rt9212 Richtek Technology Corporation, rt9212 Datasheet - Page 11

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

Output Capacitor
The output capacitor is required to maintain the DC output
voltage and supply the load transient current. The capacitor
must be selected and placed carefully to yield optimal
results and should be chosen to provide acceptable ripple
on the output voltage.
The key specification for output capacitor is its ESR. Low
ESR capacitors are preferred to keep the output voltage
ripple low. The bulk capacitor's ESR will determine the
output ripple voltage and the initial voltage drop after a
high slew-rate transient. For transient response, a
combination of low value, high frequency and bulk capacitors
placed close to the load will be required. High frequency
decoupling capacitors should be placed as close to the
power pins of the load as possible. In most cases, multiple
electrolytic capacitors of small case size perform better
than a single large case capacitor.
The capacitor value must be high enough to absorb the
inductor's ripple current. The output ripple is calculated as
:
Another concern is high ESR induced output voltage ripple
may trigger UV or OV protections will cause IC shutdown.
MOSFET
The MOSFET should be selected to meet power transfer
requirements is based on maximum drain-source voltage
(V
current, minimum on-resistance (R
management.
In high-current applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The losses can be divided into
conduction and switching losses.
Conduction losses are related to the on resistance of
MOSFET, and increase with the load current. Switching
losses occur on each ON/OFF transition. The conduction
losses are the largest component of power dissipation for
both the upper and the lower MOSFETs.
DS9212-05 March 2007
DS
), gate-source drive voltage (V
Δ
V
OUT
=
Δ
I
OUT
×
ESR
GS
DS(ON)
), maximum output
) and thermal
Preliminary
For the Buck converter the average inductor current is equal
to the output load current. The conduction loss is defined
as :
P
P
The switching loss is more difficult to calculate. The reason
is the effect of the parasitic components and switching
times during the switching procedures such as turn-on /
turn-off delays and rise and fall times. With a linear
approximation, the switching loss can be expressed as :
P
Where
V
T
T
F is switching frequency.
The total power dissipation in the switching MOSFET can
be calculate as :
P
I
P
For input voltages of 3.3V and 5V, conduction losses often
dominate switching losses. Therefore, lowering the R
of the MOSFETs always improves efficiency.
Feedback Compensation
The RT9212 is a voltage mode controller; the control loop
is a single voltage feedback path including an error amplifier
and PWM comparator as Figure 1 shows. In order to achieve
fast transient response and accurate output regulation, a
adequate compensator design is necessary. The goal of
the compensation network is to provide adequate phase
margin (greater than 45 degrees) and the highest 0dB
crossing frequency. And to manipulate loop frequency
response that its gain crosses over 0dB at a slope of -
20dB/dec.
O
Rise
Fall
CD
CD
SW
DS(OFF)
High Side Switch
Low Side Switch
2
* R
(low side switch) = I
(high side switch) = I
is fall time,
= 0.5 * V
is rise time,
DS(ON)
is drain to source voltage at off time,
*
D + 0.5 * V
= I
DS(OFF)
=
O
2
* R
* I
DS(ON)
O
O
DS(OFF)
* (T
O
2
2
* R
* R
* (1-D)
Rise
DS(ON)
DS(ON)
* I
+ T
O
* (T
* (1-D)
Fall
* D
Rise
) * F
RT9212
www.richtek.com
+ T
Fall
)* F
DS(ON)
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