L DESIGN FEATURES
TEMPERATURE (°C)
–40
QUIESCENT CURRENT (µA)
10
6
8
2
4
0
12080400
VCC = 3.6V
OUTPUT VOLTAGE (V)
0
AVERAGE INPUT CURRENT (µA)
1000
100
10
4020 3010
VCC = 3.6V
VCC VOLTAGE (V)
0
QUIESCENT CURRENT (µA)
12
10
6
8
2
4
0
161284
Boost Converters for Keep-Alive
Circuits Draw Only 8.5μA of
Quiescent Current
Introduction
Industrial remote monitoring systems
and keep-alive circuits spend most of
their time idle. Many of these systems
use batteries, so to maximize run time
power losses,even during low power
idle modes, must be minimized. Even
at no load, power supplies draw some
current to produce a regulated voltage
for keep-alive circuits.
The L T8410/-1 DC/DC boost
converter features ultralow quiescent
current and integrated high value
feedback resistors to minimize the
draw on the battery when electronics
are idle.
An entire boost converter takes very
little space, as shown in Figure 1.
Ultralow Quiescent Current
Low Noise Boost Converter
with Output Disconnect
When a micropower boost converter
is in regulation with no load, the
input current depends mainly on
two things—the quiescent current
(required to keep regulation) and the
output feedback resistor value. When
the output voltage is high, the output
feedback resistor can easily dissipate
more power than the quiescent current
of the IC. The quiescent current of the
LT8410/-1 is a low 8.5µA, while the
integrated output feedback resistors
have very high values (12.4M/0.4M).
This enables the LT8410/-1 to dissipate very little power in regulation
at no load. In fact, the LT8410/-1 can
regulate a 16V output at no load from
3.6V input with about 30µA of average
input current. Figures 2, 3 and 4 show
the typical quiescent and input current
in regulation with no load.
The LT8410/-1 controls power
delivery by varying both the peak
inductor current and switch off time.
This control scheme results in low
output voltage ripple as well as high
efficiency over a wide load range. As
shown in Figure 5, even with a small
0.1µF output capacitor, the output
ripple is typically less than 10mV. The
part also features output disconnect,
which disconnects the output voltage
from the input during shutdown. This
output disconnect circuit also sets a
maximum output current limit, allowing the chip survive output shorts.
An Excellent Choice for
High Impedance Batteries
A power source with high internal
impedance, such as a coin cell battery,
may show normal output voltage on
a voltmeter, but its voltage can collapse under heavy current demands.
This makes it incompatible with high
by Xiaohua Su
Figure 1. The LT8410/-1 is designed
to facilitate compact board layout.
switch-current DC/DC converters.
The LT8410/-1 has an integrated
power switch and Schottky diode,
and the switch current limits are very
low (25mA for the LT8410 and 8mA
for the LT8410-1). This low switch
current limit enables the LT8410/-1
to operate very efficiently from high
impedance sources, such as coin cell
batteries, without causing inrush
current problems. Figure 6 shows
the LT8410-1 charging an electrolytic
capacitor. Without any additional external circuitry, the input current for
Figure 2. Quiescent current vs
temperature—not switching
22
Figure 3. Quiescent current
vs VCC voltage—not switching
Figure 4. Average input current
in regulation with no load
Linear Technology Magazine • March 2009
1 30 1
1
2
. • +
R
R
( . ) ( )1 24 3 10 1
1
2
1 10
7 7
− • • +
− •
− −
R
R
R
R
SW CAP
GND
CHIP
ENABLE
FBP
LT8410
2.2µF
0.1µF
100µH
0.1µF
0.1µF*
