ISL9104, ISL9104A

11

FN6829.5

September 21, 2011

Overcurrent Protection

The overcurrent protection is provided on ISL9104, ISL9104A when

overload condition happens. It is realized by monitoring the CSA

output with the OCP comparator, as shown in Figure 23 on page 9.

When the current at P-Channel MOSFET is sensed to reach the

current limit, the OCP comparator is trigged to turn off the

P-Channel MOSFET immediately.

Short-Circuit Protection

ISL9104, ISL9104A 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, ISL9104, ISL9104A is disabled.

Soft-Start

The soft-start feature eliminates the in-rush 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 ISL9104, ISL9104A features low dropout operation to maximize

the battery life. When the input voltage drops to a level that

ISL9104, ISL9104A 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 P-Channel MOSFET. Minimum required input

voltage drop cross the inductor and the P-Channel MOSFET switch.

Thermal Shut Down

The ISL9104, ISL9104A 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 ISL9104, ISL9104A resumes operation starting from

the soft-start.

Applications Information

Inductor and Output Capacitor Selection

To achieve better steady state and transient response, ISL9104,

ISL9104A typically uses a 1.0µH 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 switching frequency

page 4 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 4.7µF/6.3V 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 4.7µF/6.3V ceramic capacitor (X5R or X7R) is a good starting

point for the input capacitor selection.

Output Voltage Setting Resistor Selection

For ISL9104, ISL9104A adjustable output option, the voltage

voltage values. The output voltage can be calculated using

Equation 2:

current flowing through the voltage divider resistors can be

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 minimize the

than 200k

Ω.

For adjustable output versions, C3 (shown in Figure 2 on page 2)

is highly recommended for improving stability and achieving

better transient response.

Table 1 provides the recommended component values for some

output voltage options.

ΔI

--------

–

⎝⎠

⎜⎟

⎛⎞

•

•

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

=

(EQ. 1)

-------

+

⎝⎠

⎜⎟

⎛⎞

•

=

(EQ. 2)