LINEAR TECHNOLOGY LT1301 User Manual
Applications of the LT1300 and LT1301
Micropower DC/DC Converters
Dale Eagar and Steve Pietkiewicz
INTRODUCTION
Application Note 59
January 1994
The design of battery-powered equipment can often be
quite challenging. Since few ICs can operate directly from
the end-of-life voltage from a 2-cell battery (about 1.8V),
most systems require a DC/DC converter. The system
designer often has a limited area in which to place the DC/
DC converter; associated inductors and capacitors must be
small. Surface mount components are a must and heat
sinks are out of the question! The LT1300 and LT1301
micropower DC/DC converter ICs provide new possibilities
for more efficient, compact and cost effective designs.
When designing equipment for battery-powered
operation, a number of important design constraints
should be considered. Some of these are detailed in the
check list given here:
• Design for high efficiency. A high efficiency converter
increases battery life, eliminates most heat sinks, reduces
weight and decreases PC board area. The designer
should strive for high efficiency at:
– Full Load
– Light Load
• Plan to utilize all the capacity of the battery. Can the
circuit run down to the “dead cell” voltage? Is there a
micropower shutdown mode?
• Can the DC/DC converter circuitry provide high output
power for short time intervals? Often this is a requirement
on battery-powered equipment.
• Cost. Is the complete circuit cost competitive?
• Does the design meet packaging constraints?
– Height
– PC Board Area
– Weight
The LT1300 family of DC/DC converters allows a maximum
of flexibility in the design of circuits which provide
solutions for battery-operated and other equipment
needing high efficiency, space efficient, micropower
power solutions.
INDEX TO LT1300 CIRCUITS
Figure Description Page
1 LT1300/LT1301 Block Diagram ................................................................................................................ 2
2 2-Cell to 5V DC/DC Converter Delivers >200mA with a 2V Input .............................................................. 3
8 Lower Power Applications Can Use Smaller Components. L1 is Tallest Component at 3.1mm ................ 5
11 4-Cell to 3.3V or 5V Converter Output Goes to Zero When in Shutdown .................................................. 6
13 LT1301 Delivers 12V From 3.3V or 5V Input ............................................................................................ 7
15 Flame Detector.......................................................................................................................................... 8
16, 17 Voltage Buffer ....................................................................................................................................... 8, 9
18 CCFL Driver............................................................................................................................................. 10
19 Electronic Light Stick .............................................................................................................................. 11
20 Backlight LED Driver ............................................................................................................................... 11
21 Efficiency of LED Driver .......................................................................................................................... 12
AN59-1
Application Note 59
NEW LT1300 AND LT1301 MICROPOWER
DC/DC CONVERTERS
by Steve Pietkiewicz
Introduction
The new LT1300 and LT1301 micropower DC/DC converters provide improvements in both electrical and physical
efficiency, two key areas of battery-based power supply
design. Housed in 8-lead DIP or SOIC packages, the
devices feature a 1A on-chip switch with a V
CESAT
of just
170mV. The internal oscillator frequency is set at 155kHz,
allowing the use of tiny, 5mm diameter surface mount
inductors along with standard D-case size tantalum capacitors. A complete 2-cell to 12V, 5V, or 3.3V converter
can fit in less than 0.4 square inches of PC board area.
The devices use Burst ModeTM operation to maintain high
efficiency across the full load range. The fully operating
quiescent current is only 120µ A. It can be further reduced
to 10µA by taking the SHUTDOWN pin high, which also
disables the device. The output voltage of the LT1300 can
be set at either 5V or 3.3V via the logic-controlled SELECT
pin, and the LT1301 output can be set at either 5V or 12V
using the same pin. The I
pin allows the reduction of
LIM
peak switch current and allows the use of even smaller
components. The switch current is nominally set at 1A and
can be reduced via the I
pin to approximately 400mA,
LIM
further improving efficiency in systems requiring lower
peak powers.
