ISL97801ARZ-TK Intersil, ISL97801ARZ-TK Datasheet - Page 11

• 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

Theory of Operation
General Description
The ISL97801 is a flexible, highly integrated high-power LED
driver consisting of a PWM switching controller and
integrated 36V NDMOS power FET. The device can drive up
to 8 series high-power LED's at currents up to 1A at 16V
input or 5 LEDs at current up to 350mA at 2.7V input. The
control loop can be configured as either as a boost or buck
regulator with the configuration of the buck/boostn pin,
providing an output voltage above or below the input supply
voltage, depending on the number of stacked LED's. The
controller operates from 2.7V to 16V depending on the
numbers of LEDs and current required and can be powered
by a single lithium ion battery, 5V or 12V regulated supplies
or automotive electrical systems. LED current is sensed
through a low value resistor in series with the LED. A
thermistor can be used to implement a thermal protection
scheme to limit the maximum LED temperature to a preset
desirable level.
Switching Regulator
The ISL97801 employs a current mode PWM control
scheme with a nominal switching frequency of 1MHz. This
provides fast transient response and enables the use of low
profile inductors and compact multilayer ceramic capacitors.
Settling time is optimized by the use of a simple control loop
without an error amplifier, relying instead on intrinsic gain
within the direct summing path. Due to the lower loop gain,
offset must be accounted for when setting up initial LED bias
current. Refer to the applications section of the datasheet for
further information. Figure 28 shows a block diagram of the
system.
Application Configurations
Operating Modes
The ISL97801 can operate as either a buck or boost
regulator. Hardwire BUCK/BOOSTN to GND for boost mode
or to VDC for buck mode. In buck mode the power NDMOS
drive circuit is "floated" (boot-strapped) allowing the NDMOS
gate to be driven above VIN to fully enhance the power
NDMOS. An internal Schottky diode between VDC (5V) and
VHI reduces external component count. Use a ceramic
capacitor of at least 50nF between VHI and SWS1/2 to
bootstrap VHI.
LED Load Connection
ISL97801 includes an auto-sensing FB level shift circuit that
enables the LED load to be connected to either GND or VIN.
An internal sense circuit monitors the FB pin voltage. When
the level exceeds VDC/2, the feedback reference voltage is
switched from GND to VIN. Refer to the application section
of the datasheet for typical application schematics.
11
ISL97801
Start-up
To maximize external PWM switching speed, the ISL97801
does not include an internal soft-start circuit. When VDC
exceeds the power on reset threshold, switching is delayed
for 1ms (T
through the inductor. If soft-start control is required, a
suitable application circuit is shown in Figure 30.
Light Level Control
Two light control schemes are provided:
PWM Dimming
LED color temperature varies with bias current. In
backlighting applications PWM dimming offers better control
of color temperature because current through the LED's is
kept constant. A 5V gate driver (ENL) synchronized to
EN/PWM can be used to control an external N-Ch FET and
disconnect the LED stack during the PWM off period. The
switch prevents discharge of the output capacitor by the LED
load, maintaining a constant bias independent of PWM duty
cycle. Operation at 1kHz PWM rate is shown in Figure 31
and Figure 32. The load disconnect switch improves PWM
dynamic range, linearity and color temperature control. To
1. An external PWM signal via the EN/PWM pin, providing
2. Bias current level adjustment via the LEVEL input or fixed
VBAT
Voltage
Feedback
low frequency PWM dimming.
internal bias.
FIGURE 30. EXTERNAL SOFT-START CIRCUIT
EL7801
FIGURE 29. FB REFERENCE AUTO SWITCH
DELAY
ISL97801
SWS1 SWS2
LEVEL
SHIFT
VBAT FAULT
) allowing the output capacitor to charge
SWD1
SWD2
VIN
FB
VIN
10µH
L1
R2
+
-
2k
VDC /2
4.7nF
C1
100
R1
FB
G ND
0.5
RSENSE
VOUT
April 2, 2007
FN6428.1
COUT
20µF