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

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

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

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

+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

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

The POR output also serves as a 100ms delayed Power

RSI pin needs to be directly (or indirectly through a resistor

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

700

μ V

IN

⋅

3

----------------------------

216

μ

+

⎝⎠

⎛⎞

≈

(EQ. 1)

100ms

MIN

25ns

100ms

POR

RSI

FIGURE 13. RSI AND POR TIMING DIAGRAM

V

O

0.8

1

R

1

R

2

-------

+

⎝⎠

⎜⎟

⎛⎞

×

=

(EQ. 2)

ISL97536