IR3871MTR1PBF International Rectifier, IR3871MTR1PBF Datasheet - Page 14

• 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

Figure 19. Charge Requirement during Load Step
Q
C
The output voltage drop, VDROP, initially depends
on the characteristic of the output capacitor.
VDROP is the sum of the equivalent series
inductance (ESL) of the output capacitor times the
rate of change of the output current and the ESR
times the change of the output current.
Output Capacitor Selection
Selection of the output capacitor requires meeting
voltage overshoot requirements during load
removal, and meeting steady state output ripple
voltage requirements. The output capacitor is the
most
increases the overall system cost. The output
capacitor decoupling in the converter typically
includes the low frequency capacitor, such as
Specialty Polymer Aluminum, and mid frequency
ceramic capacitors.
The first purpose of output capacitors is to provide
current when the load demand exceeds the
inductor current, as shown in Figure 19. Equation
6 shows the charge requirement for a certain load.
The advantage provided by the IR3871 at a load
step is to reduce the delay compared to a fixed
frequency control method (in microseconds or (1-
D)*Ts). If the load increases right after the PWM
signal goes low, the longest delay will be equal to
the minimum lower gate on as shown in the
Electrical Specification table. The IR3871 also
reduces the inductor current slew time, the time it
takes for the inductor current to reach equality
with the output current, by increasing the
switching frequency up to 2.5MHz. The result
reduces the recovery time.
OUT

C


V
expensive
V
DROP

1
0.5




1
2
Istep


L
V
converter

IN
 
ΔIstep

t
V
OUT
(6a)
2




component
(6b)
and
VESR is usually much greater than VESL. The
IR3871 requires a total ESR such that the ripple
voltage at the FB pin is greater than 7mV.
The second purpose of the output capacitor is to
minimize the overshoot of the output voltage
when the load decreases as shown in Figure
20. By using the law of energy before and after
the load removal, equation 7 shows the output
capacitance requirement for a load step.
Boot Capacitor Selection
The boot capacitor starts the cycle fully charged
to a voltage of VB(0).
IR3871. Choose a sufficiently small ΔV such
that VB(0)-ΔV exceeds the maximum gate
threshold voltage to turn on the high side
MOSFET.
Choose a boot capacitor value larger than the
calculated C
based on charge balance at CCM operation.
Usually the boot capacitor will be discharged to
a much lower voltage when the circuit is
operating in DCM mode at light load, due to
much longer Q2 off time and the bias current
drawn by the IC. Boot capacitance needs to be
increased if insufficient turn-on of Q1 is
observed at light load, typically larger than
0.1µF is needed. The voltage rating of this part
needs to be larger than VB(0) plus the desired
derating voltage. Its ESR and ESL needs to be
low in order to allow it to deliver the large
current and di/dt’s which drive MOSFETs most
efficiently. In support of these requirements a
ceramic capacitor should be chosen.
C
C
Figure 20. Typical Output Voltage Response
BOOT
OUT


C
V
g

BOOT
OS
L



V
2

ΔV
B
I

in equation 8. Equation 8 is
(0)
STEP
Waveform.
V
OUT

2
1
IR3871MPBF



Cg equals 0.65nF in
2
(8)
(7)
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