Theory of Operation
Figure 1 is a block diagram of the LT1300/LT1301. Refer
also to Figure 2 for associated component hookup. When
A1’s negative input, related to the SENSE pin voltage by the
appropriate resistor-divider ratio, is higher than the 1.25V
reference voltage, A1’s output is low. A2, A3 and the
oscillator are turned off, drawing no current. Only the
reference and A1 consume current, typically 120µ A. When
the voltage at A1’s negative input decreases below 1.25V,
overcoming A1’s 6mV hysteresis, A1’s output goes high,
enabling the oscillator, current comparator A2, and driver
A3. Quiescent current increases to 2mA as the device
prepares for high current switching. Q1 then turns on in a
controlled saturation for (nominally) 5.3µ s or until current
comparator A2 trips, whichever comes first. After a fixed
off-time of (nominally) 1.2µ s, Q1 turns on again. Refering
to Figure 2, the LT1300’s switching causes current to
alternately build up in L1 and dump into output capacitor
C1via D1, increasing the output voltage. When the output
is high enough to cause A1’s output to go low (Figure 1),
switching action ceases. C1 is left to supply current to the
Burst ModeTM is a trademark of Linear Technology Corporation
SENSESHUTDOWN
500k
1.25V
REFERENCE
144k
161k
GND SELECT
CURRENT
COMPARATOR
OSCILLATOR
5.3µs ON
1.2µs OFF
+
A3
DRIVER
–
A1
SLOW
COMPARATOR
ENABLE
Figure 1. LT1300/LT1301 Block Diagram
A2
V
IN
+
R1
SW
3Ω
Q2
3×
PGND
AN59 • F01
Q1
500×
–
BIAS
+
–
18mV
R2
700Ω
Q3
8.5k
I
LIM
AN59-2
Application Note 59
load until V
decreases enough to force A1’s output
OUT
high, and the entire cycle repeats. If switch current reaches
1A, causing A2 to trip, switch on-time is reduced and offtime increases slightly. This allows continuous mode
operation during bursts. Current comparator A2 monitors
the voltage across 3Ω resistor R1 which is
directly related
to inductor L1’s current. Q2’s collector current is set by
the emitter-area ratio to 0.6% of Q1’s collector current.
When R1’s voltage drop exceeds 18mV, corresponding to
1A inductor current, A2’s output goes high, truncating the
on-time portion of the oscillator cycle and increasing offtime to about 2µs as shown in Figure 3, trace A. This
programmed peak current can be reduced by tying the
I
pin to ground, causing 15µA to flow through R2 into
LIM
Q3’s collector. Q3’s current causes a 10.4mV drop in R2,
so that only an additional 7.6mV is required across R1 to
turn off the switch. This corresponds to a 400mA switch
current, as shown in Figure 3, trace B. The reduced peak
L1*
10µH
Burst ModeTM Operation
Burst ModeTM operation, a technique used by many LTC
switching regulator products, extends high efficiency
over widely varying loads.
At light load, switching regulators employing traditional
PWM regulation techniques suffer from low efficiency.
This is primarily due to relatively high quiescent (or
housekeeping) supply current and AC switching losses
resulting from constant frequency operation.
100
Burst ModeTM
SWITCHER
75
NON-Burst Mode
50
EFFICIENCY (%)
25
TM
2×
AA
CELL
SHUTDOWN
+
100µF
*SUMIDA CD54-100LC
COILCRAFT D03316-103
NC
SELECT
SHDN
I
LIM
V
IN
LT1300
SENSE
GND PGND
SW
D1
1N5817
+
Figure 2. Two-Cell to 5V DC/DC Converter Delivers >200mA
with a 2V Input
TRACE A
500mA/DIV
I
PIN
LIM
OPEN
TRACE B
500mA/DIV
I
PIN
LIM
GROUNDED
5µs/DIV
Figure 3. Switch Pin Current with I
Floating or Grounded
LIM
AN59 • F03
C1
100µF
AN59 • F02
5V OUTPUT
200mA
0
1255075
POWER (%)
100
AN59 • F1a
Figure 1a. Characteristics of Burst and Non-Burst Switchers
As seen in Figure 1a, the switching regulator not using
Burst ModeTM operation does not reach peak efficiency
until load power approaches 100%. Relatively high
fixed power drain inside the regulator accounts for the
efficiency fall-off as load is decreased. The regulator
utilizing Burst ModeTM operation, on the other hand,
maintains its high efficiency at light loads. It does this
by delivering energy to the output in discrete peak
efficiency packets. The energy packets result in a small
amount of ripple voltage (typically 50mV) on the output.
