PAM2305D
1A Step-Down DC-DC Converter
For the condition where the step-down converter
is in dropout at 100% duty cycle, the total device
dis sipation reduces to:
P
D
=I
O
R
DSONH
+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
de pend s on th e ther ma l r esi sta nc e 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:
P
D
=
T
J(MAX)
-T
A
θ
JA
2
Table 1:
Resistor selection for output voltage
setting
Vo
1.2V
1.5V
1.8V
2.5V
3.3V
R1
100k
150k
200k
380k
540k
R2
100k
100k
100k
120k
120k
100% Duty Cycle Operation
As the input voltage approaches the output
voltage, the converter turns the P-chan nel
transistor continuously on. In this mode the
output voltage is equal to the input voltage minus
th e voltag e d rop ac ros s the P - c hannel
transistor:
V
OUT
= V
IN
–I
LOAD
(R
dson
+ R
L
)
where R
dson
= P-channel switch ON resistance,
I
L O A D
= O ut pu t c u rr e n t, R
L
= I nd uc t or DC
resistance
UVLO and Soft-Start
The reference and the circuit remain reset until
the VIN crosses its UVLO threshold.
The PAM2305D 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° a
C,
reset occurs and the reset remains until the
temperature decrease to 120° at which time
C,
the circuit can be restarted.
Where TJ(max) is the maximum allowable
junction temperature 125°
A
is the ambient
C.T
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 SOT23-5
package is 250°
C/W, DFN2X2 102°
C/W, and
QFN3X3 68°
C/W, respectively. The maximum
power dissipation at T
A
= 25° can be calculated
C
by following formula:
SOT-25 package:
P
D
=(125°
C-25°
C)/250°
C/W=0.4W
DFN2*2 package:
P
D
=(125°
C-25°
C)/102°
C/W=0.984W
QFN3*3 package:
P
D
=(125°
C-25°
C)/68°
C/W=1.47W
Setting the Output Voltage
The internal reference is 0.6V (Typical). The
output voltage is calculated as below:
R1
V
O
=0.6×1+
R2
The output voltage is given by Table 1.
Power Analog Microelectronics
,
Inc
www.poweranalog.com
02/2012 Rev1.1
9
PAM [ POWER ANALOG MICOELECTRONICS ]
