PAM2308 datasheet(12/15 Pages) PAM

12 / 15 page

For the condition where the step-down converter

is in dropout at 100% duty cycle, the total device

dis sipation reduces to:

Since R

, quiescent current, and switching

losses all vary with input voltage, the total losses

should be investigated over the complete input

voltage range. The maximum power dissipation

depends on the t hermal r esistance o f I C

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:

Where TJ(max) is the maximum allowable

junction temperature 125°C.T

is the ambient

temperature and

is the thermal resistance

from the junction to the ambient. Based on the

standard JEDEC for a two layers thermal test

board, the thermal resistance

of WDFN3X3 is

60°C/W. The maximum power dissipation at T =

25°C can be calculated by following formula:

P =(125°C-25°C)/60°C/W=1.66W

The internal reference is 0.6V (Typical). The

output voltage is calculated as below:

The output voltage is given by Table 1.

Resistor selection for output voltage

setting

As the input voltage approaches the output

voltage, the converter turns the P-chan nel

transistor continuously on. In this mode the

output voltage is equal to the input voltage minus

the v oltage drop across the P - c hannel

transistor:

V= V – I

+ R )

where R

= P-channel switch ON resistance,

= Output current, R

= I nductor D C

resistance

The reference and the circuit remain reset until

the VIN crosses its UVLO threshold.

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).

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.

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.

DS(ON)

OUT

LOAD

dson

LOAD

Setting the Output Voltage

Table 1:

100% Duty Cycle Operation

UVLO and Soft-Start

Short Circuit Protection

Thermal Shutdown

æö

ç÷

èø

V =0.6×1+

1.2V

100k

1.5V

150k

100k

1.8V

200k

100k

2.5V

380k

120k

3.3V

540k

120k

DSONH

P=I R

+I V

J(MAX)

T-T

Power Analog Microelectronics Inc

www.poweranalog.com

PAM2308

Dual High-Efficiency PWM Step-Down DC-DC Coverter

07/2008 Rev 1.0

PAM2308 Datasheet(PDF) 12 Page - Power Analog Micoelectronics

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