rt9210 Richtek Technology Corporation, rt9210 Datasheet - Page 13

• Manufacturer Part Number: rt9210
• Description: Dual Synchronous Buck Dc-dc Controller Memory Vddq Termination
• 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.
DS9210-05 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
In RT9210, the VDDQ only sources current but the V
can sink and source current. When sourcing current, the
upper MOSFET supports most of the switching losses.
On the contrary, the lower MOSFET supports most of the
switching losses when V
Losses while Sourcing Current
P
+ T
P
Losses while Sinking Current
P
P
T
For input voltages of 3.3V and 5V, conduction losses often
dominate switching losses. Therefore, lowering the R
of the MOSFETs always improves efficiency.
O
Rise
Fall
Fall
CD
CD
SW
DS(OFF)
High Side Switch
Low Side Switch
High Side Switch
Low Side Switch
High Side
Low Side
2
* R
Fall
)* F
(low side switch) = I
(high side switch) = I
is fall time,
= 0.5 * V
is rise time,
)* F
DS(ON)
= I
is drain to source voltage at off time,
= I
O
O
*
2
2
* R
D + 0.5 * V
= I
= I
* R
= I
DS(OFF)
=
O
O
DS(ON)
O
DS(ON)
2
2
2
* R
* R
* R
* I
DS(ON)
DS(ON)
DS(ON)
* (1-D) + 0.5 * V
* D
O
O
DS(OFF)
TT
* (T
O
2
2
* R
is sinking.
* D + 0.5 * V
* R
* (1-D)
* (1-D)
Rise
DS(ON)
* I
DS(ON)
+ T
O
* (T
* (1-D)
Fall
* D
Rise
DS(OFF)
) * F
RT9210
DS(OFF)
www.richtek.com
+ T
* I
Fall
* I
O
)* F
* (T
O
* (T
DS(ON)
Rise
Rise
13
TT
+