rt9238 ETC-unknow, rt9238 Datasheet - Page 19

• Manufacturer Part Number: rt9238
• Description: Vrm 8.5 Pwm And Triple Linear Power System Controller
• Manufacturer: ETC-unknow

response time to a load transient. The inductor value
determines the converter’s ripple current and the
ripple voltage is a function of the ripple current. The
ripple voltage and current are approximated by the
following equations:
Increasing the value of inductance reduces the ripple
current and voltage. However, large inductance
values increase the converter’s response time to a
load transient.
One of the parameters limiting the converter’s
response to a load transient is the time required to
change the inductor current. Given a sufficiently fast
control loop design, the RT9238 will provide either
0% or 100% duty cycle in response to a load
transient. The response time is the time interval
required to slew the inductor current from an initial
current value to the post-transient current level.
During this interval the difference between the
inductor current and the transient current level must
be supplied by the output capacitor(s). Minimizing the
response time can minimize the output capacitance
required.
The response time to a transient is different for the
application of load and the removal of load. The
following equations give the approximate response
time interval for application and removal of a transient
load:
where: I
is the response time to the application of load, and
t
sure to check both of these equations at the minimum
and maximum output levels for the worst case
response time.
DS9238-01 July 2001
∆
FALL
I
=
t
RISE
V
is the response time to the removal of load. Be
IN
F
TRAN
−
S
=
×
V
V
L
L
OUT
IN
O
is the transient load current step, t
×
−
×
I
TRAN
V
V
OUT
V
OUT
IN
∆
t
FALL
V
OUT
=
=
L
O
∆
V
I
×
OUT
×
I
TRAN
ESR
RISE
Preliminary
Input Capacitor Selection
The important parameters for the bulk input capacitor
are the voltage rating and the RMS current rating. For
reliable operation, select bulk input capacitors with
voltage and current ratings above the maximum input
voltage and largest RMS current required by the
circuit. The capacitor voltage rating should be at least
1.25 times greater than the maximum input voltage.
The maximum RMS current rating requirement for the
input capacitors of a buck regulator is approximately
1/2 of the DC output load current. Worst-case RMS
current draw in a circuit employing the RT9238
amounts to the largest RMS current draw of the
switching regulator.
Use a mix of input bypass capacitors to control the
voltage overshoot across the MOSFETs. Use
ceramic
decoupling and bulk capacitors to supply the RMS
current. Small ceramic capacitors can be placed very
close to the upper MOSFET to suppress the voltage
induced in the parasitic circuit impedances.
For a through-hole design, several electrolytic
capacitors (Panasonic HFQ series or Nichicon PL
series or Sanyo MV-GX or equivalent) may be
needed. For surface mount designs, solid tantalum
capacitors can be used, but caution must be
exercised with regard to the capacitor surge current
rating. These capacitors must be capable of handling
the surge current at power-up. The TPS series
available from AVX, and the 593D series from
Sprague are both surge current tested.
MOSFET Selection/Considerations
The RT9238 requires 5 external transistors. Two N-
channel MOSFETs are employed by the PWM
converter. The GTL, AGP, and memory linear
controllers can each drive a MOSFET or a NPN
bipolar as a pass transistor. All these transistors
should be selected based upon R
gain, saturation voltages, gate supply requirements,
and thermal management considerations.
capacitance
for
the
www.richtek-ic.com.tw
high
DS(ON)
RT9238
frequency
, current
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