MB39C313 Datasheet, PDF (31/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
(2) Voltage Overshot at Boost Converter Output during Power Up
A voltage overshot appears at Boost Converter output when input voltage rise time is too fast. This overshot
voltage may damage external parts.
• Figure 4. Simplified Boost Converter of MB39C313.
VIN
Vs
MB39C313
N-DRV
P-DRV
Refer to Figure 4, consider the node voltage at power up, both gate voltage of P-type and N-type power FET
are zero. With sudden voltage change at input, current flow through inductor and charge up the output
capacitor towards input voltage. The P-type power FET will be turned off when output capacitor rise to certain
voltage. The charging current continues to flow through the Schottky diode, such that capacitor reaches its
peak voltage. As the diode blocks the reverse current, the output capacitor voltage can only be discharged
by loading elements.
To avoid this overshot voltage at power up, the rise time of input voltage should be controlled base on RLC
resonance frequency of application circuit. No load condition can be used to estimate worst case.
The LC resonance frequency is
1
2π √ LC
For typical application, L = 6.8 μH, C = 66 μF, the theoretical input rise time should be longer than 133 μs.
Margin is suggested for other parasites.
(3) GD FET Isolation
An isolation switch for Boost Converter output is suggested to break current path for application in disable
condition. The isolation switch can be controlled by GD pin. Refer to Figure 3 for its application connection.
DS04–27267–1E
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