LTC3577EUFF#PBF Linear Technology, LTC3577EUFF#PBF Datasheet - Page 41

• Manufacturer Part Number: LTC3577EUFF#PBF
• Description: IC PWR MANAGEMENT HANDHELD 44QFN
• Manufacturer: Linear Technology

Specifications of LTC3577EUFF#PBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 4.35 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -

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
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.
LED CONTROL
REGISTER
BIT
B0
B1
B2
B3
B4
B5
B6
B7
NAME
EN
GR2
GR1
MD1
MD2
PWMC1
PWMC2
SLEWLED
2
C LED Control Register
ADDRESS: 00010010
SUB-ADDRESS: 00000001
FUNCTION
Enable: 1 = Enable 0 = Off
Gradation GR[2:1]: 00 = 15ms, 01 = 460ms,
10 = 930ms, 11 = 1.85s
Mode MD[2:1]: 00 = CC Boost,
10 = PWM Boost; 01 = HV Boost
PWM CLK PWMC[2:1]: 00 = 8.77kHz,
01 = 4.39kHz, 10 = 2.92kHz, 11 = 2.19kHz
LED SW Slew Rate: 0/1 = Fast/Slow
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 20μA to 20mA with approximately
1dB per step with I
current can be approximated by the following equation:
where DAC is the decimal value programmed into the I
“LED DAC register.” For example with I
DAC[5:0] = 101010 (42 decimal) I
Table 10. I
LED DAC REGISTER
BIT
B0
B1
B2
B3
B4
B5
B6
B7
I
LED
NAME
DAC[0]
DAC[1]
DAC[2]
DAC[3]
DAC[4]
DAC[5]
N/A
N/A
=I
2
LED(FS)
C LED DAC Register
LTC3577/LTC3577-1
• 10
LED(FS)
ADDRESS: 00010010
SUB-ADDRESS: 00000010
FUNCTION
6-Bit Log DAC Code
Not Used—No Effect On Operation
Not Used—No Effect On Operation
⎛
⎝ ⎜
3 •
programmed to 20mA. The LED
DAC – 63
63
⎞
⎠ ⎟
LED
LED(FS)
equates to 2mA.
= 20mA and
41
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2
C