Asus PA246Q, Service Manual

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Service Manual PA246Q
24
#2 Converter Circuit – L4978
The L4978 is a step down monolithic power switching regulator delivering 2A at a voltage between
3.3V and 50V (selected by a simple external divider). Realized in BCD mixed technology, the device
uses an internal power D-MOS transistor (with a typical R
DS(on)
of 0.25 Ω ) to obtain very high efficiency
and high switching speed.
A switching frequency up to 300 kHz is achievable (the maximum power dissipation of the
packages must be observed). A wide input voltage range between 8V to 55V and output voltages
regulated from 3.3V to 50V cover the majority of today’s applications.
Features of this new generation of DC/DC converter include pulse-by-pulse current limit, hiccup mode
for short circuit protection, voltage feed-forward regulation, soft-start, protection against feedback loop
disconnection, inhibit for zero current consumption and thermal shutdown.
Pin Assignment of L4978
Pin No. Names Function
1 GND Ground
2 SS_INH
A logic signal (active low) disables the device (sleep mode operation).
A capacitor connected between this pin and ground determines the
soft start time. When this pin is grounded disables the device (driven
by open collector/drain).
3 OSC
An external resistor connected between the unregulated input voltage
and this pin and a capacitor connected from this pin to ground fix the
switching frequency. (Line feed forward is automatically obtained)
4 OUT Step-down regulator output
5 V
CC
Unregulated DC input voltage
6 BOOT
A capacitor connected between this pin and OUT allows to drive the
internal DMOS Transistors
7 COMP E/A output to be used for frequency compensation
8 FB
Step-down feedback input. Connecting directly to this pin results in an
output voltage of 3.3V. An external resistive divider is required for
higher output voltages.