isl8120 Intersil Corporation, isl8120 Datasheet - Page 23

• Manufacturer Part Number: isl8120
• Description: Dual/n-phase Buck Pwm Controller With Integrated Drivers
• Manufacturer: Intersil Corporation

While EN/FF is pulled to ground, a constant voltage (0.8V) is
fed into the ramp generator to maintain a minimum
peak-to-peak ramp.
Since the EN/FF pins are pulled down under fault conditions,
the pull-up resistor (R
than 5mA current from EN/FF pin. Essentially, the EN/FF
pins cannot be directly connected to VCC.
Soft-start
The ISL8120 has two independent digital pre-charged
soft-start circuitry, which has a rise time inversely
proportional to the switching frequency and is determined by
an digital counter that increments with every pulse of the
phase clock. Refer to Figure 7. The full soft-start time from
0V to 0.6V can be estimated by Equation 1.
The ISL8120 has the ability to work under a pre-charged
output (see Figure 8). The output voltage would not be
yanked down during pre-charged start-up. If the pre-charged
output voltage is greater than the final target level but prior to
113% setpoint, the switching will not start until the output
voltage reduces to the target voltage and the first PWM
pulse is generated (see Figure 9). The maximum allowable
pre-charged level is 113%. If the pre-charged level is above
113% but below 120%, the output will hiccup between 113%
(LGATE turns on) and 87% (LGATE turns off) while EN/FF is
pulled low. If the pre-charged load voltage is above 120% of
the targeted output voltage, then the controller will be
latched off and not be able to power-up. See “PRE-POR
Overvoltage Protection (PRE-POR-OVP)” on page 24 for
details.
For above target pre-charged startup, the output voltage
would not change until the end of the soft-start. If the initial
dip is below the UV level, the LGATE could be turned off. In
such an event, the body-diode drop of the low-side FET will
be sensed and could potentiality cause an OCP event for
r
t
SS
DS(ON)
R
FIGURE 6. TYPICAL 4-PHASE WITH FAULT HANDSHAKE
UP
=
1280
------------ -
f
=
SW
current sensing applications.
--------------------------------------------------------- -
I
EN_HYS
V
2-PHASE
ISL8120
EN_HYS
⋅
EN/FF1
EN/FF2
N
PHASE
UP
) should be scaled to sink no more
23
VIN
R
R
UP
DOWN
EN/FF1
EN/FF2
2-PHASE
ISL8120
(EQ. 1)
ISL8120
Power-Good
FIGURE 9. SOFT-START WITH VOUT BELOW OV BUT
CHANNEL 2 UV/OV
CHANNEL 1 UV/OV
END OF SS2
END OF SS1
-100mV
FIGURE 10. POWER-GOOD THRESHOLD WINDOW
-100mV
FIRST PWM PULSE
UV
FIGURE 8. SOFT-START WITH VOUT = UV
FIGURE 7. SOFT-START WITH VOUT = 0V
0.0V
UV
FIRST PWM PULSE
ABOVE FINAL TARGET VOLTAGE
GOOD
FIRST PWM PULSE
OR
PRE-OV (NO LATCH)
VMON1, 2
AND
t
SS_DLY
SS Settling at VREF + 100mV
SS Settling at VREF + 100mV
VOUT TARGET VOLTAGE
VOUT
≈
GOOD
t
----------- -
F
SS
384
SW
=
UV
VOUT
OV = 113%
1208
------------ -
< +20%
F
V
REF
SW
March 28, 2008
PGOOD
+13%
+9%
-9%
-13%
+20%
FN6641.0