11

,

Power Analog Microelectronics Inc

www.poweranalog.com

PAM2312

1A Step-Down DC-DC Converter

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

d e p e n d s o n t h e t h e r m a l r e si st a n c e 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(ma x) is the m aximu m allo wable

junction temper ature 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 SOT23-5

package is 250°C/W, DFN2X2 102°C/W, and

QFN3X3 68°C/W, respectively. The maximum

power dissipation at T = 25°C can be calculated

by following formula:

P =(125°C-25°C)/250°C/W=0.4W

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

th e vo lt a g e d ro p a c ro s s t h e P - c h a n n e l

transistor:

V

= V –I

(R

+ R )

where R

= P-channel switch ON resistance,

I

= O u t p u t c u rr e n t , R

= I n d u c t o r D C

resistance

The reference and the circuit remain reset until

the VIN crosses its UVLO threshold.

The PAM2312 has an internal soft-start circuit

that limits the in-rush current during start-up.

This prevents possible voltage drops of the input

volt ag e an d elim ina t es t he o ut p ut volt a ge

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 500kHz 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)

A

JA

JA

A

D

OUT

IN

LOAD

dson

L

dson

L O A D

L

θ

θ

Setting the Output Voltage

Table 1:

100% Duty Cycle Operation

UVLO and Soft-Start

Short Circuit Protection

Thermal Shutdown

æ

ö

ç

÷

è

ø

O

R1

V =0.6×1+

R2

2

D

O

DSON H

Q

IN

P =I R

+I V

J(MAX)

A

D

JA

T

-T

P =

θ

05/2010 Rev1.0

Vo

R1

R2

1.2 V

10 0 k

10 0 k

1.5 V

15 0 k

10 0 k

1.8 V

20 0 k

10 0 k

2.5 V

38 0 k

12 0 k

3.3 V

54 0 k

12 0 k