ISL6235 Datasheet, PDF (9/14 Pages) Intersil Corporation – Advanced Triple PWM Only Mode and Dual Linear Power Controller for Portable Applications

ISL6235
VSEN3
-
REF
CLK
CLK/3
DIVIDER
3:1
CLK/3
PWM
EA3
COMPARATOR
-
RAMP
RAMP
GENERATOR
GATE3
PWM
LATCH 3
SQ
RQ
CLK
t
CLK/3
t
RAMP
VEA3
t
GATE3
t
FIGURE 3. 12V BOOST OPERATION
The 33% maximum duty cycle of the converter guarantees
discontinuous inductor current and unconditional stability
over all operating conditions.
The boost converter with the limited duty cycle and
discontinuous inductor current can deliver to the load a
limited amount of power before the output voltage starts to
drop. When the duty cycle has reached Dmax, the control
loop is operating open circuit and the output voltage varies
with the output load resistance, Ro, as given by:
Vo= Vin × Dmax_{ 2}----₍--^R-_L----_xo---_F----_{)}
(EQ. 3)
Where Vin is the 5V Main voltage, Dmax = 0.33, L is the
value of the boost inductor, L3, and F = 100kHz. This
provides automatic output current limiting. When the
maximum duty cycle has been reached and for a given
inductor, a further reduction in Ro by one-half will pull the
output voltage down to 0.707 of nominal and cause an
undervoltage condition.
The 12V converter starts to operate at the same time as the
5V Main converter. The rising voltage on the 5V Main output
and the 33% duty cycle limit provides a similar soft-start, as
the 5V Main, for the 12V output.
3V ALWAYS, 5V ALWAYS Linear
Regulators
The 3.3V ALWAYS and 5V ALWAYS outputs are derived
from the battery voltage and are the first voltages available
in the notebook when power on is initiated. The 5V ALWAYS
output is generated directly from the battery voltage by a
linear regulator. It is used to power the system micro-
controller and to internally power the chip and the gate
drivers. The 3.3V ALWAYS output is generated from the 5V
ALWAYS output and may be used to power the keyboard
controller or other peripherals. The combined current
capability of these outputs is 50mA. When the 5V Main
output is greater than it’s undervoltage level, it is switched to
the 5V ALWAYS output via an internal 1.3Ω MOSFET
switch. Simultaneously, the 5V ALWAYS linear regulator is
disabled to prevent excessive power dissipation.
The rise time of the 5V ALWAYS is determined by the value
of the output capacitance on the 5V and 3.3V ALWAYS
outputs. The internal regulator is current limited to about
180mA, so the start up time is approximately:
t
=
C_OUT
×
-------⁵---^V---------
180 m A
(EQ. 4)
Where C_OUT is the sum of the capacitances on the 5V and
3.3V ALWAYS outputs.
Power Good Status
The ISL6235 monitors all the output voltages except for the
3.3V ALWAYS. A single power-good signal, PGOOD, is
issued when soft-start is completed and all monitored
outputs are within 10% of their respective set points. After
the soft-start sequence is completed, undervoltage
protection latches the chip off when any of the monitored
outputs drop below 75% of its set point.
A ‘soft-crowbar’ function is implemented for an overvoltage
on the 3.3V Main or 5V Main outputs. If the output voltage
goes above 115% of their nominal output level, the upper
MOSFET is turned off and the lower MOSFET is turned on.
This ‘soft-crowbar’ condition will be maintained until the
output voltage returns to the regulation window and then
normal operation will continue.
This ‘soft-crowbar’ and monitoring of the output, prevents the
output voltage from ringing negative as the inductor current
flows in the ‘reverse’ direction through the lower MOSFET
and output capacitors.
Over-Temperature Protection
The IC incorporates an over-temperature protection circuit
that shuts all the outputs down when the die temperature
exceeds 150^oC. Normal operation is automatically restored
when the die temperature cools to 125^oC.
Component Selection Guidelines
Output Capacitor Selection
The output capacitors for each output have unique
requirements. In general, the output capacitors should be
selected to meet the dynamic regulation requirements
including ripple voltage and load transients.
3.3V Main and 5V Main PWM Output Capacitors
Selection of the output capacitors is also dependent on the
output inductor so some inductor analysis is required to
select the output capacitors.
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