11

FN6828.1

June 26, 2009

Overcurrent Protection

The overcurrent protection is provided on ISL9103, ISL9103A

when overload condition happens. It is realized by monitoring

the CSA output with the OCP comparator, as shown in

Figure 22. When the current at P-Channel MOSFET is sensed

to reach the current limit, the OCP comparator is triggered to

turn off the P-Channel MOSFET immediately.

Short-Circuit Protection

ISL9103, ISL9103A has a Short-Circuit Protection (SCP)

comparator, which monitors the FB pin voltage for output

short-circuit protection. When the output voltage is sensed to

be lower than a certain threshold, the SCP comparator

reduces the PWM oscillator frequency to a much lower

frequency to protect the IC from being damaged.

Undervoltage Lockout (UVLO)

When the input voltage is below the Undervoltage Lock Out

(UVLO) threshold, ISL9103, ISL9103A is disabled.

Soft-Start

The soft-start feature eliminates the inrush current during the

circuit start-up. The soft-start block outputs a ramp reference

to both the voltage loop and the current loop. The two ramps

limit the inductor current rising speed as well as the output

voltage speed so that the output voltage rises in a controlled

fashion.

Low Dropout Operation

The ISL9103, ISL9103A features low dropout operation to

maximize the battery life. When the input voltage drops to a

level that ISL9103, ISL9103A can no longer operate under

switching regulation to maintain the output voltage, the

P-Channel MOSFET is completely turned on (100% duty

cycle). The dropout voltage under such condition is the

product of the load current and the ON-resistance of the

under this condition is the sum of output voltage plus the

voltage drop cross the inductor and the P-Channel MOSFET

switch.

Thermal Shut Down

The ISL9103, ISL9103A provides built-in thermal protection

function. The thermal shutdown threshold temperature is

+130°C (typ) with a 30°C (typ) hysteresis. When the internal

temperature is sensed to reach +130°C, the regulator is

completely shut down and as the temperature drops to

+100°C (typ), the ISL9103, ISL9103A resumes operation

starting from the soft-start.

Applications Information

Inductor and Output Capacitor Selection

To achieve better steady state and transient response,

ISL9103, ISL9103A typically uses a 2.2µH inductor. The

peak-to-peak inductor current ripple can be expressed in

Equation 1:

In Equation 1, usually the typical values can be used but to

have a more conservative estimation, the inductance should

consider the value with worst case tolerance; and for

Specifications” table on page 3 can be used.

To select the inductor, its saturation current rating should be

at least higher than the sum of the maximum output current

and half of the delta calculated from Equation 1. Another

more conservative approach is to select the inductor with the

current rating higher than the P-Channel MOSFET peak

current limit.

Another consideration is the inductor DC resistance since it

directly affects the efficiency of the converter. Ideally, the

inductor with the lower DC resistance should be considered

to achieve higher efficiency.

Inductor specifications could be different from different

manufacturers so please check with each manufacturer if

additional information is needed.

For the output capacitor, a ceramic capacitor can be used

because of the low ESR values, which helps to minimize the

output voltage ripple. A typical value of 10µF ceramic

capacitor should be enough for most of the applications and

the capacitor should be X5R or X7R.

Input Capacitor Selection

The main function for the input capacitor is to provide

decoupling of the parasitic inductance and to provide filtering

function to prevent the switching current from flowing back to

the battery rail. A 10µF ceramic capacitor (X5R or X7R) is a

good starting point for the input capacitor selection.

Output Voltage Setting Resistor Selection

For ISL9103, ISL9103A adjustable output option, the voltage

output voltage values. The output voltage can be calculated

using Equation 2:

current flowing through the voltage divider resistors can be

to minimize this current. On the other hand, the FB pin has

leakage current that will cause error in the output voltage

setting. The leakage current has a typical value of 0.1µA. To

ΔI

V

O

1

V

O

V

IN

---------

–

⎝⎠

⎜⎟

⎛⎞

•

Lf

S

•

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

=

(EQ. 1)

V

O

V

FB

1

R

1

R

2

-------

+

⎝⎠

⎜⎟

⎛⎞

•

=

(EQ. 2)

ISL9103, ISL9103A