IR3871MTR1PBF International Rectifier, IR3871MTR1PBF Datasheet - Page 13

• 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

CIRCUIT DESCRIPTION
COMPONENT SELECTION
Selection of components for the converter is an
iterative process which involves meeting the
specifications
performance and cost. The following sections will
guide one through the process.
Inductor Selection
Inductor selection involves meeting the steady
state output ripple requirement, minimizing the
switching loss of upper MOSFETs, meeting
transient response specifications and minimizing
the output capacitance. The output voltage
includes a DC voltage and a small AC ripple
component due to the low pass filter which has
incomplete
harmonics. Neglecting the inductance in series
with the output capacitor, the magnitude of the AC
voltage ripple is determined by the total inductor
ripple current flowing through the total equivalent
series resistance (ESR) of the output capacitor
bank.
One can use equation 4 to find the required
inductance. ΔI is defined as shown in Figure 18.
The main advantage of small inductance is
increased inductor current slew rate during a load
transient, which leads to a smaller output
capacitance requirement as discussed in the
Output Capacitor Selection section. The draw
back of using smaller inductances is increased
switching power loss in upper MOSFET, which
reduces the system efficiency and increases the
thermal dissipation.
ΔI
attenuation

T
and
ON


V
2
IN

L
trade-offs

V
of
OUT

the
(4)
switching
between
Input Capacitor Selection
The main function of the input capacitor bank is
to provide the input ripple current and fast slew
rate current during the load current step up. The
input capacitor bank must have adequate ripple
current carrying capability to handle the total
RMS current. Figure 18 shows a typical input
current. Equation 5 shows the RMS input
current. The RMS input current contains the DC
load current and the inductor ripple current. As
shown in equation 4, the inductor ripple current
is unrelated to the load current. The maximum
RMS input current occurs at the maximum
output current. The maximum power dissipation
in the input capacitor equals the square of the
maximum RMS input current times the input
capacitor’s total ESR.
The voltage rating of the input capacitor needs
to be greater than the maximum input voltage
because of high frequency ringing at the phase
node. The typical percentage is 25%.
Figure 18. Typical Input Current Waveform.
I

IN_RMS
I
OUT


Ton
Ts
1


Fs
Ts

0
f

2
 
IR3871MPBF
t
1


dt
1
3




I
ΔI
OUT



2
(5)
13