isl6236 Intersil Corporation, isl6236 Datasheet - Page 27

• Manufacturer Part Number: isl6236
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

from OUT1. C8 voltage is equal to OUT1 minus a diode
drop. When LGATE1 transitions to high, the charges from C8
will transfer to C
plus VC8. As LGATE1 transitions low on the next cycle, C12
will charge C
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 ISL6236 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
LGATE1
D
=
is the forward diode dropped across the Schottkys
V
OUT1
is the peak voltage of the LGATE1 driver
14
+
14
2 V
to its voltage minus a diode drop through
12
⋅
charges C
LGATE1
through D1b and charge it to VLGATE1
1
of Figure 68), will also increase the
–
15
4 V
27
⋅
thru D2b when LAGET1
D
DS(ON)
(EQ. 4)
ISL6236
.
For lower power dissipation, the ISL6236 uses the
on-resistance of the synchronous rectifier as the
current-sense element. Use the worst-case maximum value
for r
for the rise in r
is to allow 0.5% additional resistance for each °C of
temperature rise. The ISL6236 controller has a built-in 5µA
current source as shown in Figure . Place the hottest power
MOSEFTs as close to the IC as possible for best thermal
coupling. The current limit varies with the on-resistance of
the synchronous rectifier. When combined with the
undervoltage-protection circuit, this current-limit method is
effective in almost every circumstance.
R ILIM
A negative current limit prevents excessive reverse inductor
currents when VOUT sinks current. The negative
current-limit threshold is set to approximately 120% of the
positive current limit and therefore tracks the positive current
limit when ILIM is adjusted. The current-limit threshold is
adjusted with an external resistor for ISL6236 at ILIM. The
current-limit threshold adjustment range is from 20mV to
200mV. In the adjustable mode, the current-limit threshold
voltage is 1/10th the voltage at ILIM. The voltage at ILIM pin
is the product of 5µA * R
FIGURE 73. “VALLEY” CURRENT LIMIT THRESHOLD POINT
DS(ON)
+ +
+ +
V ILIM
V
FIGURE 74. CURRENT LIMIT BLOCK DIAGRAM
ILIM
from the MOSFET data sheet. Add some margin
DS(ON)
I
LIM
VCC
VCC
with temperature. A good general rule
(
(
(
(
VAL
ILIM
5m A
5
)
)
)
)
TIME
=
=
=
=
. The threshold defaults to
I LOAD(MAX)
I
I
I
I
LOAD
-
-
-
-
+
+
+
+
Δ
2
I
9R
9R
R
R
I PEAK
I LOAD
I LIMIT
TO CURRENT
LIMIT LOGIC
June 28, 2007
FN6373.3
Δ
I