rt9212 Richtek Technology Corporation, rt9212 Datasheet - Page 10

• Manufacturer Part Number: rt9212
• Description: Dual Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

Applications Information
Inductor
The inductor is required to supply constant current to the
output load. The inductor is selected to meet the output
voltage ripple requirements and minimize the converter's
response time to the load transient.
A larger value of inductance reduces ripple current and
voltage. However, the larger value of inductance has a larger
physical size, lower output capacitor and slower transient
response time.
A good rule for determining the inductance is to allow the
peak-to-peak ripple current in the inductor to be
approximately 30% of the maximum output current. The
inductance value can be calculated by the following
equation :
Where
V
V
F
The inductance value determines the converter's ripple
current and the ripple voltage. The ripple current is
calculated by the following equations :
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values raise
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
RT9212 will provide 0% to 100% duty cycle in response to
a load transient.
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10
Δ
RT9212
S
IN
OUT
I
OUT
is the switching frequency,
is the input voltage,
is the output voltage,
is the peak-to-peak inductor ripple current.
L
ΔI
=
=
(V
(V
V
IN
IN
IN
−
×
V
−
V
F
IN
V
OUT
S
×
OUT
×
Fs L
) V
ΔI
×
) V
×
×
OUT
OUT
OUT
Preliminary
The response time is the time required to slew the inductor
current from an initial current value to the transient current
level. The inductor limit input current slew rate during
the load transient. Minimizing the transient response time
can minimize the output capacitance required. The response
time is different for application of load and removal of load
to a transient. The following equations give the approximate
response time for application and removal of a transient
load :
Where
T
T
Input Capacitor
The input capacitor is required to supply the AC current to
the Buck converter while maintaining the DC input voltage.
The capacitor should be chosen to provide acceptable ripple
on the input supply lines. Use a mix of input bypass
capacitors to control the voltage overshoot across the
MOSFETs. Use small ceramic capacitors for high frequency
decoupling and bulk capacitors to supply the current. Place
the small ceramic capacitors close to the MOSFETs and
between the drain of Q1/Q3 and the source of Q2/Q4.
The key specifications for input capacitor are the voltage
rating and the RMS current rating. For reliable operation,
select the bulk capacitor with voltage and current ratings
above the maximum input voltage and largest RMS current.
The capacitor voltage rating should be at least 1.25 times
greater than the maximum input voltage and voltage rating
of 1.5 times is a conservative guideline. The RMS current
rating for the input capacitor of a buck regulator should be
greater than approximately 0.5 the DC load current.
Δ
Rise
Fall
I
OUT
is the response time to the removal of load,
is the response time to the application of load,
is the transient load current step.
T
Rise
=
V
L
IN
×
−
ΔI
V
OUT
OUT
,
T
DS9212-05 March 2007
Fall
=
L
×
V
ΔI
OUT
OUT