ISL6236A Datasheet, PDF (20/37 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236A
addition, SMPS2 can also use REFIN2 to track its output from
0.5V to 2.50V. The ISL6236A contains fault-protection circuits
that monitor the main PWM outputs for undervoltage and
overvoltage conditions. A power-on sequence block controls
the power-up timing of the main PWMs and monitors the
outputs for undervoltage faults. The ISL6236A includes an
adjustable low drop-out linear regulator. The bias generator
blocks include the linear regulator, 3.3V precision reference, 2V
precision reference and automatic bootstrap switchover circuit.
The synchronous-switch gate drivers are directly powered
from PVCC, while the high-side switch gate drivers are
indirectly powered from PVCC through an external capacitor
and an internal Schottky diode boost circuit.
An automatic bootstrap circuit turns off the LDO linear
regulator and powers the device from BYP if LDOREFIN is
set to GND or VCC. See Table 1.
TABLE 1. LDO OUTPUT VOLTAGE TABLE
LDO VOLTAGE
CONDITIONS
COMMENT
VOLTAGE at BYP LDOREFIN < 0.3V,
BYP > 4.63V
Internal LDO is
disabled.
VOLTAGE at BYP LDOREFIN > VCC - 1V,
BYP > 3V
Internal LDO is
disabled.
5V
LDOREFIN < 0.3V,
Internal LDO is
BYP < 4.63V
active.
3.3V
LDOREFIN > VCC - 1V,
BYP < 3V
Internal LDO is
active.
2 x LDOREFIN
0.35V < LDOREFIN < 2.25V Internal LDO is
active.
FREE-RUNNING, CONSTANT ON-TIME PWM
CONTROLLER WITH INPUT FEED-FORWARD
The constant on-time PWM control architecture is a
pseudo-fixed-frequency, constant on-time, current-mode
type with voltage feed forward. The constant on-time PWM
control architecture relies on the output ripple voltage to
provide the PWM ramp signal; thus the output filter
capacitor's ESR acts as a current-feedback resistor. The
high-side switch on-time is determined by a one-shot whose
period is inversely proportional to input voltage and directly
proportional to output voltage. Another one-shot sets a
minimum off-time (300ns typ). The on-time one-shot triggers
when the following conditions are met: the error comparator's
output is high, the synchronous rectifier current is below the
current-limit threshold, and the minimum off time one-shot
has timed out. The controller utilizes the valley point of the
output ripple to regulate and determine the off-time.
ON-TIME ONE-SHOT (t_ON)
Each PWM core includes a one-shot that sets the high-side
switch on-time for each controller. Each fast, low-jitter,
adjustable one-shot includes circuitry that varies the on-time
in response to battery and output voltage. The high-side
switch on-time is inversely proportional to the battery voltage
as measured by the VIN input and proportional to the output
voltage. This algorithm results in a nearly constant switching
frequency despite the lack of a fixed-frequency clock
generator. The benefit of a constant switching frequency is
that the frequency can be selected to avoid noise-sensitive
frequency regions, as shown in Equation 1:
t_ON
=
K_-----(_--V_{-----O----U----T-----+-----}I_{--L---O-----A----D-----}⋅_---r_{--D----S----(--O-----N----)--(--L---O-----W-----E----R----Q-----)--})
V_IN
(EQ. 1)
See Table 2 for approximate K- factors. Switching frequency
increases as a function of load current due to the increasing
drop across the synchronous rectifier, which causes a faster
inductor-current discharge ramp. On-times translate only
roughly to switching frequencies. The on-times established
in the “Electrical Specifications” table on page 4 are
influenced by switching delays in the external high-side
power MOSFET. Also, the dead-time effect increases the
effective on-time, reducing the switching frequency. It occurs
only in PWM mode (SKIP = VCC) and during dynamic output
voltage transitions when the inductor current reverses at
light or negative load currents. With reversed inductor
current, the inductor's EMF causes PHASE to go high earlier
than normal, extending the on-time by a period equal to the
UGATE-rising dead time.
TABLE 2. APPROXIMATE K-FACTOR ERRORS
SMPS
SWITCHING
APPROXIMATE
FREQUENCY K-FACTOR K-FACTOR
(kHz)
(µs)
ERROR (%)
(t_ON = GND, REF,
400
2.5
±10
or OPEN), VOUT1
(t_ON = GND),
VOUT2
500
2.0
±10
(t_ON = VCC),
VOUT1
200
5.0
±10
(t_ON = VCC, REF,
300
3.3
±10
or OPEN), VOUT2
For loads above the critical conduction point, the actual
switching frequency is:
f
=
-----^V----^O-----^U----^T-----+-----^V----^D----^R----^O-----^P----¹------
t_ON(V_IN + V_DROP2)
(EQ. 2)
where:
• V_DROP1 is the sum of the parasitic voltage drops in the
inductor discharge path, including synchronous rectifier,
inductor and PC board resistances
• V_DROP2 is the sum of the parasitic voltage drops in the
charging path, including high-side switch, inductor and PC
board resistances
• t_ON is the on-time calculated by the ISL6236A
20
FN6453.3
March 18, 2008