When not delivering these packets of energy to the
output, the regulator puts itself in a “sleep” mode with
only a voltage reference and a comparator powered up.
These two functions can be accomplished with very low
power drain. As the load is decreased to zero, even the
small amount of power consumed in sleep mode becomes significant compared to the load, resulting ultimately in decreasing efficiency.
AN59-3
Application Note 59
BATTERY
TIME (HOURS)
0
0
OUTPUT/BATTERY VOLTAGE (V)
5.0
11
AN59 • F06
9651
1.0
2.0
4.5
4.0
3.5
3.0
2.5
1.5
0.5
234 78 10
2× L91
LITHIUM
2× E91
ALKALINE
OUTPUT
switch current reduces I2R losses in Q1, L1, C1, and D1.
You can increase efficiency by doing this provided that the
accompaning reduction in full load output current is acceptable. Lower peak currents also extend alkaline battery
life due to the alkaline cells’ high internal impedance.
5V from 2 Cells
Figure 2’s circuit provides 5V from a 2-cell input. Shutdown is effected by taking the SHUTDOWN pin high. V
IN
current drops to 10µ A in this condition. This simple boost
topology does not provide output isolation and in shutdown the load is still connected to the battery via L1 and
D1. Figure 4 shows the efficiency of the circuit with a range
of input voltages, including a fresh battery (3V) and an
“almost dead” battery (2V). At load currents below a few
milliamperes, the 120µA quiescent current of the device
becomes significant, causing the fall-off in efficiency de-
90
88
86
84
82
80
EFFICIENCY (%)
78
76
74
1
10 500
LOAD CURRENT (mA)
VIN = 4.0V
VIN = 3.0V
VIN = 2.5V
VIN = 2.0V
100
AN59 • F04
tailed in Figure 4. At load currents in the 20mA to 200mA
range, efficiency flattens out in the 80% to 88% range,
depending on the input. Figure 5 details circuit operation.
V
is shown in trace A. The burst repetition pattern is
OUT
clearly shown as V
decays, then steps back up due to
OUT
switching action. Trace B shows the voltage at the switch
node. The damped, high frequency waveform at the end of
each burst is due to the inductor “ringing off,” forming an
LC tank with the switch and diode capacitance. It is not
harmful and contains far less energy than the high speed
edge which occurs when the switch turns off. Switch
current is shown in trace C. The current comparator inside
the LT1300 controls peak switch current, turning off the
switch when the current reaches approximately 1A.
Although efficiency curves present useful information, a
more important measure of battery-powered DC/DC converter performance is operating life. Figures 6 and 7 detail
battery life tests with Figure 2’s circuit at load currents of
100mA and 200mA respectively. Operating life curves are
shown using both Eveready E91 alkaline cells and new L91
“Hi-Energy” lithium cells. These lithium cells, new to the
market, are specifically designed for high drain applications. The performance advantage of lithium is about 2:1
at 100mA load current (Figure 6), increasing to 2.5:1 at
200mA load (Figure 7). Alkaline cells perform poorly at
high drain rates because their internal impedance ranges
V
OUT
A = 20mV/DIV
AC COUPLED
V
SW
B = 5V/DIV
AN59-4
I
SW
C = 1A/DIV
Figure 4. Efficiency of Figure 2’s Circuit
20µs/DIV
Figure 5. Burst ModeTM Operation in Action
AN59 • F05
Figure 6. Two Eveready L91 Lithium AA Cells Provide
Approximately Twice the Life of E91 AA Alkaline Cells at a
100mA Load Current
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