LT1073_15 Datasheet, PDF (6/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1073
APPLICATIO S I FOR ATIO
Table 1. Component Selection for Step-Up Converters
INPUT
VOLTAGE (V)
BATTERY
TYPE
OUTPUT
OUTPUT
VOLTAGE (V) CURRENT (MIN)
INDUCTOR
VALUE (µH)
1.55-1.25 Single Alkaline
3
60mA
82
1.30-1.05 Single Ni-Cad
3
20mA
180
1.55-1.25 Single Alkaline
5
30mA
82
1.30-1.05 Single Ni-Cad
5
10mA
180
3.1-2.1
Two Alkaline
5
80mA
120
3.1-2.1
Two Alkaline
5
25mA
470
3.3-2.5
Lithium
5
100mA
150
3.1-2.1
Two Alkaline
12
25mA
120
3.1-2.1
Two Alkaline
12
5mA
470
3.3-2.5
Lithium
12
30mA
150
4.5-5.5
TTL Supply
12
90mA
220
4.5-5.5
TTL Supply
12
22mA
1000
4.5-5.5
TTL Supply
24
35mA
220
G = GOWANDA
CB = CADDELL-BURNS
INDUCTOR
PART NUMBER
G GA10-822K, CB 7300-12
G GA10-183K, CB 7300-16
G GA10-822K, CB 7300-12
G GA10-183K, CB 7300-16
G GA10-123K, CB 7300-14
G GA10-473K, CB 7300-21
G GA40-153K, CB 6860-15
G GA10-123K, CB 7300-14
G GA10-473K, CB 7300-21
G GA10-153K, CB 7300-15
G GA40-223K, CB 6860-17
G GA10-104K, CB 7300-25
G GA40-223K, CB 6860-17
CAPACITOR
VALUE (µF)
NOTES
150
47
100
22
470
*
150
*
470
*
220
100
220
470
*
100
*
150
*
*Add 68Ω from I_LIM to V_IN
Measuring Input Current at Zero or Light Load
Obtaining meaningful numbers for quiescent current and
efficiency at low output current involves understanding
how the LT1073 operates. At very low or zero load current,
the device is idling for seconds at a time. When the output
voltage falls enough to trip the comparator, the power
switch comes on for a few cycles until the output voltage
rises sufficiently to overcome the comparator hysteresis.
When the power switch is on, inductor current builds up
to hundreds of milliamperes. Ordinary digital multimeters
are not capable of measuring average current because of
bandwidth and dynamic range limitations. A different
approach is required to measure the 100µA off-state and
500mA on-state currents of the circuit.
1MΩ
12V
–
LTC1050
+
1µF*
100Ω
V1
V2
+
1000µF
V_SET
*NONPOLARIZED
LT1073
CIRCUIT
1073 F01
Figure 1. Test Circuit Measures No-Load
Quiescent Current of LT1073 Converter
Quiescent current can be accurately measured using the
circuit in Figure 1. V_SET is set to the input voltage of the
LT1073. The circuit must be “booted” by shorting V2 to
V_SET. After the LT1073 output voltage has settled, discon-
nect the short. Input voltage is V2 and average input
current can be calculated by this formula:
I_IN
=
V2 – V1
100Ω
Inductor Selection
A DC/DC converter operates by storing energy as mag-
netic flux, in an inductor core and then switching this
energy into the load. Since it is flux, not charge, that is
stored, the output voltage can be higher, lower, or oppo-
site in polarity to the input voltage by choosing an appro-
priate switching topology. To operate as an efficient energy
transfer element, the inductor must fulfill three require-
ments. First, the inductance must be low enough for the
inductor to store adequate energy under the worst-case
condition of minimum input voltage and switch ON time.
The inductance must also be high enough so that maxi-
mum current ratings of the LT1073 and inductor are not
exceeded at the other worst-case condition of maximum
input voltage and ON time. Additionally, the inductor core
must be able to store the required flux, i.e., it must not
saturate. At power levels generally encountered with
LT1073-based designs, small axial-lead units with
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