ISL8102 Intersil Corporation, ISL8102 Datasheet - Page 15

• Manufacturer Part Number: ISL8102
• Description: Two-Phase Buck PWM Controller
• Manufacturer: Intersil Corporation

When each of these conditions is true, the controller
immediately begins the soft-start sequence.
Soft-Start
During soft-start, the DAC voltage ramps linearly from zero
to the programmed level. The PWM signals remain in the
high-impedance state until the controller detects that the
ramping DAC level has reached the output-voltage level.
This protects the system against the large, negative inductor
currents that would otherwise occur when starting with a pre-
existing charge on the output as the controller attempted to
regulate to zero volts at the beginning of the soft-start cycle.
The Output soft-start time, T
equal to 64 switching cycles after the ENLL has exceeded its
POR level, followed by a linear ramp with a rate determined
by the switching period, 1/F
T
2. The voltage on ENLL must be above 0.66V. The EN input
3. The driver bias voltage applied at the PVCC pins must
FIGURE 11. POWER SEQUENCING USING THRESHOLD-
SS
allows for power sequencing between the controller bias
voltage and another voltage rail. The enable comparator
holds the ISL8102 in shutdown until the voltage at ENLL
rises above 0.66V. The enable comparator has 100mV of
hysteresis to prevent bounce.
reach the internal power-on reset (POR) rising threshold.
In order for the ISL8102 to begin operation, PVCC is the
only pin that is required to have a voltage applied that
exceeds POR. Hysteresis between the rising and falling
thresholds assure that once enabled, the ISL8102 will not
inadvertently turn off unless the PVCC bias voltage drops
substantially (see Electrical Specifications).
=
CIRCUIT
FAULT LOGIC
64
------------------------------------------- -
SOFT-START
POR
+
AND
ISL8102 INTERNAL CIRCUIT
DAC
F
SENSITIVE ENABLE (ENLL) FUNCTION
SW
⋅
1280
ENABLE
COMPARATOR
SW
SS
15
+
-
0.66V
.
, begins with a delay period
EXTERNAL CIRCUIT
VCC
PVCC
ENLL
10.7kΩ
1.40kΩ
+12V
(EQ. 12)
ISL8102
For example, a regulator with 450kHz switching frequency
having REF voltage set to 1.2V has T
A 100mV offset exists on the remote-sense amplifier at the
beginning of soft-start and ramps to zero during the first 640
cycles of soft-start (704 cycles following enable). This
prevents the large inrush current that would otherwise occur
should the output voltage start out with a slight negative
bias.
During the first 640 cycles of soft-start (704 cycles following
enable) the DAC voltage increments the reference in 25mV
steps. The remainder of soft-start sees the DAC ramping
with 12.5mV steps.
The ISL8102 also has the ability to start up into a pre-
charged output as shown in Figure 12, without causing any
unnecessary disturbance. The FB pin is monitored during
soft-start, and should it be higher than the equivalent internal
ramping reference voltage, the output drives hold both
MOSFETs off. Once the internal ramping reference exceeds
the FB pin potential, the output drives are enabled, allowing
the output to ramp from the pre-charged level to the final
level dictated by the reference setting. Should the output be
pre-charged to a level exceeding the reference setting, the
output drives are enabled at the end of the soft-start period,
leading to an abrupt correction in the output voltage down to
the “reference set” level.
Fault Monitoring and Protection
The ISL8102 actively monitors output voltage and current to
detect fault conditions. Fault monitors trigger protective
measures to prevent damage to the sensitive load. One
common power good indicator is provided for linking to
external system monitors. The schematic in Figure 13
outlines the interaction between the fault monitors and the
power good signal.
FIGURE 12. SOFT-START WAVEFORMS FOR ISL8102-BASED
GND>
GND>
OUTPUT PRECHARGED
BELOW DAC LEVEL
OUTPUT PRECHARGED
MULTI-PHASE CONVERTER
ABOVE DAC LEVEL
T1
T2
T3
SS
equal to 3.55ms.
V
OUT
ENLL (5V/DIV)
October 19, 2005
(0.5V/DIV)
FN9247.0