Pam2805, Application information – Diodes PAM2805 User Manual
Page 6
PAM2805
Document number: DSxxxxx Rev. 1 - 0
6 of 9
July 2013
© Diodes Incorporated
PAM2805
A Product Line of
Diodes Incorporated
Application Information
Inductor Selection
The PAM2805 can use small value inductors due to its switching frequency of 1 MHz. The value of inductor will focus in the range of 2.2µH to
4.7µH for most PAM2805 applications. In typical high current white LED applications, it is recommended to use a 4.7µH inductor. The inductor
should have low DCR (DC resistance) to minimize the I
2
R power loss, and it requires a current rating of 2A to handle the peak inductor current
without saturating.
Capacitor Selection
An input capacitor is required to reduce the input ripple and noise for proper operation of the PAM2805. For good input decoupling, Low ESR
(equivalent series resistance) capacitors should be used at the input. At least 2.2µF input capacitor is recommended for most applications.
A minimum output capacitor value of 6.8µF is recommended under normal operating conditions, while a 10µF-22µF capacitor may be required
for higher power LED current. A reasonable value of the output capaci tor depends on the LED current. The ESR of the output capacitor is the
important parameter to determine the output voltage ripple of the converter, so low ESR capacitors should be used at the output to reduce the
output voltage ripple. The small size of ceramic capacitors is an excellent choice for PAM2805 applications. The X5R and X7R types are
preferred because they maintain capacitance over wide voltage and temperature ranges.
Diode Selection
It's indispensable to use a Schottky diode rated at 2A with the PAM2805. Using a Schottky diode with a lower forward voltage drop is better to
improve the power LED efficiency, and its voltage rating should be greater than the output voltage. SS22 is recommended Schottky diode for
rectifier.
LED Current Setting
The LED current is set by the single external Rs resistor connected to the FB pin as shown in the typical application circuit on Page 1. The typical
FB reference is internally regulated to 95mV. The LED current is 95mV/R1. It's recommended to use a 1% or better precision resistor for the
better LED current accuracy. The formula for R
S
selection is shown as follows:
V
3
V
at
mA
I
/
mV
95
R
IN
LED
S
Typically, for 1W(330mA) and 3W(750mA) LED light applications, the R
S
are 0.288
Ω and 0.127Ω respectively.
3 Modes Cycling
The PAM2805 has three modes: 100% brightness, 25% brightness and blinking (typical 8.5Hz).
The mode change is triggered by power on/off actions and cycles in the following sequence: bright, dimming, blinking and back to bright mode.
The PAM2805 will reset to the bright mode after being power off for more than 5 seconds.
Low Voltage Start-Up and Soft-Start
The PAM2805 has a build-in low voltage startup circuit for the best battery life solution. It can start up at 0.9V V
IN
typically when the preset LED
current is 200mA.
The soft-start function is made by clamping the output voltage of error amplifier with another voltage source which increases slowly from zero to
near V
IN
dur ing the soft-start period. Therefore, the duty cycle of the PWM will be increased from zero to maximum in this period. The charging
time of the inductor will be limited by the smaller duty so that the inrush current can be reduced to an acceptable value.
Over Voltage Protection
The output voltage of PAM2805 is monitored by Over Voltage Protection circuit. Once V
OUT
goes over V
OVP
, typically 4.5V, the power NMOS is
turned off and SW pin stops switching. Then, the V
OUT
is clamped to around V
OVP
.
Over Current Protection
The inductor current during charging period is detected by a current sensing circuit. When the value is larger than current limiting I
LIM
, the power
NMOS is turned off so that the inductor will be forced to leave charging stage and enter discharging stage. Therefore, the inductor peak current
will not exceed I
LIM
, whose minimum value is 2A.