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MC34844A
FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
OVER-CURRENT PROTECTION (OCP)
The boost converter also features internal Over-current
Protection (OCP) and has a user programmable Over-
voltage Protection (OVP).
The OCP operates on a cycle by cycle basis. However, if
the OCP condition remains for more than 10ms 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 I 2 C interface to 255 levels.
Zero current is achieved by turning off the LED Driver by
I 2 C (registers CHENx = 0h) for a duty cycle from 0% to 99%,
or by pulling PWM pin low regardless of the duty cycle.
I 2 C capability allows the channels to be controlled
individually or in parallel.
The duty cycle of the PWM waveform in both master and
slave modes is set using a second register on the I 2 C
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).
POWER OFF AND POWER ON LED CHANNELS
The 34844A allows the user to Power OFF and Power ON
any channel independently thru I2C/SM-BUS mode.
The POWER ON function reconnects the LED driver and
the feedback circuit to the channel to allow functionality to
that channel again.
On an opposite way when the channel is POWER OFF,
the LED driver and feedback circuit are disconnected to the
channels.
×
VSET [ V ] × 136 ICH [ RegisterValue ]
ISINK [ A ] = -------------------------------------------- -----------------------------------------------------------
Current on LED Channel (PIN and NIN mode disabled)
RISET [ Ω ] 255
Eqn. 6
This function is very useful for applications where one or
more channel has to be shutdown to avoid the output
voltages goes to OVP during the start up of the part.
Default ICH[RegisterValue]=255
In the off state, the LEDs current is set to 0 and the boost
converter stops switching.
This feature allows to drive more than 80 mA of current by
connecting the LED string to 2 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 in order 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 I 2 C interface (registers FPWM).
The default programmed value set the number of 27 kHz
clocks (40 μ 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.
The sequence to make these functions work is the
following:
To POWER ON LED channels:
1. Take PWM pin low
2. Set POWER ON bit high (MSB of Register 09)
3. Set high all Channels that should be power on by
writing “1” on CHENx bits (Registers 08 & 09)
4. Clear POWER ON bit
5. Take PWM pin high
To POWER OFF LED channels:
1. Take PWM pin low
2. Set POWER OFF bit high (MSB of Register 08)
3. Clear all Channels that should be power off by writing
“0” on CHENx bits (Registers 08 & 09)
4. Clear POWER OFF bit
5. Take PWM pin high
FPWM [ Hz ] = --------------------------------------------------------------------
PWM Frequency
20.736Mhz
FPWM [ RegisterValue ]
Eqn. 7
POWER ON bit and POWER OFF bit shouldn’t be set at
the same time in order to avoid damage to the part.
POWER ON/OFF channels should be reconfigured every
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 together.
time the part gets recovered from a POR or shutdown
condition. This also apply if the part is reenabled by software.
If the part is reenabled by software, it is recommended to
take PWM pin low, reenable the part and then follow the
corresponding sequence shown above.
34844A
Analog Integrated Circuit Device Data
46
Freescale Semiconductor
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