RT9202
Both V
SET
and V
DS
are referenced to V
IN
and a small
Internal
SS
INDUCTOR CURRENT
COUNT = 1 COUNT = 2 COUNT = 3
4V
2V
0V
OVERLOAD
APPLIED
capacitor across R
OCSET
helps V
OCSET
tracking the
variations of V
IN
due to MOSFET switching. The over-
current function will be tripped at a peak inductor current
(I
PEAK
) determined by :
I
×
R
OCSET
I
PEAK
=
OCSET
R
DS(ON)
The OC trip point varies with MOSFET's R
DS(ON)
temperature variations. The temperature coefficient of
I
OCSET
is 2500ppm that is used to compensate R
DS(ON)
temperature variations. To avoid over-current tripping in
the normal operating load range, determine the R
OCSET
resistor value from the equation above with:
1.The maximum R
SD(ON)
at the highest junction
temperature
2.The minimum I
OCSET
from the characteristics
3.Determine I
PEAK
for I
PEAK
> I
OUT(MAX)
+ (∆I)/2
where
∆I
is the output inductor ripple current.
OVER-CURRENT TRIP:
V
DS
> V
SET
i
D
×
R
DS(ON)
> I
OCSET
×
R
OCSET
OCSET
I
OCSET
40uF
DRIVE
+
OC
-
PHASE
GATE
CONTROL
0A
T0 T1
T2
TIME
T3
Figure 4
Shutdown
Pulling low the OCSET pin can shutdown the RT9202
PWM controller as shown in typical application circuit.
Inductor Selection
The RT9202 was designed for V
IN
= 5V, step-down
application mainly. Figure 5 shows the typical topology
and waveforms of step-down converter.
The ripple current of inductor can be calculated as follows:
V
IN
= +5V
R
OCSET
V
SET+
VCC
UGATE
V
DS+
i
D
IL
RIPPLE
= (5V - V
OUT
)/L
×
T
ON
Because operation frequency is fixed at 300kHz,
T
ON
= 3.33
×
V
OUT
/5V
The V
OUT
ripple is
V
OUT RIPPLE
= IL
RIPPLE
×
ESR
ESR is output capacitor equivalent series resistor
Table 1 shows the ripple voltage of V
OUT
: VIN = 5V
PWM
V
PHASE
= V
IN
- V
DS
V
OCSET
= V
IN
- V
SET
Figure 3
Under Voltage and Over Voltage Protection
The voltage at FB pin is monitored and protected against
OC (over current), UV (under voltage), and OV (over
voltage). The UV threshold is 0.5V and OV-threshold is
1.0V. Both UV/OV detection have 30µs triggered delay.
When OC or UV trigged, a hiccup re-start sequence will
be initialized, as shown in Figure 4. Only 3 times of trigger
are allowed to latch off. Hiccup is disabled during soft-
start interval.
www.richtek.com
10
DS9202-09 March 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
