PAM2306

Document number: DS36392 Rev. 3 - 2

11 of 15

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May 2021

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PAM2306

Application Information (continued)

Thermal Consideration (continued)

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





Since RDS(ON), 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 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:







JA

A

)

MAX

(

J

D

T

T

P

Where TJ(MAX) is the maximum allowable junction temperature +125°C. TA is the ambient temperature and θJA 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

θJA of WDFN3X3 is 60°C/W. The

maximum power dissipation at TA = +25°C can be calculated by following formula:

PD = (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

6

.

0

VO

The output voltage is given by Table 1.

VO

R1

R2

1.2V

100k

100k

1.5V

150k

100k

1.8V

200k

100k

2.5V

380k

120k

3.3V

540k

120k

Table 1: Resistor Selection for Output Voltage Setting

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:

VOUT = VIN - ILOAD (RDS(ON) + RL)

Where RDS(ON) = P-Channel switch ON Resistance, ILOAD = Output Current, RL = Inductor DC Resistance.

UVLO and Soft-Start

The reference and the circuit remain reset until the VIN crosses its UVLO threshold.

The PAM2306 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.