ISL6308 Datasheet, PDF (15/27 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308
Initialization
Prior to initialization, proper conditions must exist on the
ENLL, VCC, PVCC and the REF0 and REF1 pins. When the
conditions are met, the controller begins soft-start. Once the
output voltage is within the proper window of operation, the
controller asserts PGOOD.
Enable and Disable
While in shutdown mode, the PWM outputs are held in a
high-impedance state to assure the drivers remain off. The
following input conditions must be met before the ISL6308 is
released from shutdown mode.
1. The bias voltage applied at VCC must reach the internal
power-on reset (POR) rising threshold. Once this
threshold is reached, proper operation of all aspects of
the ISL6308 is guaranteed. Hysteresis between the rising
and falling thresholds assure that once enabled, the
ISL6308 will not inadvertently turn off unless the bias
voltage drops substantially (see Electrical
Specifications).
ISL6308 INTERNAL CIRCUIT EXTERNAL CIRCUIT
VCC
POR
CIRCUIT
PVCC1
+12V
ENABLE
COMPARATOR
+
-
10.7kΩ
ENLL
1.40kΩ
0.66V
SOFT-START
AND
FAULT LOGIC
FIGURE 11. POWER SEQUENCING USING THRESHOLD-
SENSITIVE ENABLE (ENLL) FUNCTION
2. The voltage on ENLL must be above 0.66V. The EN input
allows for power sequencing between the controller bias
voltage and another voltage rail. The enable comparator
holds the ISL6308 in shutdown until the voltage at ENLL
rises above 0.66V. The enable comparator has 100mV of
hysteresis to prevent bounce.
3. The driver bias voltage applied at the PVCC pins must
reach the internal power-on reset (POR) rising threshold.
In order for the ISL6308 to begin operation, PVCC1 is the
only pin that is required to have a voltage applied that
exceeds POR. However, for 2 or 3-phase operation
PVCC2 and PVCC3 must also exceed the POR
threshold. Hysteresis between the rising and falling
thresholds assure that once enabled, the ISL6308 will not
inadvertently turn off unless the PVCC bias voltage drops
substantially (see Electrical Specifications).
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_SS, begins with a delay period
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_sw.
T_SS
=
6_----4_-----+-----D_----A_-----C_------⋅_---1_---2_---8_----0_-
F_SW
(EQ. 12)
For example, a regulator with 450kHz switching frequency
having REF voltage set to 1.2V has T_SS equal to 3.55ms.
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.
The ISL6308 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.
15
FN9208.2
October 19, 2005