RT9218PS RICHTEK [Richtek Technology Corporation], RT9218PS Datasheet - Page 13

• Manufacturer Part Number: RT9218PS
• Description: 5V/12V Synchronous Buck PWM DC-DC and Linear Power Controller
• Manufacturer: RICHTEK [Richtek Technology Corporation]

V
PHASE_M
IN
OC
2. VIN_POR
UGATE will continuously generate a 10kHz colck with
1% duty cycle before V
by detecting V
falling). R
R
shows the detail actions of OCP and POR. It is highly
recommend-ed that R
3. Shutdown
Pulling low the OPS pin by a small single transistor can
shutdown the RT9218 PWM controller as shown in typical
application circuit.
Soft Start
A built-in soft-start is used to prevent surge current from
power supply input during power on. The soft-start voltage
is controlled by an internal digital counter. It clamps the
ramping of reference voltage at the input of error amplifier
and the pulse-width of the output driver slowly. The typical
soft-start duration is 3ms.
DS9218-08 March 2007
POR_H
OCSET
will keep V
-
+
+
-
OCSET
Figure 6. OCP and VIN_POR actions
1.5V
3V
0.4V
+
-
SS_Internal
10pF
OPS
must be kept lower than 37.5kΩ for large
40uA
COMP
OPS
crossing 1.5V four times (rising &
UGATE
OCSET
IN
always higher than 1.5V. Figure 6
V
SSE_Internal
OPS
RAMP_Valley
is ready. V
(2) R
(1) Internal Counter will count (V
Cparasitic
be lower than 30kΩ.
four times (rising & falling) to recognize
V
detect V
OCSET
IN
1st 2nd 3rd 4th OPS
is ready.
Cross-over
IN
IN
can be set too large. Or can
Q2
is recognized ready
is ready (counter = 1, not equal 4)
R
OCSET
DISABLE
waveform
VCORE
PHASE
OPS
> 1.5V)
1) Mode 1 (SS< Vramp_valley)
Initially the COMP stays in the positive saturation. When
SS< V
to produce duty width. So there is no PWM signal and
V
2) Mode 2 (V
When SS>V
input and produce the PWM signal and the increasing
duty width according to its magnitude above the ramp
signal. The output follows the ramp signal, SS. However
while V
SSE
saturation and declines. The takeover of SS lasts until it
meets the COMP. During this interval, since the feedback
path is broken, the converter is operated in the open loop.
3) Mode3 ( Cross-over< SS < V
When the Comp takes over the non-inverting input for PWM
Amplifier and when SSE (SS − V
the converter follows the ramp input, SSE (SS − V
Before the crossover, the output follows SS signal. And
when Comp takes over SS, the output is expected to follow
SSE (SS − V
represented as the falling of V
COMP is observed to keep its decline when it passes the
cross-over, which shortens the duty width and hence the
falling of V
output’ s response to the ramp input, SSE (SS − V
lower than that in Mode 2.
4) Mode 4 (SS > V
When SS > V
the desired V
now.
Since there is a feedback loop for the error amplifier, the
OUT
is zero.
RAMP_Valley
OUT
(SS − V
OUT
increases, the difference between V
RAMP_Valley
RAMP_Valley
GS
REF
happens.
GS
+ V
, there is no non-inverting input available
). Therefore the deviation of V
GS
signal and the soft start is completed
GS
REF
) is reduced and COMP leaves the
+ V
, the output of the converter follows
, SS takes over the non-inverting
< SS< Cross-over)
REF
)
OUT
GS
GS
for a short while. The
+ V
) < V
REF
RT9218
REF
www.richtek.com
)
, the output of
OUT
GS
GS
and
GS
) is
13
is
).