ISL8112 Intersil Corporation, ISL8112 Datasheet - Page 12

• Manufacturer Part Number: ISL8112
• Description: Main Power Supply Controllers
• Manufacturer: Intersil Corporation

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 feedforward. 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.
ON-TIME ONE-SHOT (FS)
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
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:
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 guaranteed
in the Electrical Characteristics 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
(MODE = 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 PH_ to go high earlier than normal,
extending the on-time by a period equal to the UG-rising
dead time.
t
ON
=
K V
----------------------------------------------------------------------------------------------------- -
(
OUT
+
I
LOAD
V
IN
V
⋅
IN
input and proportional to the output
r
DSON LOWERQ
12
(
)
)
(EQ. 1)
ISL8112
For loads above the critical conduction point, the actual
switching frequency is:
where:
• V
• V
• t
(FS = GND,
VREF1, or OPEN),
VSEN1
(FS = GND),
VSEN2
(FS = VCC),
VSEN1
(FS = VCC,
VREF1, or OPEN),
VSEN2
f
inductor discharge path, including synchronous rectifier,
inductor, and PC board resistances
charging path, including high-side switch, inductor, and PC
board resistances
=
ON
DROP1
DROP2
------------------------------------------------------ -
t
ON
SMPS
V
is the on-time calculated by the ISL8112.
TABLE 2. APPROXIMATE K-FACTOR ERRORS
OUT
(
V
IN
is the sum of the parasitic voltage drops in the
is the sum of the parasitic voltage drops in the
+
+
V
V
DROP1
DROP2
FREQUENCY
SWITCHING
)
(kHz)
400
500
200
300
K-FACTOR
(µs)
2.5
2.0
5.0
3.3
APPROXIMATE
ERROR (%)
K-FACTOR
November 21, 2006
±10
±10
±10
±10
(EQ. 2)
FN6396.0