IR3621F International Rectifier, IR3621F Datasheet - Page 13

• Manufacturer Part Number: IR3621F
• Description: IC PWM DUAL SYNC PREBIAS 28TSSOP
• Manufacturer: International Rectifier

Specifications of IR3621F

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 345kHz
• Duty Cycle: 86.5%
• Voltage - Supply: 5.5 V ~ 14.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 28-TSSOP
• Frequency-max: 345kHz
• Package: 28-Pin TSSOP
• Circuit: Dual Sync PWM Controller or 2Phase Single Output
• Vcc (min): 4.7
• Vcc (max): 16
• Vout (min): 0.8
• Vout (max): Vcc * 0.90
• Switch Freq (khz): programmable to 500kHz
• Pbf: PbF Option Available
• For Use With: IRDC3621 - BOARD EVAL DUAL SYNC BUCK CTRLR
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The gate drive requirement is almost the same for both
MOSFETs. Logic-level transistors 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 in a shoot-through.
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 current. 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 IRF7821 for control MOSFETs and IRF8113 for
synchronous MOSFETs. These devices provide low on-
resistance in a compact SOIC 8-Pin package.
The MOSFETs have the following data:
The total conduction losses for each output 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 a synchronous Buck converter. The syn-
chronous MOSFET turns on under zero voltage condi-
tions, therefore, the switching losses for synchronous
MOSFET can be neglected. With a linear approxima-
tion, the total switching loss can be expressed as:
P
P
P
P
P
P
ϑ = R
IRF7821
V
R
CON(TOTAL, 1.8V)
CON(TOTAL, 1.8V)
CON(TOTAL, 2.5V)
CON(TOTAL, 2.5V)
COND
COND
DSS
DS(on)
DS(ON)
= 30V
(Upper Switch) = I
DS(on)
(Lower Switch) = I
= 9mΩ
temperature dependency should be consid-
Temperature Dependency
= P
= 1.0W
= P
= 1.0W
CON(UPPER)
CON(UPPER)
LOAD
LOAD
2
2
IRF8113
V
R
+ P
DSS
+ P
DS(on)
×R
×R
CON(LOWER)
= 30V
CON(LOWER)
DS(on)
DS(on)
= 6mΩ
×D×ϑ
×(1 - D)×ϑ
GS
to pre-
www.irf.com
This resistor must be placed close to the IC, place a
small ceramic capacitor from this pin to ground for noise
rejection purposes.
These values are taken under a certain condition test.
For more details please refer to the IRF7821 data sheet.
By using equation (9), we can calculate the total switch-
ing losses.
P
P
Programming the Over-Current Limit
The over-current threshold can be set by connecting a
resistor (R
OCSet pin. The resistor can be calculated by using equa-
tion (3).
The R
should be considered for the worse case operation.
R
I
(50% over nominal output current)
This results to:
R
P
Where:
V
t
t
T = Switching Period
I
SET
From IRF7821 data sheet we obtain:
LOAD
r
f
IRF7821
t
t
SW(TOTAL,2.5V)
SW(TOTAL,1.8V)
SET
DS(on)
SW
DS(OFF)
r
f
= Fall Time
= Rise Time
= 2.7ns
= 7.3ns
=
= R
= Load Current
I
Figure 13 - Switching time waveforms.
DS(on)
O(LIM)
10%
= 6mΩ×1.5 = 9mΩ
90%
V
V
V
= Drain to Source Voltage at off time
1
DS(OFF)
GS
=R
DS
2
SET
has a positive temperature coefficient and it
= 10A×1.5 = 15A
t
d
6
= 0.18W
= 0.18W
(ON)
=6.75KΩ
) from drain of low side MOSFET to the
×
t
r
+
T
t
f
t
×
r
I
IR3621 & (PbF)
LOAD
t
d
(OFF)
---(9)
t
f
13