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RT9232BGS

Programmable Frequency Synchronous Buck PWM Controller

Richtek Technology 

RT9232B

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 ×106

RRT (Ω)

kHz

(3)

(RRT to GND)

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

reduces the switching frequency according to Equation (4)

fOSC

=

200kHz

−

33 ×106

RRT (Ω)

kHz

(4)

(RRT to VCC = 12V)

Under Voltage Protection

The under voltage protection is enabled when the RT9232B

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

RT9232B 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 OCSET 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.

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 :

ΔIL

= (VIN

− VOUT ) ×

VOUT

VIN × fOSC

×L

(5)

Generally, an inductor that limits the ripple current between

20% and 50% of output current is appropriate. Make sure

that the output inductor could handle the maximum output

current and would not saturate over the operation

temperature range.

Output Capacitor Selection

The output capacitors determine the output ripple voltage

(ΔVOUT) and the initial voltage drop after a high slew-rate

load transient. The selection of output capacitor depends

on the output ripple requirement. The output ripple voltage

is described as Equation (6).

ΔVOUT

=

ΔIL

× ESR +

1×

8

VOUT

fO2 SC × L × COUT

(1− D)

(6)

For electrolytic capacitor application, typically 90~95%

of the output voltage ripple is contributed by the ESR of

output capacitors. Paralleling lower ESR ceramic capacitor

with the bulk capacitors could dramatically reduce the

equivalent ESR and consequently the ripple voltage.

Input Capacitor Selection

Use mixed types of input bypass capacitors to control

the input voltage ripple and switching voltage spike across

the MOSFETs. The buck converter draws pulsewise

current from the input capacitor during the on time of upper

MOSFET. The RMS value of ripple current flowing through

the input capacitor is described as :

IIN(RMS) = IOUT × D × (1− D)

(7)

The input bulk capacitor must be cable of handling this

ripple current. Sometime, for higher efficiency the low ESR

capacitor is necessarily. Appropriate high frequency

ceramic capacitors physically near the MOSFETs

effectively reduce the switching voltage spikes.

DS9232B-03 March 2007

www.richtek.com

11

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