LTC3577_15 Datasheet, PDF (41/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
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 efficiency 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 efficiency.
Table 9. I²C LED Control Register
LED CONTROL
REGISTER
ADDRESS: 00010010
SUB-ADDRESS: 00000001
BIT NAME
FUNCTION
B0 EN
Enable: 1 = Enable 0 = Off
B1 GR2
B2 GR1
Gradation GR[2:1]: 00 = 15ms, 01 = 460ms,
10 = 930ms, 11 = 1.85s
B3 MD1
B4 MD2
Mode MD[2:1]: 00 = CC Boost,
10 = PWM Boost; 01 = HV Boost
B5 PWMC1
B6 PWMC2
PWM CLK PWMC[2:1]: 00 = 8.77kHz,
01 = 4.39kHz, 10 = 2.92kHz, 11 = 2.19kHz
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 fixed 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 efficient 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 efficiency.
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 20μA 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 I²C
“LED DAC register.” For example with I_LED(FS) = 20mA and
DAC[5:0] = 101010 (42 decimal) I_LED equates to 2mA.
Table 10. I²C 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
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