Pam2804, Application information – Diodes PAM2804 User Manual
Page 7
PAM2804
Document number: DSxxxxx Rev. 2 - 0
7 of 10
January 2013
© Diodes Incorporated
PAM2804
A Product Line of
Diodes Incorporated
Application Information
Setting the Output Current
The internal feedback(FB) voltage is 0.1V (Typical). The output current is calculated as below:
R
/
1
.
0
I
S
LED
=
The output Current is given by the following table.
R
S
(Ω) I
LED
(mA)
0.286 350
0.143 700
0.1 1000
As the input voltage approaches the LED forward voltage, the PAM2804 turns the P-Chan nel transistor continuously on. In this mode the
Voltage drop on LED is equal to the input voltage minus the voltage drop across the P-Channel transistor, Inductor and current resistor:
(
)
R
R
R
ILED
V
V
S
L
)
ON
(
DS
IN
LEDDROP
+
+
−
=
where R
DS(ON)
= P-Channel switch ON resistance, ILED = LED current, R
L
= Inductor DC Resistance, R
S
= Inductor DC Resistance.
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.
PCB Layout Check List
When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the PAM2804. These items are
also illustrated graphically in Figure 1. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide.
2. Does the V
FB
pin connect directly to the current sense resistor? The current sense resistor to GND trace should be kept short, direct and wide.
3. Does the (+) plate of C
IN
connect to V
IN
as closely as possible? This capacitor provides the AC current to the internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive VFB node.
5. Keep the (–) plates of C
IN
and C
OUT
as close as possible.