RT9259A
Application Information
Introduction
The RT9259A is a dual-channel DC/DC controller
specifically designed to deliver high quality power where
12V power source is available. This part consists of a
synchronous buck controller and an LDO controller. The
synchronous buck controller integrates internal MOSFET
drivers that support 12V+12V bootstrapped voltage for high
f
SW
(kHz)
1400
1200
1000
800
600
400
200
0
10
100
1000
When let open, the free running frequency is 230kHz
typically. Figure 1 shows the operation frequency vs. R
RT
for quick reference.
efficiency power conversion. The bootstrap diode is built-
in to simplify the circuit design and minimize external part
count. The LDO controller drives an external N-MOSFET
for lower power requirement.
Internal 5VDD Regulator
It is highly recommended to power the RT9259A with well-
decoupled 12V to VCC12 pin. VCC12 powers the RT9259A
control circuit, low side gate driver and bootstrap circuit for
high side gate driver. A bootstrap diode is embedded to
facilitates PCB design and reduce the total BOM cost. No
external Schottky diode is required. The RT9259A
integrates MOSFET gate drives that are powered from the
VCC12 pin and support 12V + 12V driving capability.
Converters that consist of RT9259A feature high efficiency
without special consideration on the selection of
MOSFETs.
An internal linear regulator regulates VCC12 input to a
5VDD voltage for internal control logic circuit. No external
bypass capacitor is required for filtering the 5VDD voltage.
This further facilitates PCB design and reduces the total
BOM cost.
Power On Reset
The RT9259A automatically initializes upon
applying of
input
power (at the VCC12) pin. The power on reset function
(POR) continually monitors the input bias supply voltage
at the VCC12 pin. The VCC12V POR level is typically
9.6V at VCC12V rising.
Frequency Setting and Shut Down
Connecting a resistor R
RT
from the RT_DIS pin to GND
sets the operation frequency. The relation can be roughly
expressed in the equation.
f
OSC
≅
230kHz
+
7700 (kHz)
R
RT
DS9259A-02 March 2007
R
RT
(kΩ)
(kohm)
Figure 1. RT vs. fsw at Low Frequency
Shorting the RT_DIS pin to GND with an external signal-
level MOSFET shuts down the device. This allows flexible
power sequence control for specified application. The
RT_DIS pin threshold voltage is 0.4V typically.
VIN1 Detection
The RT9259A continuously generates a 10kHz pulse train
with 1μs pulse width to turn on the upper MOSFET for
detecting the existence of VIN1 after VCC12V POR and
RT_DIS enabled as shown in Figure 2. PHASE pin voltage
is monitored during the detection duration.
If the PHASE voltage crosses 1.5V four times, VIN1
existence is recognized and the RT9259A initiates its soft
start cycle as described in next section.
V
IN1
POR_H
PHASE_M
+
-
1.5V
PHASE
UGATE
1st 2nd 3rd 4th PHASE
waveform
Internal Counter will count (V
PHASE
> 1.5V)
four times (rising & falling) to recognize
V
IN1
is ready.
Figure 2
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RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
