DESIGN FEATURES
Micropower SOT-23 Boost
with Integrated Schottky Diode
Provides Output Disconnect and
by Leonard Shtargot
Short Circuit Protection
Introduction
The LT3464 is the only micropower
boost converter in the industry to
combine a 36V NPN power switch,
power Schottky diode, and output
disconnect into an 8-lead ThinSOT.
This unprecedented level of integration saves several external components
while offering true output disconnect,
making it possible to generate outputs
of up to 34V with a zero current shutdown while using a mere 40mm2 of
board area (refer to Fig. 1).
In addition to component savings, the LT3464 offers a low typical
switch current limit of 115mA and fast
switching, a combination that allows
the use of a tiny chip inductor and tiny
ceramic capacitors. The LT3464 also
features Burst Mode control (see Figure 2), which results in highly efficient
operation over a wide range of load
currents and a low quiescent current
of only 25µA typical. The CTRL pin of
the LT3464 acts much like a dial on a
lab power supply—it allows the output
voltage to be varied, which is useful
in applications for purposes such as
LCD contrast adjustment.
L1
10µH
VIN
2.3V
TO
10V
1
C1
1µF
7
6
VIN
SW
OUT
CTRL
LT3464 CAP
8
SHDN
FB
R2
4.53M
R1
301k
Figure 1. The integrated Schottky diode and output disconnect transistor result in a tiny solution
occupying as little as 40mm2.
The LT3464’s small size and high efficiency make it an especially attractive
power solution for portable electronics
requiring long battery life and compact
circuitry. See Figure 3 for a simplified
block diagram of the LT3464.
Output Disconnect
In a simple boost circuit (Figure 4)
there exists a DC path from the input
CTRL
1
supply (VIN) through the inductor and
diode to the load (VOUT), effectively leaving the load connected to VIN during
shutdown. The resulting current drain
during shutdown is unacceptable in
many applications, requiring the addition of several external components
to isolate the load from VIN. To save
space and complexity, the LT3464 is
equipped with a PNP that completely
VIN
SW
CAP
OUT
7
6
5
3
2
VOUT
50mV/DIV
+
–
–
DELAY
S
Q
R
Q
OUT
ANTI
SAT
+
1.25V
OUT
12mV
–
20
C2
0.1µF
VOUT
20V
C1: TAIYO YUDEN LMK107 BJ105MA-T
C2: TAIYO YUDEN TMK107 BJ104MA-T
C3: TAIYO YUDEN TMK107 BJ104MA-T
L1: TAIYO YUDEN LB 2012T100MR
IL
0.1A/DIV
Figure 2. Burst Mode waveforms showing low
output ripple. The LT3464 consumes only
25µA typical when not switching.
2
4
FB
5µs/DIV
5
C3
0.1µF
GND
CPL
VIN = 5V
VOUT = 20V
ILOAD = 1mA
L = 22µH
3
SHDN 8
0.1Ω
VREF
4
GND
Figure 3. LT3464 block diagram showing integrated NPN switch, Schottky diode and output
disconnect PNP.
Linear Technology Magazine • September 2003
DESIGN FEATURES
LT3464
SIMPLE BOOST
VOUT
VIN
DISCONNECT
PNP
…
SHDN
VOUT
DRV
SW
5V/DIV
VIN = 5V
CAP PIN VOLTAGE
GND
VOUT
…
Figure 4. A simple boost circuit with LT3464’s output disconnect allows the complete solution to
draw less than 0.5µA during shutdown. The output disconnect is designed with a 25mA current
limit to protect the circuit during short circuit conditions.
disconnects the load from the Schottky
diode during shutdown (see Figures 4
and 5). During normal operation, the
control circuitry turns on the PNP and
keeps it just out of saturation, resulting in low VCE(SAT) and low quiescent
current. In addition, the disconnect
circuit has a built in current limit
25
VIN = 4V
OUTPUT VOLTAGE (V)
20
15
10
5
0
0
0.25
0.5 0.75
1
1.25 1.5
CONTROL PIN VOLTAGE (V)
1.75
Figure 6: Using the CTRL Pin as an auxiliary
reference input to control the output voltage
of 25mA to protect the chip during
a short-circuit at the output. This
feature allows the LT3464 to tolerate
an indefinite short, but care must be
taken to avoid exceeding the maximum
junction temperature.
Using the CTRL Pin
The LT3464 features an auxiliary reference input that provides an easy way
to vary the output voltage for purposes
such as LCD contrast adjustment or
display dimming. When the CTRL pin
held at or above 1.25V, the LT3464
uses the internal 1.25V reference, but
when a voltage lower than 1.25V is
applied to the CTRL pin, that voltage
becomes the new reference. Figure 6
shows the output voltage versus the
CTRL pin voltage for a 20V output
circuit. Note that the LT3464 will not
regulate the output to a voltage lower
than the input.
VIN = 5V
VOUT = 20V
ILOAD = 1mA
L = 22µH
1ms/DIV
Figure 5: The output disconnect isolates the
output load from the input supply during
shutdown. CAP Pin voltage is the output of the
Schottky in Figure 4.
LT3464 ±20V
Dual Output Converter
Figure 7 shows a single-inductor dualoutput converter for applications that
require both a positive and negative
voltage. The positive output is generated by a simple boost set up, whereas
the negative output is generated using
an inverting charge pump. Although
well regulated, the negative output will
have a slight offset from the positive
output because the external diodes
have a different on voltage when
compared to the integrated Schottky
diode.
