rt9232a Richtek Technology Corporation, rt9232a Datasheet - Page 10

• Manufacturer Part Number: rt9232a
• Description: Programmable Frequency Synchronous Buck Controller
• Manufacturer: Richtek Technology Corporation

RT9232A
Switching Frequency Setting
The default switching frequency is 200kHz when RT pin
left open. A resistor connected (R
increases the switching frequency as Equation (3).
Conversely, connecting a pull-up resistor (R
reduces the switching frequency according to Equation (4)
Under Voltage Protection
The under voltage protection is enabled when the RT9232A
is activated and SS voltage is higher than 4V. The UVP
function is specified for protecting the converter from an
instant output short circuit during normal operation. The
RT9232A continuously monitors the output voltage by
detecting the voltage on FB pin. The UVP function is
triggered and initiates the hiccup cycles when output
voltage lower than 75% of designated voltage with a 30us
delay.
Hiccup cycle turns off both upper and lower MOSFET first.
An internal 10uA current sink discharges the softstart
capacitor C
SS pin voltage touches 0V, hiccup cycle releases and
normal softstart cycle takes over. When SS voltage is
higher than 4V, the UVP function is enabled again. The
hiccup cycle restarts if the output short event still remains.
The converter is shutdown permanently after 3 times hiccup
and only restarting supply voltages can enable the
converter.
Note that triggering the POR function or EN will reset the
hiccup counter. Make sure that VCC, EN and SENSE pin
voltages are higher than their respective trip level when
output short circuit occurs or the UVP function may not
latch up the converter causing permanent damage to the
converter.
Short Circuit Protection
There is a protection implemented in RT9232A for short
circuit protection, the protection can significantly protect
the power stage from burn-out while the congenital output
www.richtek.com
10
f
f
OSC
OSC
=
=
200kHz
200kHz
SS
. SS pin voltage ramps down linearly. When
+
−
2.9
33
R
R
RT
RT
x
x
10
(
(
10
Ω
Ω
)
6
)
6
kHz
kHz
(3) (R
RT
(4) (R
) from RT pin to ground
RT
RT
to
RT
to V
) from RT pin
GND)
CC
= 12V)
circuit short as shown in Figure 2. The SCP will be
triggered while the POR is triggered 3 times including the
1st POR of system power on. While the SCP been
triggered, the fault is latched until the V
removed.
As shown as Figure 3. The POR of the chip could be
triggered by three major signal including 5V
applied for internal logic use only, 12V, and EN. The POR
will be issued if all of the 3 events are true.
Per RT9232A implementation, the EN is one of signals
will trigger SCP, and it's possible to mal-trigger SCP while
a unclear EN signals being applied. The enabling circuitry
should be took care specially while implementing the EN
circuit.
Component Selection
Components should be appropriately selected to ensure
stable operation, fast transient response, high efficiency,
minimum BOM cost and maximum reliability.
Output Inductor Selection
The selection of output inductor is based on the
considerations of efficiency, output power and operating
frequency. For a synchronous buck converter, the ripple
current of inductor (ΔI
UGATE
V
OUT
V
SS
IN
12V > V
EN > V
POR1
5V
CC_ON
EN_ON
BUS
L
) can be calculated as follows:
Figure 2
Figure 3
POR2
5V
DS9232A-02 March 2007
BUS
POR
CC
BUS
(20V/Div)
(10V/Div)
(2V/Div)
(2V/Div)
POR3
power is
which is