LTC3577-3 Datasheet, PDF (34/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
very thin (<1mm tall) ceramic capacitors ideal for use in
height-restricted designs. Table 4 shows a list of several
ceramic capacitor manufacturers.
Table 4. Ceramic Capacitor Manufacturers
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
LED BACKLIGHT/BOOST OPERATION
Introduction
The LED driver uses a constant frequency, current mode
boost converter to supply power for up to 10 series LEDs.
As shown in Figure 11 the series string of LEDs is con-
nected from the output of the boost converter (BOOST) to
the I_LED pin. Under normal operation the boost converter
BOOST output will be driven to a voltage where the I_LED
pin regulates at approximately 300mV to 400mV. The I_LED
pin is a constant current sink that is programmed via I²C
“LED DAC register”. The LED can be further controlled
using I²C to program brightness levels and soft turn-on/
turn-off effects. See the “I²C Interface” section for more
information on programming the I_LED current. The boost
converter also includes an overvoltage protection feature
to limit the BOOST output voltage as well as variable slew
rate control of the SW pin to reduce EMI.
C1
22μF
V_OUT ³⁹
LTC3577-3/
LTC3577-4
18
SW
SW
19
20
SW
L1
10μH
LPS4018-103ML
D12
ZLLS400
BOOST
R1
LED_OV ^{9 10M}
R2
3 ^20k
I_{LED_FS}
C2
1μF
C3
22nF
D1
D2
D3
D4
D5
50V
50V
22
I_LED
D10 D9 D8 D7 D6
357734 F11
Figure 11. LED Boost Application Circuit
34
LED Boost Operation
The LED boost converter is designed for very high duty
cycle operation and can boost from 3V to 40V out for load
currents up to 20mA. The boost converter also features
overvoltage protection to protect the output in case of an
open circuit in the LED string. The overvoltage protection
threshold is set by adjusting R1 in Figure 11 such that:
BOOST(MAX) = 800mV • ^R1 + LED_ OV
10 • R2
where LED_OV is approximately 1.0V.
In the case of Figure 11 BOOST(MAX) is set to 40V for a
10-LED string.
Capacitor C3 provides soft-start, limiting the inrush cur-
rent when the boost converter is first enabled. C3 provides
feedback to the I_LED pin. This feedback limits the rise time
of output voltage and the inrush current while the output
capacitor, C2, is charging.
The boost converter will be operated in either continuous
conduction mode, discontinuous conduction mode or
pulse-skipping mode depending on the inductor current
required for regulation.
LED Constant Current Sink
The LED driver uses a precision current sink to regulate the
LED current up to 20mA. The current sink is programmed
via I²C “LED DAC Register” and utilizes a 6-bit 60dB expo-
nential DAC. This DAC provides accurate current control
from 20μA to 20mA with approximately 1dB per step for
I_LED(FS) = 20mA. The LED current can be approximated
by the following equations:
I_LED
=
I_LED(FS)
•
⎛
10^⎝⎜
3
•
DAC –
63
63⎞
⎠⎟
(1)
I_LED(FS)
=
0.8V
R2
•
500
where DAC is the decimal value programmed into the I²C
“LED DAC register”. For example with I_LED(FS) = 20mA
and DAC[5:0] = 000000 (0 decimal) I_LED equates to 20μA,
while DAC[5:0] = 111111 (63 decimal) I_LED equates to
20mA. As a final example DAC[5:0] = 101010 is 42 decimal
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