PAM2842 Datasheet, PDF (13/17 Pages) Power Analog Micoelectronics – High Power LED Driver

PAM2842
High Power LED Driver
Setting the Output Limit Voltage
The OV pin is connected to the center tap of a
resistive voltage divider from the high-voltage
output to ground (see application schematic).
V_OUT-Limit
=
V_OV (1+
R_{UP )}
R_DOWN
The recommend procedure is to choose R3
=360K and R4 =12K to set Vout_limit =37.2V.
In boost and sepic circuit, when LED open or no
load, the circuit will have no feedback, if no other
measure be taken the switch voltage will be very
high and damage the switch, so this OV pin must
be set carefully.
In buck circuit, the switch voltage is always small
than input voltage, so the OV pin setting is not
important in this condition.
This OV pin is used to limit output voltage to avoid
breakdown of the switch other than to regulate
output voltage. The setting value must keep the
switch voltage below 40V.
In sepic circuit, one must notice that the switch
voltage equals Vin+Vo.
This OV pin has a hysteresis voltage detect
function, not latch-up function, so output voltage
will have a overshoot when no load or load
working voltage is high than setting limit voltage.
If the component parameter not match
appropriately, the overshoot voltage will be too
high and can demage the switch.
Several methods can decrease the overshoot
voltage:
(1) Add a small capacitor (<100pF) parallel
with the up divider resistor (See Figure 4).
(2) Use external zener to clamp the output
peak voltage (See Figure 5).
Vout
R3
Cf
OV
R4
Figure 4: add forward capacitor
Vout
COMP
ZENER
Q
R
Figure 5: Use External Zener
Note: The output limit voltage must be set higher
than working output voltage by a proper value, or
it will work abnormal in low temperature or some
other conditions.
Short LED Function
PAM2842 is a constant current driver. When one
or more LED shorted, the circuit will still work, the
output voltage is decided by LED numbers. In
boost topology, make sure the output voltage is
higher than input voltage; otherwise the unlimited
current will directly go through supply to LED and
damage the LED.
Power Dissipation
As PAM2842 integrates a power MOSFET, the
power dissipation must be considered. To a
MOSFET the power loss includes 5 sections, turn
on loss, turn off loss, conduction loss, drive loss
and output capacitor Coss loss.
P_turn-on
=
1
2
I_turn-on
V_OUT
T_r
f
P_turn-off
=
1
2
I_turn-off
V_OUT T_r
f
P_RDson
=
I R ²
RMS DSon
P_Drive = Q_gU_Drivef
P_Coss
=
1
2
C_OSS
V2
OUT
f
P_Switch = P_turn-on + P_turn-off + P_RDSon + P_drive + P_Coss
DT = q P _{ja switch}
Tr: switch rise time. Tf: switch fall time. U : _Drive gate
drive voltage. θja is relative with IC package,
heat-sink area and air flow condition etc.
Power Analog Microelectronics,Inc
www.poweranalog.com
13
09/2008 Rev 1.1