IR3871MTR1PBF International Rectifier, IR3871MTR1PBF Datasheet - Page 12

• 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
OVER CURRENT MONITOR
The over current circuitry monitors the output
current during each switching cycle. The voltage
across the synchronous MOSFET, VPHASE, is
monitored for over current and zero crossing. The
minimum LGATE interval allows time to sample
VPHASE.
The over current trip point is programmed with a
resistor from the ISET pin to PHASE pin, as
shown in equation 3, where Tj is the junction
temperature of Q2 at operation conditions, and 0.4
is the temperature coefficient (~4000 ppm/C) of
Q2 Rdson. When over current is detected, the
output gates are tri-state and SS voltage is pulled
to 0V. This initiates a new soft start cycle. If there
was a total of three OC events, the IR3871 will
disable switching. Toggling VCC or EN will allow
the next start up.
R
OVER VOLTAGE PROTECTION
The IR3871 monitors the voltage at the FB node.
If the FB voltage is above the over voltage
threshold, the gates are turned off and the
PGOOD signal is pulled low. Toggling VCC will
allow the next start up.
SET

R
DSON
20


I
A
OC

(1

T
100
j

25

0.4)
(3)
STABILITY CONSIDERATIONS
Constant-on-time control is a fast , ripple based
•
•
•
•
control scheme. Unstable operation can occur
if certain conditions are not met. The system
instability is usually caused by:
Switching noise coupled to FB input. This
causes the PWM comparator to trigger
prematurely after the 400ns minimum Q2 on-
time. It will result in double or multiple pulses
every switching cycle instead of the expected
single pulse. Double pulsing can causes
higher output voltage ripple, but in most
application it will not affect operation. This can
usually be prevented by careful layout of the
ground plane and the FB sensing trace.
Steady state ripple on FB pin being too small.
The PWM comparator in IR3871 requires
minimum 7mVp-p ripple voltage to operate
stably. Not enough ripple will result in similar
double pulsing issue described above. Solving
this may require using output capacitors with
higher ESR. Another way to solve this is to
add a ~10pF ceramic capacitor from Vout to
FB to couple more Vout ripple to the FB pin.
ESR loop instability. The stability criteria of
constant on-time is: ESR*Cout>Ton/2. If ESR
is too small that this criteria is violated then
sub-harmonic oscillation will occur. This is
similar to the instability problem of peak-
current-mode control with D>0.5. Increasing
ESR is the most effective way to stabilize the
system, but the price paid is the larger output
voltage ripple.
For applications with all ceramic output
capacitors, the ESR is usually too small to
meet
applications, external slope compensation is
necessary to make the loop stable. The ramp
injection circuit, composed of R6, C13, and
C14, shown in Figure 7 is required. The
inductor current ripple sensed by R6 and C13
is AC coupled to the FB pin through C14. The
injected ramp slope can be adjusted by
changing the RC time constant of R6 and
C13, which is similar to the circuit used for
DCR current sensing. C14 is typically chosen
by C14(R7//R8)=Ton.
the
stability
IR3871MPBF
criteria.
In
these
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