IRU3073CQ IRF [International Rectifier], IRU3073CQ Datasheet - Page 9

• Manufacturer Part Number: IRU3073CQ
• Description: SYNCHRONOUS PWM CONTROLLER WITH OVER-CURRENT PROTECTION / LDO CONTROLLER
• Manufacturer: IRF [International Rectifier]

If Di = 25%(I
The Coilcraft DO5022HC series provides a range of in-
ductors in different values, low profile suitable for large
currents. 3.3mH is a good choice for this application.
This will result to a ripple approximately 23% of output
current.
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. The ESR of the output capacitor is
calculated by the following relationship:
The Sanyo TPC series, Poscap capacitor is a good choice.
The 6TPC330M, 330mF, 6.3V has an ESR 40mV. Se-
lecting two of these capacitors in parallel, results to an
ESR of
The capacitor value must be high enough to absorb the
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
Power MOSFET Selection
The IRU3073 uses two N-Channel MOSFETs. The se-
lections 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.
Rev. 1.0
09/17/03
DSS
L = 3.125mH
Where:
DV
Di = Inductor Ripple Current
DV
This results to: ESR=26.5mV
ESR [
) exceeding the maximum input voltage (V
O
O
= 50mV and DI
= Output Voltage Ripple
20mV which achieves our low ESR goal.
DV
O
DI
DS(ON)
), then the output inductor will be:
O
O
and thermal management.
GS
---(10)
), maximum output current, On-
23% of 8A = 1.89A
DSS
GS
IN
), gate-
).
to pre-
www.irf.com
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:
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.
Choose IRF7832 for both control MOSFET and synchro-
nous MOSFET. This device provides low on-resistance
in a compact SOIC 8-Pin package.
The MOSFETs have the following data:
The total conduction losses will be:
The switching loss is more difficult to calculate, even
though the switching transition is well understood. The
reason is the effect of the parasitic components and
switching times during the switching procedures such
as turn-on / turnoff delays and rise and fall times. The
control MOSFET contributes to the majority of the switch-
ing losses in synchronous Buck converter. The synchro-
nous MOSFET turns on under zero voltage conditions,
therefore, the turn on losses for synchronous MOSFET
can be neglected. With a linear approximation, the total
switching loss can be expressed as:
The switching time waveform is shown in Figure 9.
P
P
q = R
IRF7832
V
I
R
P
P
P
Where:
V
t
t
T = Switching Period
I
D
LOAD
r
f
COND
COND
DSS
DS(ON)
CON(TOTAL)
CON(TOTAL)
SW
DS(OFF)
DS(ON)
= Rise Time
= Fall Time
= 16A @ 708C
=
= Load Current
= 30V
(Upper Switch) = I
(Lower Switch) = I
DS(ON)
V
= 4mV
temperature dependency should be consid-
= Drain to Source Voltage at off time
DS(OFF)
2
= P
= 0.38W
Temperature Dependency
CON(UPPER)
3
t
r
+
T
t
f
+ P
LOAD
LOAD
2
3
2
I
LOAD
CON(LOWER)
3R
3R
DS(ON)
DS(ON)
IRU3073
3D3q
3(1 - D)3q
---(12)
9