PAM2842 Datasheet, PDF (12/17 Pages) Power Analog Micoelectronics – High Power LED Driver

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PAM2842
Application Information (cont.)
Setting the Output Limit Voltage (cont.)
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).
Figure 4. Add Forward Capacitor
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.
PTURN−ON
=
1
2
ITURN−ON
VOUT
Tr
f
PTURN−OFF
=
1
2 ^{ITURN−ON VOUT Tr}
f
PRDS(ON) ⁼ I_{R2 MS} RDS(ON)
PDRIVE ⁼ Q_G UDRIVE ^f
PCOSS
=
1
2
C_OSS
V_{O2 UT}
f
PSWITCH ⁼ PTURN−ON ⁺ PTURNE ^{ff +} PRDS(ON) ⁺ PDRIVE ⁺ PCOSS
ΔT = θ_{JA PSWITCH}
Tr: switch rise time. Tf: switch fall time. U_DRIVE: gate drive voltage. θ_JA is relative with IC package, heat-sink area and air flow condition etc.
Above description does not consider the IC control power, so the total power will be more than calculated value.
PAM2842 has over-temperature protection. When junction temperature is over +150°C, it will shut down and auto restart when junction
temperature decrease below +120.
In high temperature circumstance application, one must pay attention to heat dissipation, or it will shut down and restart. It is recommended to
use external heat-sink and placed near to the IC surface.
PAM2842
Document number: DSxxxxx Rev. 1 - 2
12 of 17
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October 2012
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