Pam2321, Application information – Diodes PAM2321 User Manual
Page 8
PAM2321
Document number: DSxxxxx Rev. 2 - 1
8 of 11
January 2013
© Diodes Incorporated
PAM2321
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:
θ
−
=
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 TDFN3x3 package 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
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
x
6
.
0
V
O
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
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 control led by internal error
amplifier.When the feedback voltage gets higher than internal reference voltage, the device will enter into low I
Q
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:
(
)
R
R
I
V
V
L
DSON
LOAD
IN
OUT
+
−
=
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 PAM2321 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.