isl6236a Intersil Corporation, isl6236a Datasheet - Page 28

• Manufacturer Part Number: isl6236a
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

Reference and Linear Regulators (VREF3,
REF, LDO and 14V Charge Pump)
The 3.3V reference (VREF3) is accurate to ±1.5% over-
temperature, making VREF3 useful as a precision system
reference. VREF3 can supply up to 5mA for external loads.
Bypass VREF3 to GND with a 0.01µF capacitor. Leave it
open if there is no load.
The 2V reference (REF) is accurate to ±1% over-
temperature, also making REF useful as a precision system
reference. Bypass REF to GND with a 0.1µF (min) capacitor.
REF can supply up to 50µA for external loads.
An internal regulator produces a fixed 5V
(LDOREFIN < 0.2V) or 3.3V (LDOREFIN > VCC - 1V). In an
adjustable mode, the LDO output can be set from 0.7V to
4.5V. The LDO output voltage is equal to two times the
LDOREFIN voltage. The LDO regulator can supply up to
100mA for external loads. Bypass LDO with a minimum
4.7µF ceramic capacitor. When the LDOREFIN < 0.2V and
BYP voltage is 5V, the LDO bootstrap-switchover to an
internal 0.7Ω P-Channel MOSFET switch connects BYP to
LDO pin while simultaneously shutting down the internal
linear regulator. These actions bootstrap the device,
powering the loads from the BYP input voltages, rather than
through internal linear regulators from the battery. Similarly,
when the BYP = 3.3V and LDOREFIN = VCC, the LDO
bootstrap-switchover to an internal 1.5Ω P-Channel
MOSFET switch connects BYP to LDO pin while
simultaneously shutting down the internal linear regulator.
No switchover action in adjustable mode.
In Figure , the external 14V charge pump is driven by
LGATE1. When LGATE1 is low, D1a charged C
from OUT1. C
drop. When LGATE1 transitions to high, the charges from C
will transfer to C
plus VC8. As LGATE1 transitions low on the next cycle, C
will charge C
FB<Reg.Point
FIGURE 73. ULTRASONIC CURRENT WAVEFORMS
FB<REG.POINT
0A
14
8
voltage is equal to OUT1 minus a diode
to its voltage minus a diode drop through
12
through D1b and charge it to VLGATE1
40μs (MAX)
ON-TIME (t
ON-TIME (t
28
ON
)
ON )
)
Zero-Crossing
ZERO-CROSSING
Detection
DETECTION
INDUCTOR
CURRENT
8
sourced
12
ISL6236A
8
D2a. Finally, C
switched to high. CP output voltage is:
where:
• V
• V
SECFB is used to monitor the charge pump through resistive
divider. In an event when SECFB dropped below 2V, the
detection circuit force the highside MOSFET (SMPS1) off
and the lowside MOSFET (SMPS1) on for 300ns to allow CP
to recharge and SECFB rise above 2V. In the event of an
overload on CP where SECFB can not reach more than 2V,
the monitor will be deactivated. Special care should be taken
to ensure enough normal voltage ripple on each cycle as to
prevent CP shut-down. The SECFB pin has ~17mV of
hysteresis, so the ripple should be enough to bring the
SECFB voltage above the threshold by ~3x the hystersis, or
(2V + 3*17mV) = 2.051V. Reducing the CP decoupling
capacitor and placing a small ceramic capacitor (10pF to
47pF) in parallel with the upper leg of the SECFB resistor
feedback network (R
robustness of the charge pump.
Current-Limit Circuit (ILIM ) with r
Temperature Compensation
The current-limit circuit employs a "valley" current-sensing
algorithm. The ISL6236A uses the on-resistance of the
synchronous rectifier as a current-sensing element. If the
magnitude of the current-sense signal at PHASE is above
the current-limit threshold, the PWM is not allowed to initiate
a new cycle. The actual peak current is greater than the
current-limit threshold by an amount equal to the inductor
ripple current. Therefore, the exact current-limit
characteristic and maximum load capability are a function of
the current-limit threshold, inductor value and input and
output voltage.
CP
FIGURE 74. “VALLEY” CURRENT LIMIT THRESHOLD POINT
LGATE1
D
=
is the forward diode dropped across the Schottkys
V
OUT1
is the peak voltage of the LGATE1 driver
+
14
2 V
⋅
charges C
LGATE1
I
LIM
1
of Figure ), will also increase the
(
(
(
(
VAL
–
)
)
)
)
TIME
15
=
=
=
=
4 V
I LOAD(MAX)
I
I
I
⋅
I
LOAD
thru D2b when LGATE1
D
-
-
-
-
Δ
2
I
I PEAK
I LOAD
I LIMIT
DS(ON)
June 28, 2007
FN6453.1
(EQ. 4)
Δ
I