IR3871MTR1PBF International Rectifier, IR3871MTR1PBF Datasheet - Page 15

• Manufacturer Part Number: IR3871MTR1PBF
• Description: IC REG SYNC BUCK 8A 17-QFN
• Manufacturer: International Rectifier
• Series: SupIRBuck™r
• Type: Step-Down (Buck)r

Specifications of IR3871MTR1PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.5 ~ 5 V
• Current - Output: 8A
• Frequency - Switching: Adj to 1MHz
• Voltage - Input: 3 ~ 26 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 17-PowerVQFN
• Part Status: Active
• Package: PQFN / 5 x 6
• Circuit: Single Output
• Iout (a): 8
• Switch Freq (khz): 0 - 1000
• Input Range (v): 3.0 - 26
• Output Range (v): 0.5 - 12
• Ocp Otp Uvlo Pre-bias Soft Start And: Constant On-Time + OVP no OTP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: IR3871MTR1PBFTR

L
DESIGN EXAMPLE
Design Criteria:
Input Voltage, VIN, = 6V to 21V
Output Voltage, VOUT = 1.25V
Switching Frequency, FS = 400KHz
Inductor Ripple Current, 2ΔI = 3A
Maximum Output Current, IOUT = 6A
Over Current Trip, IOC = 9A
Overshoot Allowance, VOS = VOUT + 50mV
Undershoot Allowance, VDROP = 50mV
Pick a standard value 158 kΩ, 1% resistor.
Find R
The R
expected to increase by a factor of 1.4 over
temperature.
standard resistor.
Find a resistive voltage divider for V
Choose the soft start capacitor:
Once the soft start time has chosen, such as
1000us to reach to the reference voltage, a 22nF
for CSS is used to meet 1000us.
Choose
specification:
Find R
R
resistors.
R
R
V
2



FF
SET
FB
= 1.33kΩ, R
1.0
1.25
V


21

OUT
SET
FF
1

DSON
V
R
V
V
1.4
V
H
:
R
2
IN


:

an



2
20
3


V

10m
2Δ
A
R
21
of the lower MOSFET could be
IN
1.25
pF
1
20

V
inductor
I
400K


Therefore, pick a 6.49kΩ, 1%
1
V


V

-

F
V
1.25
= 1.96 kΩ, both 1% standard
400K
OUT
A
OUT
s

9
Hz
A


V
0.5
Hz

to
V


6.3K
meet
156
K

OUT

the
= 1.25V:
design
I
Choose an inductor with the lowest DCR and
AC power loss as possible to increase the
overall system efficiency. For instance, choose
an FDU0650-R82M manufactured by TOKO.
The inductance of this part is 820nH and has
4.2mΩ DCR. Ripple current needs to be
recalculated using the chosen inductor.
IN_RMS
A
accommodates 6 Arms of ripple current at
300KHz. Due to the chemistry of multilayer
ceramic capacitors, the capacitance varies over
temperature and operating voltage, both AC and
DC. One 10µF capacitor is recommended. In a
practical solution, one 1µF capacitor is required
along with 10µF. The purpose of the 1µF
capacitor is to suppress the switching noise and
deliver high frequency current.
Choose an output capacitor:
To meet the undershoot specification, select a
set of output capacitors which has an equivalent
ESR of 10mΩ (50mV/5A). To meet the
overshoot specification, equation 7 will be used
to calculate the minimum output capacitance.
As a result, 160µF will be needed for 5A load
remover. Combine those two requirements, one
can choose a set of output capacitors from
manufactures
Polymer
POSCAP
(EEFSL0D151R)
recommended. This capacitor has 9mΩ ESR
which leaves margin for the voltage drop of the
ESL during load step up. The typical ESL for
this capacitor is around 2nH.
Choose an input capacitor:
ΔI
Panasonic


6
2
A

21
Capacitor)
1.25

V
from
1.25

V
21
such as
0.82u

V

V
21
10µF

from
V
H
Sanyo.
-
1
IR3871MPBF

from
1.25
400K

1
3
SP-Cap
V

(ECJ3YB1E106M)



Panasonic

1.75
Hz
Panasonic
6
A
A

A
1.75
(Specialty



2
150µF

A
1.5
15
or
is
A