ISL8112 Intersil Corporation, ISL8112 Datasheet - Page 19

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

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.
For lower power dissipation, the ISL8112 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 ISL8112 controller has a built-in 5µA
current source as shown in Figure 25. 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
FIGURE 24. “VALLEY” CURRENT LIMIT THRESHOLD POINT
DS(ON)
FIGURE 25. CURRENT LIMIT BLOCK DIAGRAM
V ILIM
+ +
+ +
ILIM_
ILIM_
from the MOSFET data sheet. Add some margin
DS(ON)
VCC
VCC
with temperature. A good general rule
I
I LIM
I LIM
I LIM
LIM
5µA
TIME
TIME
(
(
(
(
VAL
19
I LOAD(MAX)
I LOAD(MAX)
I LOAD(MAX)
I LOAD(MAX)
)
=
I
LOAD
+
+
+
+
-
9R
9R
R
R
Δ
2
I
I
I
I
I PEAK
I PEAK
I PEAK
I PEAK
I LOAD
I LOAD
I LOAD
I LOAD
I LIMIT
I LIMIT
I LIMIT
I LIMIT
TO CURRENT
LIMIT LOGIC
Δ I
ISL8112
I
I
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 ISL8112 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
100mV when ILIM_ is connected to VCC. The logic
threshold for switch-over to the 100mV default value is
approximately VCC - 1V.
The PC board layout guidelines should be carefully
observed to ensure that noise and DC errors do not corrupt
the current-sense signals at PH_.
MOSFET Gate Drivers (UG_, LG_)
The UG_ and LG_ gate drivers sink 2.0A and 3.3A
respectively of gate drive, ensuring robust gate drive for
high-current applications. The UG_ floating high-side
MOSFET drivers are powered by diode-capacitor charge
pumps at BOOT_. The LG_ synchronous-rectifier drivers are
powered by PVCC.
The internal pull-down transistors that drive LG_ low have a
0.6Ω typical on-resistance. These low on-resistance
pulldown transistors prevent LG_ from being pulled up
during the fast rise time of the inductor nodes due to
capacitive coupling from the drain to the gate of the low-side
synchronous-rectifier MOSFETs. However, for high-current
applications, some combinations of high- and low-side
MOSFETs may cause excessive gate-drain coupling, which
leads to poor efficiency and EMI-producing shoot-through
currents. Adding a 4.7Ω resistor in series with BOOT_
increases the turn-on time of the high-side MOSFETs at the
expense of efficiency, without degrading the turn-off time
(Figure 26).
FIGURE 26. REDUCING THE SWITCHING-NODE RISE TIME
ISL8112
5V
5V
5V
5V
BOOT_
UG_
PH_
C BOOT
4.7Ω
ILIM
. The threshold defaults to
Q1
Q1
Q1
Q1
VIN
VIN
VIN
VIN
November 21, 2006
OUT_
FN6396.0