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LP8556 Datasheet(PDF) 14 Page - Texas Instruments

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Part # LP8556
Description  High-Efficiency LED Backlight Driver for Tablets
PDF  52 Pages
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

LP8556 Datasheet(HTML) 14 Page - Texas Instruments

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background image
Light
Load
SW
R
R
OCP
R
S
R
6
+
-
+
-
Osc/
ramp
gm
-
+
VREF
VBOOST
VFB
Active Load
Startup
OVP
Edge Rate
Control
Spread
Spectrum
R
LP8556
SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012
www.ti.com
Boost Converter Overview
OPERATION
The LP8556 boost DC/DC converter generates a 7V to approximately 43V boost output voltage from a 2.7V to
36V boost input voltage. The boost output voltage minimum, maximum value and range can be set digitally by
pre-configuring EPROM memory (VBOOST_RANGE, VBOOST and VBOOST_MAX fields).
The converter is a magnetic switching PWM mode DC/DC boost converter with a current limit. It uses CPM
(current programmed mode) control, where the inductor current is measured and controlled with the feedback.
During startup, the soft-start function reduces the peak inductor current. LP8556 has an internal 20 MHz
oscillator which is used for clocking the boost. The following figure shows the boost block diagram.
Figure 17. LP8556 Boost Converter Block Diagram
SETTING BOOST SWITCHING FREQUENCY
The LP8556 boost converter switching frequency can be set either by an external resistor (BOOST_FSET_EN =
1 selection), RFSET, or by pre-configuring EPROM memory with the choice of boost frequency (BOOST_FREQ
field). Table 1 summarizes setting of the switching frequency. Note that the RFSET is shared for setting the PWM
dimming frequency in addition to setting the boost switching frequency. Setting the boost switching frequency
and PWM dimming frequency using an external resistor is separately shown in Table 5.
Table 1. Configuring Boost Switching Frequency via EPROM
RFSET [Ω]
BOOST_FSET_EN
BOOST_FREQ[1:0]
fSW [kHz]
don't care
0
00
312
don't care
0
01
625
don't care
0
10
1250
don't care
0
11
undefined
(1)
1
don't care
(1)
(1)
See Table 5
14
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Copyright © 2012, Texas Instruments Incorporated
Product Folder Links: LP8556



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