rt9218b Richtek Technology Corporation, rt9218b Datasheet - Page 12

• Manufacturer Part Number: rt9218b
• Description: Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

V
PHASE_M
IN
OC
RT9218B
2. VIN_POR
UGATE will continuously generate a 10kHz colck with 1%
duty cycle before V
detecting V
R
will keep V
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 RT9218B 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.
www.richtek.com
12
POR_H
OCSET
must be kept lower than 37.5kΩ for large R
OCSET
-
+
+
-
Figure 6. OCP and VIN_POR actions
1.5V
OPS
OPS
3V
0.4V
+
-
SS_Internal
always higher than 1.5V. Figure 6 shows
10pF
be lower than 30kΩ.
crossing 1.5V four times (rising & falling).
40uA
COMP
IN
UGATE
is ready. V
V
SSE_Internal
OPS
RAMP_Valley
(1) Internal Counter will count (V
(2) R
Cparasitic
four times (rising & falling) to recognize
V
detect V
OCSET
IN
1st 2nd 3rd 4th OPS
IN
is ready.
Cross-over
is recognized ready by
IN
can be set too large. Or can
Q2
is ready (counter = 1, not equal 4)
R
OCSET
DISABLE
waveform
VCORE
PHASE
OPS
OCSET
> 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
(SS − V
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
OUT
is zero.
increases, the difference between V
RAMP_Valley
GS
) is reduced and COMP leaves the saturation
OUT
RAMP_Valley
RAMP_Valley
GS
REF
happens.
GS
+ V
, there is no non-inverting input available
). Therefore the deviation of V
signal and the soft start is completed
GS
REF
+ V
, the output of the converter follows
, SS takes over the non-inverting
< SS< Cross-over)
REF
)
OUT
DS9218B-09 March 2007
GS
GS
+ V
) < V
for a short while. The
REF
REF
)
, the output of
OUT
and SSE
GS
GS
GS
) is
is
).