RT8239A/B/C
19
DS8239A/B/C-06 October 2012
www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Figure 5. Charge pump circuit connected to SECFB
MOSFET Gate Driver (UGATEx, LGATEx)
The high side driver is designed to drive high current, low
R
DS(ON)
N-MOSFET(s). When configured as a floating driver,
5V bias voltage is delivered from the LDO5 supply. The
average drive current is also calculated by the gate charge
at V
GS
= 5V times switching frequency. The instantaneous
drive current is supplied by the flying capacitor between
BOOTx and PHASEx pins. A dead time to prevent shoot
through is internally generated from high side MOSFET
off to low side MOSFET on and low side MOSFET off to
high side MOSFET on.
The low side driver is designed to drive high current low
R
DS(ON)
N-MOSFET(s). The internal pull down transistor
that drives LGATEx low is robust, with a 1.5
Ω
typical on-
resistance. A 5V bias voltage is delivered from the LDO5
supply. The instantaneous drive current is supplied by an
input capacitor connected between LDO5 and GND.
For high current applications, some combinations of high
and low side MOSFETs may cause excessive gate drain
coupling, which leads to efficiency killing, EMI producing,
shoot through currents. This is often remedied by adding
a resistor in series with BOOTx, which increases the turn
on time of the high side MOSFET without degrading the
turn-off time. See Figure 6.
Figure 6. Increasing the UGATEx Rise Time
Soft-Start
The RT8239A/B/C provides an internal
soft-start function
to prevent large inrush current and output voltage overshoot
when the converter starts up. The soft-start (SS)
automatically begins once the chip is enabled. During soft-
start, the internal current limit circuit gradually ramps up
the inductor current from zero. The maximum current limit
value is set externally as described in previous section.
The soft-start time is determined by the current limit level
and output capacitor value. The current limit threshold ramp
up time is typically 2ms from zero to 200mV after
ENTRIPx is enabled. A unique PWM duty limit control
that prevents output over voltage during soft-start period
is designed specifically for FBx floating.
UVLO Protection
The RT8239A/B/C has LDO5 under voltage lock out
protection (UVLO). When the LDO5 voltage is lower than
4.05V (typ.) and the LDO3 voltage is lower than 2.2V (typ.),
both switch power supplies are shut off. This is a non-
latch protection.
Power Good Output (PGOOD)
PGOOD is an open-drain type output and requires a pull
up resistor. PGOOD is actively held low in soft-start,
standby, and shutdown. It is released when both output
voltages are above 90% of the nominal regulation point
for RT8239A. For RT8239B/C, besides requiring both
output voltages to be above 90% of nominal regulation
point, the SECFB threshold must also be above 50% of
nominal regulation point in order for PGOOD to be released.
The PGOOD signal goes low if either output turns off or is
10% below its nominal regulation point.
SECFB
LGATE1
VOUT1
D1
D2
D3
C3
D4
C1
C2
C
F
R
CP1
C4
R
CP2
Charge Pump
BOOTx
R
BOOT
UGATEx
V
IN
PHASEx
controller will enter Ultrasonic Mode. Special care should
be taken to ensure that enough normal ripple voltage is
present on each cycle to prevent charge pump shutdown.
The robustness of the charge pump can be increased by
reducing the charge pump decoupling capacitor and placing
a small ceramic capacitor, C
F
(47pF to 220pF), in parallel
with the upper leg of the SECFB resistor feedback network,
R
CP1
, as shown below in Figure 5.
