PAM2861 Datasheet, PDF (8/12 Pages) Diodes Incorporated – 1A LED DRIVER WITH INTERNAL SWITCH

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PAM2861
Application Information
Setting Nominal Average Output Current with External Resistor R_S
The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (R_S) connected between VIN
and I_SENSE and is given by:
IOUT(NOM) ^{ 0.1}
RS
The table below gives values of nominal average output current for several preferred values of current setting resistor (R_S) in the typical
application circuit shown on page 1.
R_S (Ω)
0.1
0.13
0.15
0.3
Nominal Average Output
Current (mA)
1000
760
667
333
The above values assume that the VSET pin is floating and at a nominal voltage of VREF (1.25V). Note that R_S = 0.1Ω is the minimum allowed
value of sense resistor under these conditions to maintain switch current below the specified maximum value. It is possible to use different
values of R_S if the VSET pin is driven from an external voltage.
Capacitor Selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and
lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input
supply.
A minimum value of 4.7µF is acceptable if the input source is close to the device, but higher values will improve performance at lower input
voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended. Capacitors with
Y5Vdielectric are not suitable for decoupling in this application and should NOT be used.
Inductor Selection
Recommended inductor values for the PAM2861 are in the range 33µH to 100µH. Higher values of inductance are recommended at higher
supply voltages in order to minimize errors due to switching delays, which in increased ripple and lower efficiency. Higher values of inductance
also result in a smaller change in output current over the supply voltage range. The inductor should be mounted as close to the device as
possilbe with low resistance connections to the LX and VIN pins. The chosen coil should have a saturation current higher than the peak output
current and a continuous current rating above the required mean output current.
The inductor value should be chosen to maintain operating duty cycle and switch ‘on’/’off’ times within the specified limits over the supply voltage
and load current range. The following equations can be used as a guide.
LX Switch ‘On’ time
TON

VIN ^
LI
VLED ^ ILED ^(RS ^ RL ^ RLX⁾
Lx Switch ‘Off’ time
TOFF

VLED ^
LI
VD ^ ILED ^(RS ^ RL⁾
Where: L is the coil inductance; R_L is the coil resistance; R_S is the current sense resistance I_LED is the required LED current; ΔI is the coil peak-
peak ripple current (Internally set to 0.25 x I_LED); V_IN is the supply voltage; V_LED is the total LED forward voltage; R_LX is the switch resistance
(0.3Ω nominal); V_D is the diode forward voltage at the required load current
PAM2861
Document number: DSxxxxx Rev. 1 - 1
8 of 12
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July 2013
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