ISL78100_15 Datasheet, PDF (11/19 Pages) Intersil Corporation – High Power LED Driver

ISL78100
Theory of Operation
General Description
The ISL78100 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 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 LEDs. 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 ISL78100 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 “Application Configurations” on page 11
of the datasheet for further information. Figure 28 shows a
block diagram of the system.
Application Configurations
Operating Modes
The ISL78100 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
ISL78100 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.
VIN
VOLTAGE
FEEDBACK
LEVEL
SHIFT
FB
GND
+
-
ISL78100
VDC/2
FIGURE 29. FB REFERENCE AUTO SWITCH
Start-up
To maximize external PWM switching speed, the ISL78100
does not include an internal soft-start circuit. When VDC
exceeds the power-on reset threshold, switching is delayed
for 1ms (t_DELAY) allowing the output capacitor to charge
through the inductor. If soft-start control is required, a
suitable application circuit is shown in Figure 30.
VBAT
VBAT FAULT
ISL78100 ^VIN
SWD1
SWD2
FB
SWS1 SWS2
10µH
L1
C1
4.7nF
VOUT
COUT
20µF
R1
R2 ¹⁰⁰
2k
0.5
RSENSE
FIGURE 30. EXTERNAL SOFT-START CIRCUIT
Light Level Control
Two light control schemes are provided:
1. An external PWM signal via the EN/PWM pin, providing
low frequency PWM dimming.
2. Bias current level adjustment via the LEVEL input or fixed
internal bias.
PWM Dimming
LED color temperature varies with bias current. In
backlighting applications, PWM dimming offers better control
of color temperature because current through the LEDs is
kept constant. A 5V gate driver (ENL) synchronized to
EN/PWM can be used to control an external N-Channel 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 further improve the linearity of PWM
dimming, an internal timer delays system shutdown via
EN/PWM for 50ms.
11
FN6626.1
December 24, 2013