FAN5037 Datasheet, PDF (9/11 Pages) Fairchild Semiconductor – Adjustable Switching Regulator Controller

PRODUCT SPECIFICATION
FAN5037
Power and Ground Connections
The connection of VCCA to the 5V power supply plane
should be short and bypassed with a 0.1µF directly at the
VCCA pin of the FAN5037. The ideal connection would be a
via down to the 5V power plane. A similar arrangement
should be made for the VCCP pin that connects to +12V.
Each ground should have a separate via connection to the
ground plane below.
A 12V power supply is used to bias the VCCP. A 47Ω resis-
tor is used to limit the transient current into VCCP. A 1uF
capacitor filter is used to filter the VCCP supply and source
the transient current required to charge the MOSFET gate
capacitance. This method provides sufficiently high gate bias
voltage to the MOSFET (VGS), and therefore reduces
RDS(ON) of the MOSFET and its power loss.
Figure 4 provides about 5V of gate bias which works well
when using typical logic-level MOSFETs. Non-logic-level
MOSFETs should not be used because of their higher
RDS(ON).
MOSFET Gate Bias
+5V
+12V
47Ω
VCCP
1µF
GNDP
Q1
L2
D1
R_SENSE
V_OUT
C_BULK
Figure 4. 12V Gate Bias Configuration
REV. 1.0.3 9/26/01
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