V
OUT
= 16V
V
IN
2.5V to 16V
VCCV
OUT
V
REF
SHDN
604K
412K
*HIGHER VALUE CAPACITOR IS REQUIRED
WHEN THE VIN IS HIGHER THAN 5V
LOAD CURRENT (mA)
0.01
V
OUT
PEAK-TO-PEAK RIPPLE (mV)
10
8
2
6
4
0
1010.1
VIN = 3.6V
LOAD CURRENT (mA)
0.01
EFFICIENCY (%)
100
50
60
70
80
90
40
100100.1 1
VIN = 12V
VIN = 5V
VIN = 3.6V
Figure 5. General purpose bias with wide input voltage and low output voltage ripple
SW CAP
GND
FBP
LT8410-1
C1
2.2µF
C3
10000µF
C4
0.1µF
L1
220µH
C2
1.0µF
V
OUT
= 16V
V
IN
2.5V to 16V
VCCV
OUT
V
REF
SHDN
R1
604k
R2
412k
TURN ON/OFF
C1: 2.2μF, 16V, X5R, 0603
C2: 1.0μF, 25V, X5R, 0603*
C3: 10000μF, Electrolytic Capacitor
C4: 0.1μF, 16V, X7R, 0402
L1: COILCRAFT LPS3008-224ML
* HIGHER CAPACITANCE VALUE IS REQUIRED FOR
C2 WHEN THE VIN IS HIGHER THAN 12V
SHDN VOLTAGE
2V/DIV
V
OUT
VOLTAGE
10V/DIV
INPUT CURRENT
5mA/DIV
INDUCTOR
CURRENT
10mA/DIV
VIN = 3.6V 20s/DIV
R1
R2
R3
CONNECT TO
SHDN PIN
ENABLE VOLTAGE
the entire charging cycle is less than
8mA.
LT8410/-1 to fit almost anywhere.
Figure 1 shows the size of a circuit
similar to that shown in Figure 4,
Tiny Footprint with
Small Ceramic Capacitors
Available in a tiny 8-pin 2mm × 2mm
illustrating how little board space
is required to build a full featured
LT8410/-1 application.
DFN package, the LT8410/-1 is internally compensated and stable for
a wide range of output capacitors. For
most applications, using 0.1µF output
capacitor and 1µF input capacitor is
sufficient. An optional 0.1µF capacitor
at the V
pin implements a soft-start
REF
feature. The combination of small
package size and the ability to use
small ceramic capacitors enable the
SHDN Pin Comparator and
Soft-Start Reset Feature
An internal comparator compares the
SHDN pin voltage to an internal volt-
age reference of 1.3V, giving the part
a precise turn-on voltage level. The
SHDN pin has built-in programmable
hysteresis to reject noise and tolerate
slowly varying input voltages. Driving
DESIGN FEATURES L
the SHDN pin below 0.3V shuts down
the part and reduces input current to
less than 1µA. When the part is on, and
the SHDN pin voltage is close to 1.3V,
0.1µA current flows out of the SHDN
pin. A programmable enable voltage
can be set up by connecting external
resistors as shown in Figure 7.
The turn-on voltage for the con-
figuration is:
and the turn-off voltage is:
where R1, R2 and R3 are resistance
in Ω. Programming the turn-on/turn-
off voltage is particularly useful for
applications where high source impedance power sources are used, such as
energy harvesting applications.
By connecting an external capacitor (typically 47nF to 220nF) to the
V
pin, a soft-start feature can be
REF
implemented. When the part is brought
continued on page 29
Figure 7. Programming the enable
voltage by using external resistors
Linear Technology Magazine • March 2009
Figure 6. Capacitor charger with the LT8410-1 and charging waveforms
23
DESIGN FEATURES L
V V V
BE Q BE Q BE( ) ( )1 2
= =
dV
dT
dV
dT
mV
C
BE Q BE Q( ) ( )1 2
2
= =
°
V V
R
R
V
CTRL REF BE
= −
8
7
PTC
dV
dT
RRmV
C
CTRL
= = •
872
°
R
R
V
V
mV
CVdV dT
REF
BE
OUT
OUT
8
7
2
=
+ •
°
R
R
V
dV
dT
mV
C
V
mV
C
V
BE
OUT
OUT
REF
1
2
2
2
1=
• + •
•
°
°
−
dV
dT
dV
dT
mV
C
BE Q BE Q( ) ( )1 2
2
= =
°
TEMPERATURE (°C)
–50
V
APD
(V)
60
42
58
54
50
46
56
52
48
44
40
7525 125500 100–25
OUTPUT CURRENT (A)
0.1
EFFICIENCY (%)
100
90
80
70
60
50
40
0.5 0.90.3 0.7 1.1 1.3
VIN = 15V
VIN = 8V
VIN = 30V
Authors can be contacted
at (408) 432-1900
and expensive solution than typical
microprocessor-controlled methods.
