PAM2842 Datasheet, PDF (10/17 Pages) Power Analog Micoelectronics – High Power LED Driver

PAM2842
High Power LED Driver
Application Information
Topology Selection
When maximum power supply voltage is below
than minimum load voltage, select the boost
topology. When minimum power supply voltage is
high than maximum load voltage, select buck
topology. When load voltage range is small and
between the power supply voltage, select sepic
topology.
Table-1: Voltage condition Vs Topology
Condition
Vin_max < Vo_min
Vin_min > Vo_max
Vo Ì Vin
Topology
Boost
Buck
Sepic
Inductor Selection
The inductance, peak current rating, series
resistance, and physical size should all be
considered when selecting an inductor. These
factors affect the converter's operating mode,
efficiency, maximum output load capability,
transient response time, output voltage ripple,
and cost.
The maximum output current, input voltage,
output voltage, and switching frequency
determine the inductor value. Large inductance
can minimizes the current ripple, and therefore
reduces the peak current, which decreases core
losses in the inductor and I2R losses in the entire
power path. However, large inductor values also
require more energy storage and more turns of
wire, which increases physical size and I2R
copper losses in the inductor. Low inductor values
decrease the physical size, but increase the
current ripple and peak current. Finding the best
inductor involves the compromises among circuit
efficiency, inductor size, and cost.
When choosing an inductor, the first step is to
determine the operating mode: continuous
conduction mode (CCM) or discontinuous
conduction mode (DCM). When CCM mode is
chosen, the ripple current and the peak current of
the inductor can be minimized. If a small-size
inductor is required, DCM mode can be chosen. In
DCM mode, the inductor value and size can be
minimized but the inductor ripple current and peak
current are higher than those in CCM.
For the large power application, if chose DCM, the
peak current will be very large, it will have great
electrical stress on the components, so we chose
CCM.
When work in CCM mode, a reasonable ripple
current is chosen to
ΔI_L=0.4I_L
For the boost topology,
I_L
=
I_O
1- D
D = ^{VO - VIN}
V_O
DI_L
=
V_IN(V_O - V_IN )
LFV_O
D: duty cycle, Io: output current, F: switching
frequency.
From above equation we can get the inductance:
L
=
2.5V_I2N(V_O -
FI_O V_O2
V_IN )
The inductor's current rating should be higher
than
I_L
+
DI_L
2
For the buck topology, I_L=I_O
D = ^VO
V_IN
DI_L
=
(V_IN - V_O )V_O
LFV_IN
so
L = ^{2.5VO(VIN - VO )}
FI_O V_IN
For the sepic topology, L1=L2
I_L1
=
I_O
D
1- D
I = _L2 I_O
D = ^VO
V_IN + V_O
Power Analog Microelectronics,Inc
www.poweranalog.com
10
09/2008 Rev 1.1