Datasheet

LTC3577/LTC3577-1
41
3577fa
OPERATION
Bits B2, B3, and B4 set the operating modes of the step-
down switching regulators (bucks). Writing 1 to any of
these three registers will put that respective buck converter
in the high effi ciency Burst Mode operation, while a 0 will
enable the low noise pulse-skipping mode operation.
The B5 and B6 bits adjust the slew rate of all SW pins
together so they all slew at the same rate. It is recom-
mended that the fastest slew rate (B6:B5 = 00) be used
unless EMI is an issue in the application as slower slew
rates cause reduced effi ciency.
Table 9. I
2
C LED Control Register
LED CONTROL
REGISTER
ADDRESS: 00010010
SUB-ADDRESS: 00000001
BIT NAME FUNCTION
B0 EN Enable: 1 = Enable 0 = Off
B1 GR2
Gradation GR[2:1]: 00 = 15ms, 01 = 460ms,
10 = 930ms, 11 = 1.85s
B2 GR1
B3 MD1
Mode MD[2:1]: 00 = CC Boost,
10 = PWM Boost; 01 = HV Boost
B4 MD2
B5 PWMC1
PWM CLK PWMC[2:1]: 00 = 8.77kHz,
01 = 4.39kHz, 10 = 2.92kHz, 11 = 2.19kHz
B6 PWMC2
B7 SLEWLED LED SW Slew Rate: 0/1 = Fast/Slow
Table 9 shows the second byte of data that can be written
to at sub-address 0x01. This byte of data is referred to as
the “LED control register.”
Bit B0 enables and disables the LED boost circuitry. Writing
a 1 to B0 enables the LED boost circuitry, while writing a
0 disables the LED boost circuitry.
Bits B1 and B2 are the LED gradation which sets the ramp
up and down time of the LED current when enabled or
disabled. The gradation function allows the LEDs to turn
on/off gradually as opposed to an abrupt step.
Bits B3 and B4 set the operating mode of the LED boost
circuitry. The operating modes are: B4:B3 = 00 LED
constant current (CC) boost operation; B4:B3 = 10 LED
PWM boost operation; B4:B3 = 01 fi xed high voltage (HV)
output boost operation; B4:B3 = 11 not supported, do not
use. See the “LED Backlight/Boost Operation” section for
more information on the operating modes.
Bits B5 and B6 set the PWM clock speed as shown in
Table 9 of the “LED Backlight/Boost Operation” section.
Bit B7 sets the slew rate of the LED boost SW pin. Setting
B7 to 0 results in the fastest slew rate and provides the
most effi cient mode of operation. Setting B7 to 1 should
only be used in cases where EMI due to SW slewing is an
issue as the slower slew rate causes a loss in effi ciency.
See the “LED Backlight/Boost Operation” section for more
detailed operating information.
Table 10 shows the third byte of data that can be written
to at sub-address 0x02. This byte of data is referred to as
the “LED DAC register.” The LED current source utilizes a
6-bit 60dB exponential DAC. This DAC provides accurate
current control from 20A to 20mA with approximately
1dB per step with I
LED(FS)
programmed to 20mA. The LED
current can be approximated by the following equation:
I
LED
=I
LED(FS)
•10
3 •
DAC – 63
63
⎛
⎝
⎜
⎞
⎠
⎟
where DAC is the decimal value programmed into the I
2
C
“LED DAC register.” For example with I
LED(FS)
= 20mA and
DAC[5:0] = 101010 (42 decimal) I
LED
equates to 2mA.
Table 10. I
2
C LED DAC Register
LED DAC REGISTER
ADDRESS: 00010010
SUB-ADDRESS: 00000010
BIT NAME FUNCTION
B0 DAC[0] 6-Bit Log DAC Code
B1 DAC[1]
B2 DAC[2]
B3 DAC[3]
B4 DAC[4]
B5 DAC[5]
B6 N/A Not Used—No Effect On Operation
B7 N/A Not Used—No Effect On Operation