LTC3577_15 Datasheet, PDF (37/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
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 then 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, PWM-
NUM[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 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 I_LED pin. The purpose of
this is to have some current flowing in the LEDs at all times.
This helps to reduce the magnitude of the voltage swing
on the I_LED pin as the current is pulsed on and off.
Fixed Boost Output
Setting MD1 to 1 and MD2 to 0 selects the fixed high voltage
boost mode. This mode can be used to generate output
voltages at or greater than V_OUT. When configured as a
boost converter the I_LED pin becomes the feedback pin,
and will regulate the output voltage such that the voltage
on the I_LED pin is 800mV.
Figure 17 shows a fixed 12V output generated using the
boost converter in the fixed high voltage boost mode. Any
output voltage up to 40V may be programmed by select-
ing appropriate values for the R1 and R2 voltage divider
from the equation:
V_BOOST
=
0.8V
•
⎛
⎝⎜
R1
R2
+
1^{⎞⎠⎟}
Values for R2 should be kept below 24.3k to keep the pole
at I_LED beyond cross over.
The boost is designed primarily as a high voltage, high duty
cycle converter. When operating with a lower boost ratio,
a larger output capacitor, 10μF, should be used. Operating
with a very low duty cycle will cause cycle skipping which
will increase ripple.
C1
22μF
V_OUT
39
3
I_{LED_FS}
LTC3577
18
SW
SW
19
20
SW
L4
10μH
LPS4018-103ML
D12
ZLLS400
R1
I_LED ^{22 800mV VREF}
301k
R2
LED_OV ⁹
21.5k
3577 F17
BOOST
C2
10μF
10V
Figure 17. Fixed 12V/75mA Boost Output Application
3577fa
37