XRP7613IDBTR-F Datasheet, PDF (9/14 Pages) Exar Corporation – 1.2A 36V Step Down High Brightness LED Driver

XRP7613
1.2A 36V Step Down High Brightness LED Driver
SETTING THE LED CURRENT
The output current I_LED of XRP7613 can be set
by the external sense resistor R_SET. The
relationship between I_LED and R_SET is
V_SET can be determined from graphs in figures
6-8. As an example for the operating
conditions I_LED=350mA, V_IN=24V, 3xLED;
V_SET=105mV from figure 6.
OPERATING FREQUENCY
The operating frequency of the XRP7613 can
be calculated from the following equation
where f_S is the operating frequency, T_ON is the
switch on time and T_OFF is the switch off time.
The switch on time can be approximated from
the following equation
The switch off time can be approximated from
the following equation
where V_IN is the input voltage
V_LED is the total LED forward voltage
I_LED is the LED average current
R_SET is current sense resistance
R_L is inductor resistance
R_DS(ON) is switch on resistance (0.5Ω typ.)
L is the inductor value
ΔI_L is the inductor peak to peak current
V_D is diode forward voltage at the LED average
current.
The recommended operating frequency should
not exceed 1MHz.
DIMMING CONTROL
The XRP7613 offers two ways of achieving LED
dimming: standard PWM dimming and analog
dimming. The EN/DIM input pin is used not
only to control the XRP7613 shutdown but also
the PWM and analog dimming functions.
If dimming and/or shutdown controls are not
required, the EN/DIM pin can be left floating
for automatic turn on upon application of
proper V_IN.
PWM Dimming
A logic-level PWM signal applied to the EN/DIM
pin can be used for PWM dimming control of
the LEDs. This external signal turns the
MOSFET gate drive on and off, thereby
modulating the average current delivered to
the LED proportional to the duty cycle of the
PWM signal. The EN/DIM signal will shutdown
the XRP7613 when EN/DIM = L and turn-on
the XRP7613 when DIM = H. The DIM signal
needs to be greater than 1.3V minimum to
turn-on and less than 200mV to fully turn-off
the device.
The maximum allowed PWM dimming
frequency that can be applied is 40 KHz.
Analog Dimming
The average current delivered to the LED, ie
the LED brightness, can also be controlled by
applying a variable DC voltage signal to the
EN/DIM pin.
A DC voltage greater than 1.25V will drive
output LED current to 100% of the LED
current as set by the external sense resistor
R_SET while a voltage lower than 200mV will
shutdown the XRP7613. When analog dimming
is required, the DC voltage range of EN/DIM
should be between 0.4V to 1.25V in order
modulating the average current delivered to
the LED accordingly.
© 2012 Exar Corporation
9/13
Rev. 1.2.0