ISL78171 Datasheet, PDF (26/29 Pages) Intersil Corporation – 6-Channel, 50mA Automotive LED Driver with Ultra-high Dimming Ratio and Phase Shift Control

ISL78171
The peak current can be derived from the voltage across the
inductor during the Off period, as shown in Equation 23:
IL_peak = V_O  I_O   85%  V_I  + 1  2V_I  V_O – V_I   L  V_O  f_S 
(EQ. 23)
The value of 85% is an average term for the efficiency
approximation. The first term is average current that is inversely
proportional to the input voltage. The second term is inductor
current change that is inversely proportional to L and f_S. As a
result, for a given switching frequency and minimum input
voltage at which the system operates, the inductor I_SAT must be
chosen carefully.
Output Capacitors
The output capacitor smooths the output voltage and supplies
load current directly during the conduction phase of the power
switch. Output ripple voltage consists of discharge and charge of
the output capacitor during FET ON and OFF time and the voltage
drop due to flow through the ESR of the output capacitor. The
ripple voltage can be shown as Equation 24:
V_CO = I_O  C_O  D  f_S  + I_O  ESR
(EQ. 24)
The conservation of charge principle shown in Equation 22 also
indicates that, during the boost switch Off period, the output
capacitor is charged with the inductor ripple current, minus a
relatively small output current in boost topology. As a result, the
user must select an output capacitor with low ESR and adequate
input ripple current capability.
Note: Capacitors have a voltage coefficient that makes their
effective capacitance drop as the voltage across them increases.
C_OUT in Equation 24 assumes the effective value of the capacitor
at a particular voltage and not the manufacturer’s stated value,
measured at 0V.
Output Ripple
V_Co, can be reduced by increasing Co or f_SW, or using small ESR
capacitors. In general, ceramic capacitors are the best choice for
output capacitors in small to medium sized LCD backlight
applications due to their cost, form factor and low ESR.
A larger output capacitor will also ease the driver response
during PWM dimming Off period due to the longer sample and
hold effect of the output drooping. The driver does not need to
boost harder in the next On period that minimizes transient
current. The output capacitor is also needed for compensation,
and, in general 2x4.7µF/50V ceramic capacitors are suitable for
notebook display backlight applications.
Schottky Diode
A high-speed rectifier diode is necessary to prevent excessive
voltage overshoot. Schottky diodes are recommended because
of their fast recovery time, low forward voltage and reverse
leakage current, which minimize losses. The reverse voltage
rating of the selected Schottky diode must be higher than the
maximum output voltage. Also the average/peak current rating
of the Schottky diode must meet the output current and peak
inductor current requirements.
Applications
High Current Applications
Each channel of the ISL78171 can support up to 30mA (50mA at
V_IN = 12V). For applications that need higher current, multiple
channels can be grouped to achieve the desired current
(Figure 42). For example, the cathode of the last LED can be
connected to CH0 through CH2; this configuration can be treated
as a single string with 90mA current driving capability.
V_OUT
CH0
CH1
CH2
FIGURE 42. GANGING MULTIPLE CHANNELS FOR HIGH CURRENT
APPLICATIONS
Low Voltage Operations
The ISL78171 VIN pin can be separately biased from the LED
power input to allow low-voltage operation. For systems that have
only a single supply, V_OUT can be tied to the driver VIN pin to allow
initial start-up (Figure 43). The circuit works as follows: when the
input voltage is available and the device is not enabled, V_OUT
follows V_IN with a Schottky diode voltage drop. The V_OUT
boot-strapped to the VIN pin allows initial start-up, once the part
is enabled. Once the driver starts up with V_OUT regulating to the
target, the VIN pin voltage also increases. As long as V_OUT does
not exceed 26.5V and the extra power loss on V_IN is acceptable,
this configuration can be used for input voltage as low as 3.0V.
The Fault Protection FET feature cannot be used in this
configuration.
For systems that have dual supplies, the VIN pin can be biased
from 5V to 12V, while input voltage can be as low as 2.7V
(Figure 44). In this configuration, VBIAS must be greater than or
equal to VIN to use the fault FET.
Submit Document Feedback ₂₆
FN8602.0
June 15, 2015