ISL6413

FN9129 Rev 0.00

Page 10 of 13

Oct 29, 2003

Shutdown

Driving the EN_LDO pin low will put LDO1 and LDO2 into the

shutdown mode. Driving the EN_PWM pin low will put the

PWM into shutdown mode. Pulling the EN_PWM and EN_LDO

both pins low simultaneously, puts the complete chip into

shutdown mode, and supply current drops to 15µA typical.

Protection Features for the LDOs

Current Limit

The ISL6413 monitors and controls the pass transistor’s gate

voltage to limit the output current. The current limit for LDO1 is

330mA and LDO2 is 250mA. The output can be shorted to

ground without damaging the part due to the current limit and

thermal protection features.

Thermal Overload Protection

Thermal overload protection limits total power dissipation in the

the thermal sensor sends a signal to the shutdown logic,

turning off the pass transistor and allowing the IC to cool. The

pass transistor turns on again after the IC’s junction

temperature typically cools by 20°C, resulting in a pulsed

output during continuous thermal overload conditions. Thermal

overload protection protects the ISL6413 against fault

conditions. For continuous operation, do not exceed the

absolute maximum junction temperature rating of +150°C.

Operating Region and Power Dissipation

The maximum power dissipation of ISL6413 depends on the

thermal resistance of the IC package and circuit board, the

temperature difference between the die junction and ambient

air, and the rate of air flow. The power dissipated in the device

is:

PT = P1 + P2 + P3, where

The maximum power dissipation is:

the thermal resistance from the junction to the surrounding

environment.

The ISL6413 package features an exposed thermal pad on its

underside. This pad lowers the thermal resistance of the

package by providing a direct heat conduction path from the

die to the PC board. Additionally, the ISL6413’s ground

(GND_LDO and PGND) performs the dual function of providing

an electrical connection to system ground and channeling heat

away. Connect the exposed backside pad direct to the

GND_LDO ground plane.

Applications Information

LDO Regulator Capacitor Selection and Regulator

Stability

Capacitors are required at the ISL6413 LDO Regulators’ input

and output for stable operation over the entire load range and

the full temperature range. Use >1µF capacitor at the input of

source impedance of the input supply. Larger capacitor values

and lower ESR provides better PSRR and line transient

response. The input capacitor must be located at a distance of

returned to a clean analog ground. Any good quality ceramic

capacitor can be used as an input capacitor.

The output capacitor must meet the requirements of minimum

amount of capacitance and ESR for both LDO’s. The ISL6413

is specifically designed to work with small ceramic output

capacitors. The output capacitor’s ESR affects stability and

output noise. Use an output capacitor with an ESR of 50m

 or

less to insure stability and optimum transient response. For

stable operation, a ceramic capacitor, with a minimum value of

There is no upper limit to the output capacitor value. Larger

capacitor can reduce noise and improve load transient

response, stability and PSRR. Higher value of output capacitor

(10µF) is recommended for LDO2 when used to power VCO

circuitry in wireless chipsets. The output capacitor should be

board inductances and the other end of the capacitor should

be returned to a clean analog ground.

PWM Regulator Component Selection

INDUCTOR SELECTION

A 10µH minimum output inductor is used with the ISL6413

PWM section. Values larger then 15µH or less then 10µH may

cause stability problems because of the internal compensation

of the regulator. The important parameters of the inductor that

need to be considered are the current rating of the inductor

and the DC resistance of the inductor. The dc resistance of the

inductor will influence directly the efficiency of the converter.

Therefore, an inductor with lowest dc resistance should be

selected for highest efficiency.