1-Cell Li-Ion to 16V
Boost Converter
Figures 8 and 9 show that the LT3464
performs well in applications that need
a high output voltage at a relatively
continued on page 24
L1
47µH
VIN
2.3V TO 10V
VIN
1
SW
OUT
CTRL
LT3464 CAP
8
D2
L1
22µH
VIN
2.3V TO 10V
6
7
C1
1µF
C5
D1
0.33µF
–VOUT
–20V
C4
0.33µF
SHDN
FB
3
5
2
C3
0.22µF
C2
0.33µF
GND
4
C1: TAIYO YUDEN LMK107 BJ105MA-T
C2, C4, C5: TAIYO YUDEN GMK212 BJ334MG-T
C3: TAIYO YUDEN UMK212 BJ224MG-T
L1: MURATA LQH32CN470K
D1, D2: CENTRAL CMDSH-3
Figure 7. ±20V Dual output converter
Linear Technology Magazine • September 2003
VOUT
20V
R2
4.53M
R1
301k
C1
1µF
1
7
6
VIN
SW
OUT
CTRL
LT3464 CAP
8
SHDN
FB
3
5
2
C3
0.22µF
C2
0.33µF
GND
VOUT
16V
3.48M
294k
4
C1: TAIYO YUDEN LMK107 BJ105MA-T
C2: TAIYO YUDEN GMK212 BJ334MG-T
C3: TAIYO YUDEN EMK107 BJ224MA-T
L1: MURATA LQH32CN220K
Figure 8. Li-Ion to 16V boost converter
21
DESIGN FEATURES
VLT6600 OUT = IDAC •
1+
A
R2 R2
+
R1 R3
WHERE A =
“I”
DIGITAL
INPUT
R3
432Ω
+
–
“Q”
DIGITAL
INPUT
1
7
0.1µF
2
8
R2 402Ω
R3
432Ω
3.3V
3.3V
R3
432Ω
+
R2
402Ω
R1
66.5
–5V
R2 402Ω
R3
432Ω
3.3V
+
4
VCC
LT6600-10
LT5503
3.3V
BI
–5
+
390Ω
8.2pF
2.7nH
0°
90°
MODIN
3.3V
0.1µF
7
0.1µF
3
–
6
1
R1
66.5Ω
LTC1666/7
DIN (Q CHANNEL DAC) IOUT
10mA FS
–
R2
402Ω
R1
66.5
–5V
5V
3.3V
0.1µF
R1
66.5Ω
LTC1666/7
DIN (I CHANNEL DAC) IOUT
10mA FS
402Ω FOT THE LT6600-10 AND LT6600-20
1580Ω FOR THE LT6600-2.5
3.3V
3.3V
5V
{
2
8
MOI
1.2pF
3.3V
+
4
LT6600-10
–5
+
8.2pF
VGA
1.2pF
3
–
18nH
2.7nH
BQ
GND
DMODE
MixEN
ModEN
GC1
GC2
6
Figure 7. Using the LT6600 as a transimpedance amplifier and smoothing filter in a base station application.
mode level when it converts the singleended input to differential. In Figure
6, the input signal is referenced to
ground and the signal presented to
the ADC is referenced to VCM.
To illustrate the excellent dynamic
range of the LT6600, consider Figure 6
with a 1MHz input signal of 800mVPP
amplified by an LT6600-2.5. With RIN
= 402Ω, the amplifier provides 12dB
of voltage gain. The signal presented
to the ADC converter is 3.2VP–P. The
distortion components will be at least
82dB below the fundamental, and the
signal-to-noise ratio will be 81dB in a
5MHz bandwidth.
The differential output DAC is another application where the LT6600
excels. Figure 7 shows the LT6600
acting as a transimpedance amplifier
and a 4th order smoothing filter, in
LT3464, continued from page 21
90
80
VIN = 8.4V
EFFICIENCY (%)
VIN = 4.2V
70
low delay distortion in the passband
(Figure 2), making for an outstanding
DAC smoothing solution.
low current. As shown in Figure 9,
high efficiency is maintained with low
output currents.
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call the
LTC literature service number:
Conclusion
60
50
40
30
0.01
a base station application. The input
common mode range of the LT6600
accommodates the compliance range
of the DAC. The output common mode
voltage of the LT6600 is set to optimize
the performance of the LT5503 direct
I/Q modulator. The resistors between
the DAC and the LT6600 allow the user
to adjust the transimpedance gain. The
LT6600 and LT5503 are operating on
a 3.3V power supply.
To illustrate the optimized filtering
of the LT6600, consider the case where
the DAC in Figure 7 has a sample rate
of 50Msps and the baseband signal
information extends to 10MHz. By using an LT6600-10, the attenuation of
the images near 40MHz would be more
than 50dB (filter response plus sin(x)/x
attenuation). The excellent rejection
in the stopband is combined with
0.1
1
10
LOAD CURRENT (mA)
100
The LT3464 in the ThinSOT package
produces an ultra compact boost
solution featuring high efficiency,
low quiescent current, true output
disconnect, and low external parts
count.
Conclusion
The LT6600 differential filter-amplifiers are the most compact ADC
anti-aliasing and DAC smoothing
solutions available in the 2.5MHz
to 20MHz range. The combination of
low noise, low distortion, and precision
response are impossible to replicate
with discrete designs. The LT6600 is
pin compatible with standard differential output op amps and performs
all of the same functions. The LT6600
improves the design of any system
requiring differential signal buffering
and filtering.
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
Figure 9. Efficiency for the circuit in Figure 7
24
Linear Technology Magazine • September 2003