LTC3577-4_15 Datasheet, PDF (35/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
LED Gradation
The LED driver features an automatic gradation circuit.
The gradation circuit ramps the LED current up when
the LED driver is enabled and ramps the current down
when the LED driver is disabled. The DAC is enabled and
disabled with the EN bit of the I²C “LED control register”.
The gradation function is automatic when enabling and
disabling the LED driver; only the gradation speed needs to
be programmed to use this function. The gradation speed
is set by the GR1 and GR2 bits of the I²C “LED control
register” which allows transitions times of approximately
15ms, one-half second, one second and two seconds.
See the “I²C Interface” section for more information. The
gradation function allows the LEDs to turn on and off
gradually as opposed to an abrupt step.
LED PWM vs Constant Current Operation
The LED driver provides both linear LED current mode as
well as PWM LED current mode. These modes are selected
through the MD1 and MD2 bits of the I²C “LED control
register”. When both bits are “0” the LED boost converter
is in constant current (CC) mode and the I_LED current sink
is constant whose value is set by the DAC[5:0] bits of the
I²C“LED DAC register”.
Setting MD1 to “0” and MD2 to “1” selects the LED PWM
mode. In this mode the LED driver is pulsed using an
internally generated PWM signal. The PWM mode may be
used to reduce the LED intensity for a given programmed
current.
When dimming via PWM the LED driver and boost converter
are both turned on and off together. This allows some
degree of additional control over the LED current, and in
some cases may offer a more efficient method of dimming
since the boost could be operated at an optimal efficiency
point and pulsed for the desired LED intensity.
The PWM mode, if enabled, is set up using 3 values,
PWMNUM [3:0] and PWMDEN [3:0] in the I²C “LED PWM
Register” and PWMCLK, set by PWMC2 and PWMC1 in
the I²C “LED Control Register.”
Duty Cycle = ^PWMNUM
PWMDEN
Frequency = ^PWMCLK
PWMDEN
Table 6. PWM Clock Frequency
PWMC2
PWMC1
0
0
0
1
1
0
1
1
PWMCLK
8.77kHz
4.39kHz
2.92kHz
2.19kHz
Using the PWM control, a 4-bit internally generated PWM is
possible as additional dimming. Using these control bits a
number of PWM duty cycles and frequencies are available
in the 100Hz to 500Hz range. This range was selected to
be below the audio range and above the frequency where
the PWM is visible.
For example, given PWMC2 = 1, PWMC1 = 0,
PWMNUM[3:0] = 0111 and PWMDEN[3:0] = 1100 then
the duty cycle will be 58.3% and PWM frequency will be
243Hz.
If PWMNUM is set to 0 then the duty cycle will be 0% and
the current sink will effectively be off. If PWMNUM is ever
programmed to a value larger than PWMDEN the duty
cycle will be 100% and the current sink will effectively be
constant. PWMDEN and PWMNUM may both be changed
to result in 73 different duty cycle possibilities and 41 dif-
ferent PWM frequencies between 8.77kHz and 100Hz.
When PWM mode is enabled a small (2μA) standby current
source is always enabled on the LED pin. The purpose of
this is to have some current flowing in the LED’s at all times.
This helps to reduce the magnitude of the voltage swing
on the LED pin as the current is pulsed on and off.
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