LINEAR TECHNOLOGY LTC3587 Technical data
L DESIGN FEATURES
L1
15µH
L4
10µH
I
FB3
EN/SS3
EN/SS1
FLT
SW2 GND FB2
CAP1
FB1
V
OUT1
LT3587
V
FB3VOUT3
CAP3 SW3 V
IN
SW1
C6
1µF
C1
10µF
C
FB1
2.7pF
R
FB1
1M
CCD POSITIVE
15V
50mA
V
VIN
2.5V TO 6V
C2
2.2µF
L2
15µH
L3
15µH
R
FB2
1M
CCD NEGATIVE
–8V
100mA
LED DRIVER
20mA UP TO 24V
V
VIN
2.5V TO 6V
C7
22µF
D
S2
D
S3
C4
2.2µF
R
VFB3
1.65M
(OPTIONAL)
D
S1
R
IFB3
8.06k
C5
100nF
C3
100nF
C1: MURATA GRM21BR61C106KE15L
C2: MURATA GRM188R61C225KE15D
C3, C5: MURATA GRM033R60J104KE19D
C4: MURATA GRM21BR71E225KA73L
C6: MURATA GRM155R61A105KE15D
C7: TAIYO YUDEN LMK212BJ226MG-T
C
FB1
: MURATA GRM1555C1H2R7BZ01D
C
FB2
: MURATA GRM1555C1H6R8BZ01D
L1, L2, L3: SUMIDA CDRH2D18/HP-150N
L4: TOKO 1071AS-100M
DS1, DS2, DS3: IR IR05H40CSPTR
C
FB2
6.8pF
±32V Triple-Output Supply for LCDs,
CCDs and LEDs Includes Fault
Protection in a 3mm × 3mm QFN
by Eko T. Lisuwandi
Introduction
The task of designing a battery powered system with multiple high voltage
supplies is a daunting one. In such
systems board space is at a premium
and high efficiency is required to
extend battery life. Supplies must be
sequenced in start-up and shut-down,
and multiple supplies must be able to
maintain regulation without interaction across supplies.
The LT3587 is a 1-chip solution
that combines three switching regulators and three internal high voltage
switches to produce two high voltage
boost converters and a single high voltage inverter. The LT3587 is designed to
run from inputs ranging from 2.5V to
6V, making it ideal for battery powered
systems. Small package size and low
component count produces a small,
efficient solution. Typical applications
include digital still and video cameras,
high performance portable scanners
and display systems, PDAs, cellular
phones and handheld computers that
have high voltage peripherals such as
CCD sensors, LED backlights, LCD
displays or OLED displays.
Features
To keep the component count low, the
LT3587 integrates three high voltage
power switches capable of switching
0.5A, 1A and 1.1A at up to 32V in
a 3mm × 3mm QFN package. Each
of the positive channels includes an
output disconnect to prevent a direct
DC path from input to output when
the switches are disabled. The LT3587
also includes a bidirectional fault pin
(FLT), which can be used for fault
indication (output) or for emergency
shutdown (input).
The LT3587 offers a wide output
range, up to 32V for the positive channels (channels 1 and 3) and –32V for
the inverter (channel 2). Channel 3
is configurable as either a voltage or
current regulator. When configured as
a current regulator, channel 3 uses a
1-wire output that requires no current
sense or high current ground return
lines, easing board layout. A single
resistor programs each of the three
channels output voltage levels and/or
the channel 3 output current level.
Intelligent soft-start allows for
sequential soft-start of channel 1 followed by the inverter negative output
using a single capacitor. Internal
sequencing circuitry disables the
inverter until channel 1 output has
reached 87% of its final value.
20
Figure 1. Solution for a Li-Ion powered camera provides positive and negative
supplies for biasing a CCD imager and an LED driver for a 5-LED backlight
Triple-Output Supply for CCD
Imager and LED Backlight
Figure 1 shows a typical application
providing a positive and negative voltage bias for a CCD imager and a 20mA
current bias for an LED backlight. All
three channels of the LT3587 use a
constant frequency, current mode control scheme to provide voltage and/or
current regulation at the output.
