MB39C313 Datasheet, PDF (8/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
■ FUNCTIONAL DESCRIPTIONS
VLOGIC : Buck Converter
The Buck converter is a fixed frequency PWM control asynchronous converter with integrated NMOS power
switch. It features voltage mode control with input feed forward to improve line regulation performance. The
converter is internally compensated and is designed to work with ceramic output capacitor. The main switch
of the converter is a 3.2 A rated power NMOS with gate drive circuit reference to SWB pin (source terminal
of the NMOS power FET). The gate drive circuit is powered from an internal 4 V regulator and is bootstrapped
from SWB pin via an external capacitor to achieve driving capability beyond the supply rail.
Soft Start (Buck Converter)
The Buck converter has build in soft start control to limit the inrush current at start up. The soft start cycle
start after EN1 is asserted and the duration is internally set to 1 ms. During the soft start cycle, the second
non-inverting input of the error amplifier, refer to the block diagram, ramps up from 0 V.
Thus, the Buck converter output ramps up in a control manner. The soft start cycle ends when the voltage
on the second non-inverting input of the error amplifier rises above the reference voltage of 1.213 V.
Short Circuit Protection (Buck Converter)
The Buck converter is protected from short circuit fault by internal cycle-to-cycle current limit. In addition,
the switching frequency is reduced to limit the power dissipation during the fault condition.
The switching frequency reduction depends on the voltage on FBB pin. When the voltage of FBB pin is below
0.9 V and 0.6 V, the switching frequency reduces to 1/2 and 1/4 of the normal value respectively.
The switching frequency becomes normal automatically if the normal situation was resumed.
VS : Boost Converter
The Boost converter features fixed frequency pulse width modulated (PWM) control with integrated NMOS
power switch. The switching frequency can be set to either 500 kHz or 750 kHz via the FREQ pin. The
converter operates as an asynchronous Boost converter with external Schottky diode. The use of voltage
mode control with input feed forward improves line regulation performance. In addition, the converter is
designed with external frequency compensation that allows flexibility on selecting external component values.
A PMOS switch with on resistance of 10 Ω connects between SW and OS pin so that it operates in parallel
with the external Schottky diode. At high loading current, most of the inductor current flows through the
external Schottky diode. At light load, the PMOS switch provides a conduction path that allows the inductor
current flow in reverse direction. As a result, the converter stays in continuous conduction mode for most of
the load current range and allows the use of simple frequency compensation scheme.
Soft Start (Boost Converter)
A build in soft start circuit with an external capacitor connects to SS pin provides soft start function for the
Boost converter to prevent high inrush current during start up. The SS pin provides a constant charging
current so that soft start time is adjustable by changing the capacitance value of an external capacitor. During
start up, the output voltage of the Boost converter is controlled by the SS pin until the voltage on SS pin is
higher than the voltage on FB pin and the soft start cycle ends.
Over Voltage Protection (Boost Converter)
The Boost converter has build in over voltage protection to prevent MB39C313 from being damaged due to
excessive voltage stress under fault conditions such as FB pin is left floating or short to ground.
The protection circuitry monitors the Boost converter output via OS pin and shut down the NMOS power
FET that connects to SW pin when the voltage on OS pin is higher than 18.7 V. As a result, the inductor
current start to fall and the output of the Boost converter follows. The Boost converter resumes normal
operation when the voltage at OS pin falls below the protection threshold.
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DS04–27267–1E