ISL8200AMMREP View Datasheet(PDF) - Renesas Electronics
Part Name
Description
Manufacturer
ISL8200AMMREP Datasheet PDF : 24 Pages
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ISL8200AMM
FIRST PWM PULSE SS SETTLING AT VREF + 100mV
INIT. VOUT
VOUT TARGET VOLTAGE
-100mV
FIGURE 26. SOFT-START WITH VOUT < TARGET VOLTAGE
FIRST PWM PULSE
OV = 113%
VOUT TARGET VOLTAGE
FIGURE 27. SOFT-START WITH VOUT BELOW OVERVOLTAGE BUT
ABOVE FINAL TARGET VOLTAGE
Voltage Feedforward
The voltage applied to the FF pin is fed to adjust the sawtooth
amplitude of the channel. The amplitude the sawtooth is set to is
1.25 times the corresponding FF voltage when the module is
enabled. This configuration helps to maintain a constant gain
(GM = VIN • DMAX/VRAMP) and input voltage to achieve
optimum loop response over a wide input voltage range. The
sawtooth ramp offset voltage is 1V (equal to 0.8V*1.25), and the
peak of the sawtooth is limited to VCC - 1.4V. With VCC = 5.4V, the
ramp has a maximum peak-to-peak amplitude of VCC - 2.4V (equal
to 3V); so the feed-forward voltage effective range is typically 3x
as the ramp amplitude ranges from 1V to 3V.
A 384 cycle delay is added after the system reaches its rising
POR and prior to the soft-start. The RC timing at the FF pin should
be sufficiently small to ensure that the input bus reaches its
static state and the internal ramp circuitry stabilizes before
soft-start. A large RC could cause the internal ramp amplitude
not to synchronize with the input bus voltage during output
start-up or when recovering from faults. A 1nF capacitor is
recommended as a starting value for typical applications. The
voltage on the FF pin needs to be above 0.8V prior to soft-start
and during PWM switching to ensure reliable regulation. In a
typical application, FF pin can be shorted to the EN pin.
RUP
=
---V----E----N----_---H----Y---S-----
N x I E N _HYS
RDOWN = V----E---R-N---U-_---FP---T---H---V--–--E---V-N--E--_--N-R---_-E--R-F---E----F-
where N is number of EN pins connected together
VEN_FTH = VEN_RTH – VEN_HYS
VRAMP = max(VCC_FF GRAMP, VCC - 1.4V - VRAMP_OFFSET
VCC_FF = max(0.8V, VFF
VIN
RUP
SYSTEM DELAY
RDOWN
VCC
0.8V
FF
EN
GRAMP = 1.25
VCC_FF
VCC - 1.4V
LIMITER
0.8V
VRAMP_OFFSET = 1.0V
UPPER LIMIT
SAWTOOTH
AMPLITUDE
(VRAMP)
LOWER LIMIT
(RAMP OFFSET)
EN_POR
IEN_HYS = 30µA
OV, OT, OC, AND PLL LOCKING FAULTS
FIGURE 28. SIMPLIFIED ENABLE AND VOLTAGE FEEDFORWARD CIRCUIT
FN8287 Rev 2.00
June 3, 2015
Page 15 of 24
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