PAM2303
3A Low Noise 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
DSON H
+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:
T
J(MAX)
-T
A
P
D
=
θ
JA
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 QFN3X3-16
68°
C/W and SOP-8(EP) 90°
C/W respectively.
The maximum power dissipation at T
A
= 25° can
C
be calculated by following formula:
P
D
=(125°
C-25°
C)/68°
C/W=1.47W(QFN3X3-16)
P
D
=(125°
C-25°
C)/90°
C/W=1.11W(SOP-8)
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.
R1
V
O
=0.6×1+
R2
Table 1:
Resistor recommend for output voltage
setting
2
In this mode, the device has two states, working
state and idle state. First, the device enters into
wor k in g s tat e c ont ro ll ed by in ter n al e r ro r
amplifier.When the feedback vol tage 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-chan nel
transistor continuously on. In this mode the
output voltage is equal to the input voltage min us
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 PAM2303 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 make the output voltage
rise up smoothly.
Short Circuit Protection
The switch peak current is limited cycle-by-cycle
to a typical value of 4A. 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 more
smaller than current limit.
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.
Vo
1.2V
1.5V
1.8V
2.5V
3.3V
R1
150k
225k
300k
475k
680k
R2
150k
150k
150k
150k
150k
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.
Power Analog Microelectronics
,
Inc
www.poweranalog.com
11/2011 Rev1.2
9
PAM [ POWER ANALOG MICOELECTRONICS ]
