PAM2303

3A Low Noise 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 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 QFN3X3-16

68°C/W and SOP-8(EP) 90°C/W respectively.

The maximum power dissipation at T = 25°C can

be calculated by following formula:

P =(125°C-25°C)/68°C/W=1.47W(QFN3X3-16)

P =(125°C-25°C)/90°C/W=1.11W(SOP-8)

The internal reference is 0.6V (Typical). The

output voltage is calculated as below:

The output voltage is given by Table 1.

Resistor recommend for output voltage

setting

When load current decreases, the peak switch

current in Power-PMOS will be lower than skip

current threshold and the device will enter into

Pulse Skipping Mode.

In this mode, the device has two states, working

state and idle state. First, the device enters into

w o r k in g s t a t e c o n t ro ll e d b y in t e r n a l e r ro r

amplifier.When the feedback vol tage gets higher

than internal reference voltage, the device will

enter into low I

idle state with most of internal

blocks disabled. The output voltage will be

reduced by loading or leakage current. When the

feedback voltage gets lower than the internal

reference voltage, the convertor will start a

working state again.

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 min us

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 PAM2303 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 make the output voltage

rise up smoothly.

The switch peak current is limited cycle-by-cycle

to a typical value of 4A. 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 more

smaller than current limit.

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

D

Q

OUT

IN

LOAD

dson

L

dson

L O A D

L

θ

θ

Setting the Output Voltage

Table 1:

Pulse Skipping Mode (PSM) Description

100% Duty Cycle Operation

UVLO and Soft-Start

Short Circuit Protection

Thermal Shutdown













O

R1

V =0.6×1+

R2

Vo

R1

R2

1.2V

150k

150k

1.5V

225k

150k

1.8V

300k

150k

2.5V

475k

150k

3.3V

680k

150k

2

D

O

DSON H

Q

IN

P =I R

+I V

J(MAX)

A

D

JA

T

-T

P =

θ

9

,

Power Analog Microelectronics Inc

www.poweranalog.com

11/2011 Rev1.2