LTC3577-3 Datasheet, PDF (40/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
Table 8. Buck Control Register
BUCK CONTROL
REGISTER
ADDRESS: 00010010
SUB-ADDRESS: 00000000
BIT NAME
FUNCTION
B0 N/A
Not Used—No Effect on Operation
B1 N/A
Not Used—No Effect on Operation
B2 BK1BRST
Buck1 Burst Mode Enable
B3 BK2BRST
Buck2 Burst Mode Enable
B4 BK3BRST
Buck2 Burst Mode Enable
B5 SLEWCTL1
B6 SLEWCTL2
Buck SW Slew Rate: 00 = 1ns,
01 = 2ns, 10 = 4ns, 11 = 8ns
B7 N/A
Not Used—No Effect on Operation
Bits B2, B3, and B4 set the operating modes of the
step-down switching regulators (bucks). Writing a 1 to
any of these three registers will put that respective buck
converter in the high efficiency Burst Mode of operation,
while a 0 will enable the low noise “pulse-skipping” mode
of 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 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”.
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 = 1.85 Seconds
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
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 con-
stant 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 5 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 “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.
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