rt9232b Richtek Technology Corporation, rt9232b Datasheet - Page 11

• Manufacturer Part Number: rt9232b
• Description: Programmable Frequency Synchronous Buck Controller
• Manufacturer: Richtek Technology Corporation

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 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 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 V
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.
DS9232B-03 March 2007
f
f
OSC
OSC
=
=
200kHz
200kHz
SS
. SS pin voltage ramps down linearly. When
+
−
3
2.9
R
R
3
RT
RT
×
×
10
(
(
10
Ω
Ω
6
)
)
6
kHz
kHz
(3) (R
RT
(4) (R
CC
) from RT pin to ground
, EN and OCSET pin
RT
RT
RT
to
to V
) from RT pin
GND)
CC
= 12V)
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
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
(ΔV
load transient. The selection of output capacitor depends
on the output ripple requirement. The output ripple voltage
is described as Equation (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 :
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.
I
Δ
Δ
IN(RMS)
I
V
L
OUT
OUT
=
(V
) and the initial voltage drop after a high slew-rate
=
IN
=
Δ
I
−
OUT
I
L
V
×
OUT
ESR
×
)
D
×
+
×
V
L
8
(1
1
IN
) can be calculated as follows :
×
−
×
V
f
D)
OUT
f
OSC
2
OSC
V
×
×
OUT
L
L
×
RT9232B
C
OUT
www.richtek.com
(1
−
D)
(6)
(5)
(7)
11