PAM2308

Document number: DSxxxxx Rev. 1 - 1

11 of 15

www.diodes.com

January 2013

© Diodes Incorporated

PAM2308

A Product Line of

Diodes Incorporated

For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:

V

I

R

I

P

IN

Q

H

)

ON

(

DS

2

O

D

+

=

voltage range. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surrounding airflow

and temperature difference between junction and ambient. The maximum power dissipation can be calculated by the following formula:

θ

−

=

JA

A

)

MAX

(

J

D

T

T

P

P = (125°C - 25°C) /60°C/W = 1.66W

Setting the Output Voltage

The internal reference is 0.6V (Typical). The output voltage is calculated as below:

⎟

⎠

⎞

⎜

⎝

⎛ +

=

2

R

1

R

1

x

6

.

0

The output voltage is given by Table 1.

Table 1: Resistor selection for output voltage setting.

R1

R2

1.2V

100k

100k

1.5V

150k

100k

1.8V

200k

100k

2.5V

380k

120k

3.3V

540k

120k

100% Duty Cycle Operation

As the input voltage approaches the output voltage, the converter turns the P-Channel transistor continuously on. In this mode the output voltage

is equal to the input voltage minus the voltage drop across the P-Channel transistor:

()

R

R

I

V

V

L

DSON

LOAD

IN

OUT

+

−

=

UVLO and Soft-Start

The PAM2308 has an internal soft-start circuit that limits the in-rush current during start-up. This prevents possible voltage drops of the input

voltage and eliminates the output voltage overshoot. The soft-start acts as a digital circuit to increase the switch current in several steps to the

P-Channel current limit (1500mA).

Short Circuit Protection

The switch peak current is limited cycle-by-cycle to a typical value of 1500mA. In the event of an output voltage short circuit, the device operates

with a frequency of 400kHz and minimum duty cycle, therefore the average input current is typically 200mA.

Thermal Shutdown

When the die temperature exceeds +150°C, a reset occurs and the reset remains until the temperature decrease to +120°C, at which time the

circuit can be restarted.