The positive CCD bias is configured as a simple non-synchronous
boost converter. Its output voltage is
set to 15V via the feedback resistor
R
. The 15µH inductor (L1) is sized
FB1
for a maximum load of 50mA. The
negative CCD bias is configured as a
non-synchronous ´Cuk converter. Its
output voltage is set at –8V using the
feedback resistor R
Linear Technology Magazine • January 2009
. The two 15µH
FB2
DISCONNECT
CONTROL
TO INTERNAL
CIRCUIT
LT3587
M2
M3
CAP3
V
OUT3
V
OUT1
CAP1
C1
C4
OVERVOLTAGE
PROTECTION
DISCONNECT
CONTROL
SHDN3
SHDN1
M1
I
FB3
R
IFB3
I
VIN
500mA/DIV
V
VOUT1
10V/DIV
V
NEG
10V/DIV
V
EN/SS1
2V/DIV
400µs/DIV
0V
0V
0V
0mA
I
VIN
500mA/DIV
V
VOUT1
10V/DIV
V
NEG
10V/DIV
V
EN/SS1
2V/DIV
4ms/DIV
0mA
0V
0V
Figure 2. Start-up waveforms with no soft-start capacitor, and with a 10nF soft-start capacitor
(L2 and L3) inductors are sized for a
maximum load of 100mA.
The LED backlight driver is configured as an output-current-regulated
boost converter. Its output current is
set at 20mA using the current programming resistor R
. The 10µH
IFB3
inductor (L4) is sized for a typical load
of 20mA at up to 24V. Note the optional
voltage feedback resistor, R
VFB3
, on
the LED driver. This resistor acts as
a voltage clamp on the LED driver
output, so that if one of the LED fails
open, the voltage on the LED driver
output is clamped to 24V.
voltage on the EN/SS1 pin is at least
600mV. This ensures that channel 2
starts up after channel 1. Channel 1
and channel 2 regulation loops are
free running with full inductor current
when the voltage at the EN/SS1 pin
is above 2.5V.
In a similar fashion, a capacitor
from the EN/SS3 pin to ground (C5
in Figure 1) sets up a soft-start ramp
for channel 3. When the voltage at the
EN/SS3 pins goes above 200mV, regulation loop for channel 3 is enabled.
When the voltage at the EN/SS3 pin
is above 2V, the regulation loop for
channel 3 is free running with full
Soft-Start
inductor current.
All channels feature soft-start (a slow
voltage ramp from zero to regulation)
to prevent potentially damaging large
inrush currents at start-up. Softstart is implemented via two separate
soft-start control pins: EN/SS1 and
EN/SS3. The EN/SS1 pin controls
Start-Up Sequencing
The LT3587 also includes internal
sequencing circuitry that inhibits the
channel 2 from operating until the
feedback voltage of channel 1 (at the
FB1 pin) reaches about 1.1V (about
the soft-start for channel 1 and the
inverter, while the EN/SS3 pin controls
the soft-start for channel 3. Both of
these soft-start pins are pulled up with
a 1µA internal current source.
A capacitor from the EN/SS1 pin
to ground (C3 in Figure 1) programs
a soft-start ramp for channel 1 and
channel 2 (the inverter). As the 1µA
current source charges up the capacitor, the regulation loops for channel
1 and channel 2 are enabled when
the EN/SS1 pin voltage rises above
200mV. During start-up, the peak
switch current for channel 1 proportionally rises with the soft-start voltage
ramp at the EN/SS1 pin. The inverter
switch current also follows the voltage
ramp at the EN/SS1 pin, but its switch
current ramp does not start until the
Linear Technology Magazine • January 2009
Figure 3. Partial block diagram of the LT3587 showing the disconnect PMOS for channels 1 and 3
DESIGN FEATURES L
87% of the final voltage). The size of the
soft-start capacitor controls channel
2 start-up behavior.
If there is no soft-start capacitor,
or a very small capacitor, then the
negative channel starts up immediately with full inductor current when
the positive output reaches 87% of its
final value. If a large soft-start capacitor is used, then the EN/SS1 voltage
controls the inverter channel past
the point of regulation of the positive
channel. Figure 2 shows the start-up
sequencing without soft-start and with
a 10nF soft-start capacitor.
Output Disconnect
Both of the positive channels (channels
1 and 3) have an output disconnect
between their respective CAP and V
pins. This disconnect feature prevents
a DC path from forming between V
and V
through the inductors when
OUT
switching is disabled (Figure 1).
For channel 1, this output disconnect feature is implemented using a
PMOS (M1) as shown in the partial block
diagram in Figure 3. When turned on,
M1 normally provides a low resistance,
low power dissipation path for delivering output current between the CAP1
pin and the V
pin. M1 is on as long
OUT1
as the voltage difference between CAP1
and V
is greater than 2.5V. This al-
IN
lows the positive bias to stay high as
long as possible while the negative bias
discharges during turn off.
OUT
21
IN
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