Pam2312, Application information – Diodes PAM2312 User Manual
Page 11
PAM2312
Document number: DSxxxxx Rev. 2 - 0
11 of 14
January 2013
© Diodes Incorporated
PAM2312
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 TSOT25 package is
250°C/W, DFN2X2 102°C/W, and QFN3X3 68°C/W, respectively. The maximum power dissipation at T
A
= +25°C can be calculated by following
formula:
P
D
= (125°C - 25°C) /250°C/W = 0.4W
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
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 PAM2312 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 500kHz 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.