Datasheet ISL97634 (Intersil) - 8

link to page 8 link to page 8 link to page 8 link to page 8 link to page 8 link to page 8 link to page 8 ISL97634 TABLE 1. Applications PART NO. OVP MAX ILED Analog Dimming ISL97634IRT14Z 14V 70mA Analog dimming is usually not recommended because of the ISL97634IRT18Z 18V 50mA brightness non-linearity at low levels dimming. However, some systems are EMI or noise sensitive that analog dimming may be ISL97634IRT26Z 26V 30mA more suitable than PWM dimming under those situations. The Shutdown ISL97632 is part of the same family as the ISL97634 and has been designed with a serial interface to give access to 32 When PWM/EN is taken low the ISL97634 enters into the separate dimming levels. Alternatively analog dimming can be power-down mode where the supply current is reduced to less achieved by applying a variable DC voltage (VDim) at the FB pin than 1µA. The device resumes normal when the PWM/EN goes (see Figure 15) to adjust the LED current. As the DC dimming high. signal voltage increases above VFB, the voltage drop on R1 and R Components Selection 2 increases and the voltage drop on RSET decreases. Thus, the LED current decreases as shown in Equation 6: The input capacitance is typically 0.22µF. The output capacitor V   +  – V  should be in the range of 0.22µF to 1µF. X5R or X7R type of I FB R1 R2 Dim R1 = ------------------------------------- LED R  (EQ. 6) ceramic capacitors of the appropriate voltage rating are 2 RSET recommended. If VDIM is taken below FB, the inverse will happen and the When choosing an inductor, make sure the average and peak brightness will increase. current ratings are adequate by using Equations 3, 4 and 5 (80% The DC dimming signal voltage can be a variable DC voltage from efficiency assumed): a POT, a DCP (Digitally Controlled Potentiometer), or a DC voltage generated by filtering a high frequency PWM control signal. I  I LED VOUT = -----------------
L1
LAVG 0.8  V (EQ. 3) IN
22µH
1 I = + -  
VIN
LPK ILAVG I
LEDs
2 L (EQ. 4)
VIN LX 3.3V VOUT
V   – 
C1 C2
I IN VOUT VIN = -------------------------
ISL97634
L L  V  (EQ. 5)
1µF 0.22µF
OUT fOSC
R1
Where:
PWM FB 3.3k R GND SET R2
• I
4.75
L is the peak-to-peak inductor current ripple in Amps • L is the inductance in H
VDim
• fOSC is the switching frequency, typically 1.45MHz FIGURE 15. ANALOG DIMMING CONTROL APPLICATION CIRCUIT The ISL97634 supports a wide range of inductance values (10µH As brightness is directly proportional to LED currents, V to ~82µH). For lower inductor values or lighter loads, the boost Dim may be calculated for any desired “relative brightness” (F) using inductor current may become discontinuous. For high boost Equation 7: inductor values, the boost inductor current will be in continuous mode. R  R  V 2 = ---- 1    + ---- (EQ. 7) Dim V – F R FB 1 R   In addition to the inductor value and switching frequency, the 1 2 input voltage, number of LEDs and the LED current also affects Where F = ILED (dimmed)/ILED (undimmed). whether the converter operates in continuous conduction or These equations are valid for values of R1 and R2 such that both discontinuous conduction mode. Both operating modes are R1>>RSET and R2>>RSET. allowed and normal. The discontinuous conduction mode yields lower efficiency due to higher peak current. The analog dimming circuit can be tailored to a desired relative brightness for different VDim ranges using Equation 8. Compensation V –  R  R Dim_max VFB 1 = ---------------------------------- (EQ. 8) The product of the output capacitor and the load create a pole 2 V   FB 1 – Fmin while the inductor creates a right half plane zero. Both of these attributes degrade the phase margin but the ISL97634 has Where VDim_max is the maximum VDim voltage and Fmin is the internal compensation network that ensures the device operates minimum relative brightness (i.e., the brightness with VDim_max reliably under the specified conditions. The internal applied). compensation and the highly integrated functions of the i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k ISL97634 make it a design friendly device to be used in high i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k volume, high reliability applications. FN6264 Rev 4.00 Page 8 of 12 August 27, 2013