XRP7613IDBTR-F Datasheet, PDF (8/14 Pages) Exar Corporation – 1.2A 36V Step Down High Brightness LED Driver

XRP7613
1.2A 36V Step Down High Brightness LED Driver
APPLICATION INFORMATION
HYSTERETIC OPERATION
The XRP7613 is a hysteretic step-down LED
driver. It uses ±15% double-ended hysteresis
to regulate the average LED current to the
value programmed by RSET (refer to figure 1).
The ±15% hysteresis is achieved with
resistors R2 and R3 in the block diagram
shown in figure 2. Average internal current
through R1, R2 and R3 is given by
I_INT(AVG)=0.1V/R1. Note that voltage across
RSET must be the same as voltage across R1.
Therefore average LED current should be
I_LED(AVG)=0.1V/R_SET. During off time FETs N1
and N2 are off. Inductor current I_L ramps
down through the external Schottky diode and
voltage at ISEN decreases. This, in turn,
causes the I_INT to decrease. When I_INT falls
15% below I_INT(AVG), comparator is triggered on
(note that this should correspond to I_LED falling
15% below I_LED(AVG)). N1 and N2 turn on and
on time commences. N2 shorts R3 and
thereby requires a higher I_INT in order to
trigger the comparator off. N1 shorts the
inductor to ground, I_L ramps up and voltage at
ISEN increases. This causes the I_INT to
increase. When I_INT rises 15% above I_INT(AVG),
comparator is triggered off and the cycle
repeats.
TURN ON AND TURN OFF DELAY
As explained above when I_INT falls 15% below
I_INT(AVG), the comparator is triggered on.
However, it takes 280ns (nominal) before N1
turns on and LX transitions from high to low
voltage (refer to figure 20). The turn on delay
time results in inductor current ripple ΔI_L to
exceed the -15% hysteresis set by the internal
control. Because this delay imposes a lower
bound on the N1 off time, it has been called
“Minimum Switch OFF Time” in the electrical
specifications table.
When I_INT rises 15% above I_INT(AVG), the
comparator is triggered off. There is, however,
a delay of 180ns before N1 turns off and LX
transitions from low to high voltage. The turn
off delay time results in ΔI_L exceeding the
+15% hysteresis set by the internal control.
Because this delay imposes a lower bound on
the N1 on time, it has been called “Minimum
Switch ON Time” in the electrical specifications
table.
Thus the delay times will cause the switching
frequency to be lower than expected because
the turn on and turn off time will take longer
to complete. Graphs of typical switching
frequency versus V_IN for various operating
conditions are shown in figures 16-18.
The delay times, under some operating
conditions, may force the average current to
deviate from I_LED(AVG)=0.1V/R_SET if they cause
asymmetric hysteresis. As an example in
figure 20 the positive hysteresis is higher than
the negative hysteresis and there is a positive
offset. Average current is higher than
0.1V/R_SET. The effect of delay times on
average current has been taken into account
by measuring the voltage across R_SET for
various operating conditions. Graphs of V_SET
versus V_IN are shown in (figures 6-8).
+15%
IL(avg)
0.1V/RSET
-15%
LX
Turn on delay
= 280ns
Turn off delay
= 180ns
Fig. 20: Effect of Delay Times on Inductor and LED
Current Ripple and Average Current
SHUTDOWN CONTROL
A shutdown control function is provided
through the EN/DIM input pin. Connecting the
EN/DIM input pin to ground or to a DC voltage
lower than 200mV for longer than 20ms will
completely shut down the XRP7613. In this
state, the quiescent current is less than 35μA
and the internal reference, error amplifier,
comparators, and biasing circuitry completely
turned off.
© 2012 Exar Corporation
8/13
Rev. 1.2.0