ISL97801ARZ-TK Intersil, ISL97801ARZ-TK Datasheet - Page 14

• Manufacturer Part Number: ISL97801ARZ-TK
• Description: IC LED DRIVER AUTOMOTIVE 20-QFN
• Manufacturer: Intersil
• Type: Automotiver

Specifications of ISL97801ARZ-TK

• Topology: PWM, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Frequency: 850kHz ~ 1.15MHz
• Voltage - Supply: 2.7 V ~ 16 V
• Voltage - Output: 32V
• Mounting Type: Surface Mount
• Package / Case: 20-VQFN Exposed Pad, 20-HVQFN, 20-SQFN, 20-DHVQFN
• Operating Temperature: -40°C ~ 105°C
• Current - Output / Channel: 1A
• Internal Switch(s): Yes
• Efficiency: 90%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ISL97801ARZ-TK

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 configuaration 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:
where:
and
V
D
I
LPK
RIPPLE
=
V
------------------------------- -
=
100mV
200mV
200mV
100mV
200mV
200mV
OUT
50mV
50mV
VFB
VFB
V
------------ -
1 D
OUT
I
–
=
O
–
--------------- -
C
V
+
OUT
I
IN
O
(
------------------------------------
V
OUT
×
2
------ -
F
D
×
S
–
L
+
V
100mA
350mA
100mA
350mA
50mA
50mA
I
IN
I
I
OUT
OUT
LPK
1A
1A
)
×
14
×
(
----------------- -
1 D
ESR
–
f
s
)
TABLE 2. BOOST MODE COMPENSATION. 2.7V OPERATION
Electrolytic
Electrolytic
Electrolytic
Electrolytic
Electrolytic
Electrolytic
Electrolytic
Electrolytic
TABLE 3. BOOST MODE COMPENSATION 6V OPERATION
VOUT (V)
VOUT (V)
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
LED’s
LED’s
141µF
141µF
94µF
40µF
94µF
60µF
94µF
60µF
94µF
40µF
60µF
60µF
94µF
40µF
ILIM
(EQ. 4)
(EQ. 5)
(EQ. 6)
7
2
7
2
ISL97801
47µF
20µF
60µF
47µF
40µF
47µF
20µF
47µF
60µF
47µF
60µF
47µF
40µF
10.5
ILIM
10.5
3
3
In buck mode:
where:
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 ISL97801 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.
V
D
RIPPLE
=
V
--------------- -
V
40µF
40µF
47µF
40µF
40µF
60µF
47µF
40µF
OUT
ILIM
ILIM
14
14
IN
4
4
=
(
---------------------------------------------- -
V
IN
2
–
×
20µF
40µF
47µF
40µF
20µF
40µF
60µF
V
17.5
ILIM
17.5
ILIM
f
OUT
5
5
s
×
L
)
×
D
×
20µF
40µF
20µF
40µF
40µF
ILIM
ILIM
ILIM
⎛
⎝
21
21
-------------------------- -
f
6
6
s
×
C
D
OUT
DMAX
+
20µF
40µF
40µF
24.5
ILIM
ILIM
24.5
ILIM
ESR
7
7
⎞
⎠
April 2, 2007
DMAX
20µF
40µF
40µF
ILIM
ILIM
ILIM
(EQ. 7)
(EQ. 8)
28
28
FN6428.1
8
8