ir3637s International Rectifier Corp., ir3637s Datasheet - Page 7

• Manufacturer Part Number: ir3637s
• Description: 1% Accurate Synchronous Pwm Controller
• Manufacturer: International Rectifier Corp.

For higher efficiency, low ESR capacitor is recommended.
Two capacitors of Sanyo's TPB series PosCap with
150µF, 6.3V, 40mΩ ESR and 1.4A ripple current will
meet the ripple current requirement.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low inductor
value causes large ripple current, resulting in the smaller
size, faster response to a load transient but poor effi-
ciency and high output noise. Generally, the selection of
inductor value can be reduced to desired maximum ripple
current in the inductor (∆i). The optimum point is usually
found between 20% and 50% ripple of the output cur-
rent.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
V
L = (V
Where:
V
V
∆i = Inductor Ripple Current
f
∆t = Turn On Time
D = Duty Cycle
If ∆i = 40%(I
L = 1.2µH
The Coilcraft DO3316 series provides a range of induc-
tors in different values, low profile suitable for large cur-
rents, 1.5µH, 8A(Isat) is a good choice for this applica-
tion.
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements.
Rev. 1.1
06/16/05
I
Where:
D is the Duty Cycle, D=V
I
I
For I
S
RMS
RMS
OUT
IN
IN
OUT
= Switching Frequency
= Maximum Input Voltage
- V
= I
is the output current for each channel.
is the RMS value of the input capacitor current.
OUT
= Output Voltage
IN
OUT
OUT
=6A and D=0.36, the I
- V
= L×
OUT
D×(1-D)
O
), then the output inductor will be:
)×
∆t
∆i
V
IN
; ∆t = D×
×∆i×f
V
OUT
OUT
S
---(3)
/V
RMS
1
f
IN.
S
=2.8A
---(5)
; D =
V
V
OUT
IN
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The ESR of the output capacitor is calculated by the
following relationship:
The Sanyo TPB series, PosCap capacitor is a good
choice. The 6TPB150M 150µF, 6.3V has an ESR 40mΩ.
Selecting two of these capacitors in parallel, results to
an ESR of ≅ 20mΩ which achieves our low ESR goal.
Power MOSFET Selection
The IR3637 uses two N-Channel MOSFETs. The selec-
tions criteria to meet power transfer requirements is
based on maximum drain-source voltage (V
source drive voltage (V
resistance R
The MOSFET must have a maximum operating voltage
(V
The gate drive requirement is almost the same for both
MOSFETs. Logic-level transistor can be used and cau-
tion should be taken with devices at very low V
vent undesired turn-on of the complementary MOSFET,
which results a shoot-through current.
The total power dissipation for MOSFETs includes con-
duction and switching losses. For the Buck converter
the average inductor current is equal to the DC load cur-
rent. The conduction loss is defined as:
P
P
ϑ = R
The R
ered for the worst case operation. This is typically given
in the MOSFET data sheet. Ensure that the conduction
losses and switching losses do not exceed the package
ratings or violate the overall thermal budget.
COND
COND
DSS
ESR ≤
Where:
∆V
∆I
∆V
Results to ESR=20.8mΩ
) exceeding the maximum input voltage (V
DS(ON)
DS(ON)
O
(Upper Switch) = I
(Lower Switch) = I
O
O
= Inductor Ripple Current
=50mV and ∆I
= Output Voltage Ripple
Temperature Dependency
temperature dependency should be consid-
∆V
DS(on)
∆I
O
O
and thermal management.
GS
O
=2.4A
), maximum output current, On-
---(4)
2
LOAD
2
LOAD
× R
× R
IR3637SPBF
DS(on)
DS(on)
× D × ϑ
× (1 - D) × ϑ
DSS
GS
IN
), gate-
to pre-
).
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