ISL78100_15 Datasheet, PDF (14/19 Pages) Intersil Corporation – High Power LED Driver

ISL78100
Rectifier Diode
A high speed rectifier diode is necessary to prevent
excessive voltage overshoot, especially in the boost
configuration. Low forward voltage and reverse leakage
current will minimize losses, making Schottky diodes the
preferred choice. Similarly to the inductor, a diode with a
suitable current rating to handle current limit in the
configuration must be used.
Output Capacitor
The output capacitor acts to smooth the output voltage and
in the boost configuration supplies load current directly
during the conduction phase of the power switch. Ripple
voltage consists of two components, the first due to charging
and discharging of the capacitor; the second due to IR drop
across the ESR of the capacitor by inductor ripple current.
In boost mode:
V_RIPPLE = _C-----_O-^I--^O--_U----_T-  -_F-^D--_S-- + I_LPK  ESR
(EQ. 4)
where:
D = -^V----^O---_V-^U----_OT----_U-–---_T--^V----^I--^N--
(EQ. 5)
and
I_LPK = ₁----^I-–-^O----_D--- + ^---^V----^O-----^U₂----^T-_----–-_L---^V----^I--^N-----^  ^---¹-----_f^–-_s---^D-----^
(EQ. 6)
In buck mode:
V_RIPPLE
=
_---V_{----I--N---2--–-----}V_{---f-O-s----U----T-L--}_----_-----D_--



f^--_s^-----^----C^-D----_O^----_U^-----_T^-
+
E
S
R_
(EQ. 7)
where:
D = -^V---_V-^O---_I-^U-_N---^T--
(EQ. 8)
For a low ESR ceramic capacitor, output ripple is dominated
by the charging and discharging of the output capacitor.
Care should be taken to ensure the voltage rating of the
capacitor exceeds the maximum output voltage.
Compensation
The ISL78100 employs a direct summing control loop with
current feedback. No error amplifier is used in the system.
The arrangement provides fast transient response and
makes use of the output capacitor to compensate the loop.
The effect of the pole associated with the inductor is
minimized by the current feedback. The number of LEDs,
their DC bias current and the value of feedback resistor alter
loop stability due to their effect on feedback factor, which is
heavily influenced by the small signal impedance of the
LEDs. Generally, higher numbers of LEDs, lower bias levels
and smaller values of feedback resistor will require smaller
output capacitors to achieve loop stability. A combination of
low ESR electrolytic and ceramic capacitors may be used to
reduce implementation costs.
VFB
50mV
100mV
200mV
200mV
I_OUT
50mA
100mA
350mA
1A
TABLE 2. BOOST MODE COMPENSATION. 2.7V OPERATION
VOUT (V)
7
10.5
14
17.5
LED’s
2
3
4
5
Electrolytic
94µF
47µF
Ceramic
40µF
20µF
40µF
20µF
Electrolytic
94µF
Ceramic
60µF
60µF
40µF
40µF
Electrolytic
94µF
47µF
47µF
47µF
Ceramic
60µF
40µF
40µF
40µF
Electrolytic
ILIM
ILIM
ILIM
ILIM
Ceramic
21
6
20µF
40µF
ILIM
ILIM
24.5
7
DMAX
28
8
DMAX
ILIM
ILIM
ILIM
ILIM
VFB
50mV
100mV
200mV
200mV
I_OUT
50mA
100mA
350mA
1A
TABLE 3. BOOST MODE COMPENSATION 6V OPERATION
VOUT (V)
7
10.5
14
17.5
LED’s
2
3
4
5
Electrolytic
94µF
47µF
Ceramic
40µF
20µF
40µF
20µF
Electrolytic 141µF
47µF
Ceramic
60µF
60µF
60µF
40µF
Electrolytic 141µF
47µF
47µF
Ceramic
60µF
60µF
40µF
60µF
Electrolytic
94µF
47µF
ILIM
ILIM
Ceramic
40µF
40µF
21
6
20µF
40µF
40µF
ILIM
24.5
7
20µF
40µF
40µF
ILIM
28
8
20µF
40µF
40µF
ILIM
14
FN6626.1
December 24, 2013