unless VCC goes below the stop voltage of 8V.
VCC
Idelay
Vref
IFB
VO
Vfb
4
OSC
H11A817A
D1 D2
CB
2.5R
KA431
+
Vfb* R
-
SenseFET
Gate
driver
FSDM0465RS Rev: 00
VSD
OLP
Rsense
Figure 18. Pulse Width Modulation (PWM) Circuit
CVcc
VDC
VCC
3
Vstr
6
8V/12V
VCC good
Istart
Vref
Internal
Bias
FSDM0465RS Rev: 00
Figure 17. Internal Startup Circuit
2. Feedback Control: FSDM0465RS 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 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.
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
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
the SenseFET is limited.
2.2 Leading-Edge Blanking (LEB): At the instant the
internal SenseFET is turned on, there usually exists a
high-current spike 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
FSDM0465RS employs a leading edge blanking (LEB)
circuit. This circuit inhibits the PWM comparator for a
short time (tLEB) after the SenseFET is turned on.
3. Protection Circuit: The FSDM0465RS 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
the fault condition occurs, switching is terminated and
the SenseFET 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
FSDM0465RS resumes its normal operation. In this
manner, the auto-restart can alternately enable and
© 2008 Fairchild Semiconductor Corporation
FSDM0465RS Rev. 1.0.1
11
www.fairchildsemi.com
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