PAM2308
Dual High-Efficiency PWM Step-Down DC-DC Coverter
For the condition where the step-down converter
is in dropout at 100% duty cycle, the total device
dissipation reduces to:
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:
2
P =IO RDSONH +IQV
D
IN
Since RDS(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:
VOUT = VIN – ILOAD (Rdson + RL)
where Rdson = P-channel switch ON resistance,
= Output current, RL = Inductor DC
ILOAD
resistance
UVLO and Soft-Start
TJ(MAX) -TA
The reference and the circuit remain reset until
the VIN crosses its UVLO threshold.
P =
D
θJA
Where TJ(max) is the maximum allowable
junction temperature 125°C.TA is the ambient
temperature and θJA 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 θJA of WDFN3X3 is
60°C/W. The maximum power dissipation at TA =
25°C can be calculated by following formula:
The PAM2308 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
PD=(125°C-25°C)/60°C/W=1.66W
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.
Setting the Output Voltage
The internal reference is 0.6V (Typical). The
output voltage is calculated as below:
R1
R2
æ
ö
V
O
=0.6×1+
è
Thermal Shutdown
ç
÷
ø
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.
The output voltage is given by Table 1.
Table 1: Resistor selection for output voltage
setting
Vo
R1
R2
1.2V
1.5V
1.8V
2.5V
3.3V
100k
150k
200k
380k
540k
100k
100k
100k
120k
120k
Power Analog Microelectronics,Inc
www.poweranalog.com
07/2008 Rev 1.0
12
PAM [ POWER ANALOG MICOELECTRONICS ]
