Diodes PAM2303 - Manual (Page 9)

PAM2303
Document number: DSxxxxx Rev. 1 - 2
9 of 14
www.diodes.com
November 2012
© Diodes Incorporated
PAM2303
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Thermal Consideration (cont.)
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
P
D
= I
O
2
R
DSON H
+I
Q
V
IN
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:
θ
−
=
JA
A ) MAX ( J
D
T T
P
Where TJ(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 QFN3X3-16 68°C/W
and SOP-8(EP) 90°C/W, respectively. The maximum power dissipation at T
A
= +25°C can be calculated by the 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)
Setting the Output Voltage
The internal reference is 0.6V (Typical). The output voltage is calculated as below:
The output voltage is given by Table 1.
⎟
⎠
⎞
⎜
⎝
⎛ +
× =
2 R
1 R
1 6 . 0 V
O
Table 1: Resistor recommended for output voltage setting
V
O
R1 R2
1.2V 150k 150k
1.5V 225k 150k
1.8V 300k 150k
2.5V 475k 150k
3.3V 680k 150k
Pulse Skipping Mode (PSM) Description
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 working state cont rolled by internal error
amplifier.When the feedback voltage 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.
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
DSON
+ R
L
)
where R
DSON
= 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 PAM2303 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 make the output voltage rise up smoothly.