Datasheet ISL97634 (Intersil) - 9

link to page 9 link to page 5 link to page 9 link to page 9 link to page 8 link to page 10 ISL97634 Efficiency Improvement follower action of M1, limiting the maximum voltage on the ISL97634 LX pin to below V Figure 2 shows the efficiency measurements during PWM IN, but allowing the output voltage to go much higher than the breakdown limit on the LX pin. The switch operation. The choice of the inductor has a significant impact on current limit and maximum duty cycle will not be changed by this the power efficiency. Equation 4 shows the higher the inductance, setup, so input voltage wil need to be carefully considered to make the lower the peak current, therefore, the lower the conduction sure that the required output voltage and current levels are and switching losses. On the other hand, it has also a higher series achievable. Because the source of M1 is effectively floating when resistance. Nevertheless, the efficiency improvement effect by the internal LX switch is off, the drain-to-source capacitance of M1 lowering the peak current is greater than the resistance increases may be sufficient to capacitively pull the node high enough to break with larger value of inductor. Efficiency can also be improved for down the gate oxide of M1. To prevent this, V systems that have high supply voltages. Since the ISL97634 can OUT should be connected to V only supply from 2.4V to 5.5V, V IN, allowing the internal Schottky diode to limit the IN must be separated from the peak voltage. This will also hold the VOUT pin at a known low high supply voltage for the boost circuit as shown in Figure 16 and voltage, preventing the built in OVP function from causing problems. the efficiency improvement is shown in Figure 17. This OVP function is effectively useless in this mode as the real output voltage is outside its intended range. If the user wants to
C3 0.22µF D1
implement their own OVP protection (to prevent damage to the output capacitor), they should insert a zener diode from VOUT to the
Vs = 12V L1 D2
FB pin. In this setup, it would be wise not to use the FBSW to FB
1 2
switch, as otherwise, the zener diode will have to be a high power
22µH C1 1µF D3
one capable of dissipating the entire LED load power. Then the LED stack can then be connected directly to the sense resistor via a
D4
10k resistor to FB. A zener can be placed from VOUT to the FB pin
VIN = 2.7V TO 5.5V VIN LX
allowing an overvoltage event to pull-up on FB with a low
C2 0.1µF VOUT D5
breakdown current (and thus low power zener diode) as a result
ISL97634
of the 10k resistor.
FBSW D6 FB PWM
/
EN L1 V D0 IN = 2.7V TO 5.5V 1 2 GND R1 4 C1 10µH or 22µH C3 4.7µF 10BQ100 1µF M1 FQT13N06L SK011C226KAR
FIGURE 16. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
VIN VOUT C2 LX
EFFICIENCY OPERATION
0.1µF ISL97634 FBSW
.
90 FB VS = 12V PWM/ENGND R1 6.3 VS = 9V 85 (%) 80 IENCY VIN = 4V EFFIC 7 LEDs 75 L1 = 22µH R1 = 4
FIGURE 18. HIGH VOLTAGE LED DRIVER USING A CASCODE
fPWM 70 0 5 10 15 20 25 30
SEPIC Operation
ILED (mA)
For applications where the output voltage is not always above the FIGURE 17. EFFICIENCY IMPROVEMENT WITH 9 AND 12V INPUTS input voltage, a buck or boost regulation is needed. A SEPIC (Single-Ended Primary Inductance Converter) topology, shown in Operation with VOUT above 26V Figure 19, can be considered for such an application. A single cell Li-ion battery operating a cellular phone backlight or For LED backlighting applications that need an output voltage above flashlight is one example. The battery voltage is between 2.5V 26V, the voltage range of the ISL97634 is not sufficient. However, and 4.2V, depending on the state of charge. On the other hand, the ISL97634 can be used as an LED controller with an external the output may require only one 3V to 4V medium power LED for protection MOSFET connected in cascode fashion to achieve higher illumination because the light guard of the backlight assembly is output voltage as shown in Figure 18. A 60V logic level N-Channel optimized for cost efficiency trade-off reason. MOSFET is configured such that its drain ties between the inductor and the anode of Schottky diode, its gate ties to the input, and its In fact, a SEPIC configured LED driver is flexible enough to allow source ties to the ISL97634 LX node connecting to the drain of the the output to be well above or below the input voltage, unlike the internal switch. When the internal switch turns on, it pulls the source previous example. Another example is when the number of LEDs of M1 down to ground and LX conducts as normal. When the and input requirements are different from platform to platform, a internal switch turns off, the source of M1 will be pulled up by the common circuit and PCB that fit all the platforms in some cases FN6264 Rev 4.00 Page 9 of 12 August 27, 2013