PAM2309 Datasheet, PDF (11/16 Pages) Power Analog Micoelectronics – 1A Step-Down DC-DC Converter

PAM2309
1A Step-Down DC-DC Converter
For the condition where the step-down converter
is in dropout at 100% duty cycle, the total device
dissipation reduces to:
P_D =I_O2R_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
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:
P_D
=
T_J(MAX) -T_A
θ_JA
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 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°C can be calculated
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 0.5V (Typical). The
output voltage is calculated as below:
VO=0.5×1^æçè +
R1 ö
R2 ^÷ø
The output voltage is given by Table 1.
Table 1: Resistor selection for output voltage
setting
Vo
1.2V
1.5V
1.8V
2.5V
3.3V
R1
330k
360k
470k
510k
680k
R2
240k
180k
180k
130k
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:
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 VIN crosses its UVLO threshold.
The PAM2309 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°C, a
reset occurs and the reset remains until the
temperature decrease to 120°C, at which time
the circuit can be restarted.
Power Analog Microelectronics,Inc
www.poweranalog.com
11
08/2009 Rev 1.1