Diodes PAM2306D - Manual (Page 8)

PAM2306D
Document number: DSxxxxx Rev. 1 - 1
8 of 12
www.diodes.com
November 2012
© Diodes Incorporated
PAM2306D
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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 R
DS(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 T
J(MAX)
is the maximum allowable junction temperature 125°C. T
A
is the ambient temperature and θ
JA
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 θ
JA
of WDFN3X3 is 60°C/W.
The maximum power dissipation at T
A
= +25°C can be calculated by following formula:
P
D
= (125°C - 25°C)/60°C/W = 1.66W
Selecting the Output Voltage
The internal reference is 0.6V (Typical). The output voltage is calculated as below:





 
 
2 R
1 R
1 6 . 0 V
O
The output voltage is given by Table 1.
Table 1: Resistor selection for output voltage setting.
V
O
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:
V
OUT
= V
IN
- I
LOAD
(R
DS(ON)
+ R
L
)
where R
DS(ON)
= P-Channel switch ON resistance, I
LOAD
= Output Current, R
L
= Inductor DC Resistance
UVLO and Soft-Start
The reference and the circuit remain reset until the V
IN
crosses its UVLO threshold.
The PAM2306D 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.