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RT9232

Programmable Frequency Synchronous Buck PWM Controller

Richtek Technology 

RT9232

Switching Frequency Setting

The default switching frequency is 200kHz when RT pin

left open. A resistor connected (RRT) from RT pin to ground

increases the switching frequency as Equation (3).

fOSC = 200kHz + 2.9 x 106 kHz (3)

RRT (Ω)

(RRT to GND)

Conversely, connecting a pull-up resistor (RRT) from RT pin

reduces the switching frequency according to Equation (4)

fOSC = 200kHz − 33 x 106 kHz

RRT (Ω)

(4)

(RRT to VCC = 12V)

Under Voltage Protection

The under voltage protection is enabled when the RT9232

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

RT9232 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 CSS. SS pin voltage ramps down linearly. When

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 RT9232 for short

circuit protection, the protection can significantly protect

the power stage from burn-out while the congenital output

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 VCC power is

removed.

(2V/Div)

SS

UGATE

(20V/Div)

(2V/Div)

VIN

VOUT POR1

POR2

Figure 2

(10V/Div)

POR3

As shown as Figure 3. The POR of the chip could be

triggered by three major signal including 5VBUS which is

applied for internal logic use only, 12V, and EN. The POR

will be issued if all of the 3 events are true.

Per RT9232 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.

5VBUS

12V > VCC_ON

EN > VEN_ON

5VBUS

POR

Figure 3

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 (ΔIL) can be calculated as follows:

www.richtek.com

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DS9232-06 March 2007

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