Analog Integrated Circuit Device Data



Freescale Semiconductor

17

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

OVERCURRENT PROTECTION (OCP)

The boost converter also features internal overcurrent protection (OCP) and has a user programmable overvoltage protection (OVP).

The OCP operates on a cycle by cycle basis. However, if the OCP condition remains for more than 10 ms then the device turns off the

LED Drivers, the Boost goes to Sleep mode and the output FAULT pin goes into high-impedance. The device can only be restarted by

recycling the enable or creating a Power On Reset (POR).

CURRENT MIRROR

The programmable current mirror matches the current in 10 LED strings to within 2%. The maximum current is set using a resistor to

GND from the ISET pin. This can be scaled down using the I2C interface to 255 levels.

Zero current is achieved by turning off the LED Driver by I2C (registers CHENx = 0h) for a duty cycle from 0% to 99%, or by pulling

PWM pin low regardless of the duty cycle.

I2C capability allows the channels to be controlled individually or in parallel.

Current on LED Channel (PIN and NIN mode disabled)

Eqn. 1

Default ICH[RegisterValue]=255

In the off state, the LEDs current is set to 0 and the boost converter stops switching.

This feature allows driving more than 80 mA of current by connecting the LED string to two or more LED channels in parallel. For

example; if the application requires to drive a channels at 160 mA, then the bottom of each LED string should be connected to two

channels to duplicate the current capability (Example: CH0+CH1 = 160 mA).

PWM GENERATOR

The PWM generator can operate in either master or slave modes, as set by the M/~S pin.

In master mode, the internal PWM generator frequency is programmed through the I2C interface (registers FPWM). The default

programmed value set the number of 27 kHz clocks (40

s) in one PWM cycle. The 18-bit resolution allows minimum PWM frequencies

of 110 Hz to be programmed. The resulting frequency is output on the CK pin.

PWM Frequency

Eqn. 2

In slave mode, the CK pin acts as an input. The internal digital PLL uses this frequency as the PWM frequency. By setting one device

as master, and connecting the CK output to the input on a number of slave configured devices, all PWM frequencies are synchronized.

The duty cycle of the PWM waveform in both master and slave modes is set using a second register on the I2C interface (register

DPWM), and can be controlled from 100% duty cycle to 1/256 tPWM = 0.39%. Zero percent of duty cycle is achieved by turning LED drivers

off (register CHENx = 0h) or pulling PWM pin low.

An external PWM can also be used. The PWM input is 'AND'ed with the internal signal. By setting the serial interface to 100% duty

cycle (default), the external pin has full control of the PWM duty cycle. This pin can also be used to modulate the LED at a lower frequency

than the PWM dimming frequency (DHC Minimum pulse width = 400 ns).

V

OUT

Method 1

Method 2

A0/SEN

A0/SEN

OVP = VZENER2 + 6.5 V

OVP = 6.5 V [(RUPPER / RLOWER) + 1] + (100E-6 x RUPPER)

RUPPER

RLOWER

VZENER2

V

OUT

ISINK A



VSET V

 136



RISET





------------------------------------------ ICH RegisterValue



255

-----------------------------------------------------------



=

FPWM Hz



20.736Mhz

FPWM RegisterValue



--------------------------------------------------------------------

=