PAM2841 Datasheet, PDF (9/14 Pages) Power Analog Micoelectronics – 1.5ASW Current, 40V Precision WLED Driver

PAM2841
1.5A SW Current, 40V Precision WLED Driver
Application Information
Inductor Selection
to the device.
The selection of the inductor affects steady state
operation as well as transient behavior and loop
stability. These factors make it the most important
component in power regulator design. There are
three important inductor specifications, inductor
value, DC resistance and saturation current.
Considering inductor value alone is not enough.
The inductor value determines the inductor ripple
current. Choose an inductor that can handle the
necessary peak current without saturation, the
inductor DC current given by:
Iin_dc=Vout*Iout/(Vin*η) η=efficiency.
Inductor values can have ±20% tolerance with no
current bias. When the inductor current
approaches saturation level, its inductance can
decrease 20% to 35% from the 0A value
depending on how the inductor vendor defines
saturation current. Using an inductor with a
smaller inductance value causes discontinuous
PWM when the inductor current ramps down to
zero before the end of each switching cycle. This
reduces the boost converter's maximum output
current, causes large input voltage ripple and
reduces efficiency. Large inductance value
provides much more output current and higher
conversion efficiency. For these reasons, an
inductor within 4.7μH to 22μH value range is
recommended.
Schottky Diode Selection
The high switching frequency of the PAM2841
demands a high-speed rectification for optimum
efficiency. Ensure that the diode average and
peak current rating exceeds the average output
current and peak inductor current. In addition, the
diode's reverse breakdown voltage must exceed
the open protection voltage.
Input and Output Capacitor Selection
Input Capacitor
At least a 1μF input capacitor is recommended to
reduce the input ripple and switching noise for
normal operating conditions. Larger value and
lower ESR (Equivalent Series Resistance) may
be needed if the application require very low input
ripple. It follows that ceramic capacitors are a
good choice for applications. Note that the input
capacitor should be located as close as possible
Output Capacitor
The output capacitor is mainly selected to meet
the requirement for the output ripple and loop
stability. This ripple voltage is related to the
capacitor's capacitance and its equivalent series
resistance (ESR). A output capacitor of 1μF
minimum is recommended and maybe need a
larger capacitor. The total output voltage ripple
has two components: the capacitive ripple
caused by the charging and discharging on the
output capacitor, and the ohmic ripple due to the
capacitor's equivalent series resistance (ESR):
V =V +V _RIPPLE
RIPPLE(C)
RIPPLE(ESR)
V ≈ _RIPPLE(C)
I ) ²
OUT
1/2*(L/(C_OUT*((V_OUT(MA -_X) V ) _IN(MIN) )))*(I^2PEAK-
V =I *R _{RIPPLE(ESR) PEAK ESR(COUT)}
Where I_PEAK is the peak inductor current.
Multilayer ceramic capacitors are an excellent
choice as they have extremely low ESR and are
available in small footprints. Capacitance and
ESR variation with temperature should be
considered for best performance in applications
with wide operating temperature ranges.
Dimming Control
There are 4 different types of dimming control
methods:
1). Using an External PWM Signal to EN Pin
With the PWM signal applied to the EN pin, the
PAM2841 is alternately turned on or off by the
PWM signal. The LEDs operate at either zero or
full current. The average LED current changes
proportionally with the duty cycle of the PWM
signal. A 0% duty cycle turns off the PAM2841 and
leads to zero LED current. A 100% duty cycle
generates full current.Also the recommend
dimming frequency is between 100Hz and 200Hz.
I =I * _{AVE STATE} (T_ON –T_STARTUP)/(T + _ON T_OFF)
where T : _ON on time of a period
T : _STARTUP 0.85ms
T : _OFF off time of a period
I : _STATE on state current (full current)
Power Analog Microelectronics,Inc
www.poweranalog.com
9
08/2008 Rev 1.3