6
FN6279.1
November 2, 2007
Applications Information
Product Description
The ISL97536 is a synchronous, integrated FET 1A
step-down regulator which operates from an input of 2.5V to
6V. The output voltage is user-adjustable with a pair of
external resistors.
The internally-compensated controller makes it possible to
use only two ceramic capacitors and one inductor to form a
complete, very small footprint 1A DC/DC converter.
PWM Operation
In PWM switching mode, the P-Channel MOSFET and
N-Channel MOSFET always operate complementary. When
the P-Channel MOSFET is on and the N-Channel MOSFET
off, the inductor current increases linearly. The input energy
is transferred to the output and also stored in the inductor.
When the P-Channel MOSFET is off and the N-Channel
MOSFET on, the inductor current decreases linearly, and
energy is transferred from the inductor to the output. Hence,
the average current through the inductor is the output
current. Since the inductor and the output capacitor act as a
low pass filter, the duty cycle ratio is approximately equal to
V
O
divided by V
IN
.
The output LC filter has a second order effect. To maintain
the stability of the converter, the overall controller must be
compensated. This is done with the fixed internally
compensated error amplifier and the PWM compensator.
Because the compensations are fixed, the values of input
and output capacitors are 10µF to 40µF ceramic and
inductor is 1.5µH to 2.2µH.
Start-Up and Shutdown
When the EN pin is tied to V
IN
, and V
IN
reaches
approximately 2.4V, the regulator begins to switch. The
inductor current limit is gradually increased to ensure proper
soft-start operation.
When the EN pin is connected to a logic low, the ISL97536 is
in the shutdown mode. All the control circuitry and both
MOSFETs are off, and V
OUT
falls to zero. In this mode, the
total input current is less than 1µA.
When the EN reaches logic HI, the regulator repeats the
start-up procedure, including the soft-start function.
Current Limit and Short-Circuit Protection
The current limit is set at about 1.5A for the PMOS. When a
short-circuit occurs in the load, the preset current limit
restricts the amount of current available to the output, which
causes the output voltage to drop as load demand
increases. When the output voltage drops 30mV below the
reference voltage, the converter will shutdown for a period of
time, approximated by Equation 1, and then restart. If the
overcurrent condition still exists, it will repeat the
shutdown-wait-restart event. This is called a “hiccup” event.
The average power dissipation is reduced, thereby reducing
the likelihood of damage current and thermal conditions in the
IC as shown in Equation 1.
Thermal Shutdown
Once the junction reaches about +145°C, the regulator shuts
down. Both the P-Channel and the N-Channel MOSFETs
turn off. The output voltage will drop to zero. With the output
MOSFETs turned off, the regulator will cool down. Once the
junction temperature drops to about +130°C, the regulator
will perform a normal restart.
Thermal Performance
The ISL97536 is available in a fused-lead MSOP10.
Compared with regular MSOP10 package, the fused-lead
package provides lower thermal resistance. The θ
JA
is
+100°C/W on a 4-layer board and +125°C/W on 2-layer
board. Maximizing the copper area around the pins will
further improve the thermal performance.
RSI/POR Function
When powering up, the open-collector Power-On-Reset
output holds low for about 100ms after V
O
reaches the
preset voltage. When the active-HI reset signal RSI is
issued, POR goes to low immediately and holds for the
same period of time after RSI comes back to LOW. The
output voltage is unaffected. (Please refer to the timing
diagram). When the function is not used, connect RSI to
ground and leave open the pull-up resistor R
4
at POR pin.
The POR output also serves as a 100ms delayed Power
Good signal when the pull-up resistor R
4
is installed. The
RSI pin needs to be directly (or indirectly through a resistor
R
6
) connected to Ground for this to function properly.
Output Voltage Selection
Users can set the output voltage of the variable version with
a resistor divider, which can be chosen based on Equation 2:
tHICCUP
700μ V
IN
⋅
3
----------------------------
216μ+
⎝⎠
⎛⎞
≈
(EQ. 1)
100ms
MIN
25ns
100ms
POR
RSI
V
O
FIGURE 13. RSI AND POR TIMING DIAGRAM
V
O
0.8 1
R
1
R
2
-------
+
⎝⎠
⎜⎟
⎛⎞
×=
(EQ. 2)
ISL97536