The simplest scheme uses a resistor
divider from the V
pin to the CTRL
REF
pin, where the top resistor in the divider is an NTC (negative temperature
coefficient) resistor. While simple,
this method suffers from nonlinear
temperature coefficient of the NTC
resistor. A more precise method uses
a transistor network as shown in Figure 7. The PTC (Positive Temperature
Coefficient) of the CTRL pin voltage is
Figure 8. Temperature response
of the circuit shown in Figure 7
realized by an emitter follower of Q1
and a VBE multiplier of Q2.
Assuming:
and
then the CTRL pin voltage is
with
Given V
at room and dV
OUT
OUT
/DT,
the R1/R2 and R8/R7 can be calculated as follows
Resistors R5–R9 are selected to make
I(Q1) = I(Q2) ≈ 10µA, and
Simulation using LTspice always
gives a good starting point. The circuit
shown in Figure 7 is designed to have
V
= 50V (V
APD
dV
/dT = 100mV/°C (dV
APD
= 55V) at room and
OUT
OUT
/dT =
100mV/°C). The measured temperature response is shown in Figure 8,
which is very close to the design
target.
Conclusion
The LT3571 is a highly integrated,
compact solution to APD bias supply
design. It provides a useful feature set
and the flexibility to meet a variety of
challenging requirements, such as low
noise, fast transient response speed,
and temperature compensation. With
a high level of integration and superior performance, the LT3571 is the
natural choice for APD bias supply
design.
L
LT8410, continued from page 23
out of shutdown, the V
discharged for 70µs with a strong pull
down current, and then charged with
10µA to 1.235V. This achieves soft
start since the output is proportional
to V
. Full soft-start cycles occur
REF
even with short SHDN low pulses
since V
REF
part is enabled.
In addition, the LT8410/-1 features
a 2.5V to 16V input voltage range, up
LT3653/63, continued from page 21
of handling 60V transients. Figure 4
shows the circuit efficiency at multiple
input voltages.
The current limit of the application
is set to 1.2A, therefore, the power path
components are sized to handle 1.2A
maximum. To reduce the application
footprint, the LT3663 includes internal
compensation and a boost diode. The
RUN pin, when low, puts the LT3663
into a low current shutdown mode.
Linear Technology Magazine • March 2009
pin is first
REF
is discharged when the
to 40V output voltage and overvoltage
protection for CAP and V
OUT
.
Conclusion
The LT8410/-1 is a smart choice
for applications which require low
quiescent current and low input current. The ultralow quiescent current,
combined with high value integrated
feedback resistors, keeps the average
input current very low, significantly
Figure 4. Efficiency of the circuit in Figure 3
extending battery operating time.
Low current limit internal switches
(8mA for the LT8410-1, 25mA for the
LT8410) make the part ideal for high
impedance sources such as coin cell
batteries. The LT8410/-1 is packed
with features without compromising
performance or ease of use and is
available in a tiny 8-pin 2mm × 2mm
package.
L
Conclusion
The accurate programmable output
current limit of the LT3653 and
LT3663 eliminates localized heating
from an output overload, reduces the
maximum current requirements on the
power components, and makes for a
robust power supply solutions.
L
29
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