
Linear Technology Magazine • February 1995
9
DESIGN FEATURES
Typical Applications
Fixed Output 3.3V
and 5V Converter
A fixed LTC1267 application cir-
cuit creating 3.3V/2A and 5V/2A is
shown in Figure 3. The operating
efficiency, shown in Figure 2, exceeds
90% for both the 3.3V and 5V sec-
tions. The 3.3V section of the circuit
in Figure 3 comprises the main switch
Q1, synchronous switch Q2, induc-
tor L1, and current shunt R
SENSE3
.
The 5V section is similar and com-
prises Q3, Q4, L2, and R
SENSE5
. Each
current-sense resistor (R
SENSE
) moni-
tors the inductor current and is used
to set the output current according to
the formula I
OUT
= 100mV/R
SENSE
.
Advantages of current control include
excellent line and load transient
rejection, inherent short-circuit pro-
tection, and controlled startup
currents. Peak inductor currents for
L1 and L2 are limited to 150mV/
R
SENSE
or 3.0A. The EXT V
CC
pin is
connected to the 5V output, increas-
ing efficiency at high input voltages.
The maximum input voltage is lim-
ited by the MOSFETs and should not
exceed 28V.
Adjustable Output
3.6V and 5V Converter
The adjustable output LTC1267-
ADJ shown in Figure 4 is configured
as a 3.6V/2.5A and 5V/2A converter.
The resistor divider composed of R1
and R2 sets the output voltage ac-
cording to the formula V
OUT
= 1.25V
(1 + R2/R1). The input voltage range
for this application is 5.5V to 28V.
Conclusion
The LTC1267 adds even more ver-
satility to Linear Technology’s family
of high-efficiency step-down regula-
tor controllers. Providing for up to
40V input voltage, the LTC1267
allows the use of higher voltage wall
adapters. The 28-pin SSOP package
and associated external components
make dual output voltage, high-effi-
ciency DC-to-DC conversion feasible
in the extremely small board space
available in today’s portable
electronics.
Description
Both regulator blocks in the
LTC1267 use a constant off-time cur-
rent-mode architecture. This results
in a power supply that has very high
efficiency over a wide load current
range, fast transient response, and
very low dropout. The LTC1267 is
ideal for applications that require 3.3V
and 5V to be implemented with the
highest conversion efficiencies over a
wide load current range in a small
board space. The LTC1267-ADJ has
two externally adjustable outputs,
which allow remote load sensing and
user-customized output voltages.
Each regulator section employs a
pair of external, complementary
MOSFETs and a user-programmable
current sense resistor for setting the
operating current level to optimize
performance for each application. A
master shutdown pin turns off both
main outputs and the 4.5V LDO. Both
outputs in the LTC1267 have indi-
vidual shutdown capability, whereas
the LTC1267-ADJ has a shutdown
LTC1142HV or LTC1142HV-ADJ can
be used.) At low input voltages, the
internal 4.5V low-dropout regulator
stays in regulation with only a 5V
input voltage, extracting the maxi-
mum possible energy from the battery
pack.
All members of the LTC1142/
LTC1267 family are capable of 100%
duty cycle, providing very low drop-
out operation (lower than that of most
linear low-dropout regulators), and
all have built-in current limiting. As
the input voltage on the LTC1267
drops, the loop extends the on-time
for the P-channel switch (off-time is
constant), thereby keeping the in-
ductor ripple current constant.
Eventually the on-time extends so far
that the P-channel MOSFET is on at
DC or 100% duty cycle. Load and line
regulation are excellent for a wide
variety of conditions, including
making the transition from Burst
Mode™ operation to continuous-
mode operation.
operation as the LTC1142HV (see the
comparison in Table 1). The LTC1267
automatically switches to Burst
Mode™ operation at low output cur-
rents to maintain greater than 90%
efficiency over two decades of load
current range. The wide operating
range is illustrated by the typical
efficiency curve of Figure 2. Battery
life is extended by providing high
efficiencies at load currents from a
few milliamps (when the device is in
standby or sleep modes) to Amps (un-
der full power conditions).
pin for only one of its two outputs.
The higher input-voltage capabil-
ity of the LTC1267 is required by
battery-powered systems that use
many cells in series to provide more
power and longer battery life for high-
performance portable systems. For
12-cell and larger applications, the
AC adapter voltage can be as high as
30V, well below the 40V maximum of
the LTC1267, allowing operation di-
rectly from the AC adapter. (If the
application uses an AC adapter volt-
age of 18V or less, the dual output
All members of the LTC1142/LTC1267
family are capable of 100% duty cycle,
providing very low dropout operation —
lower than that of most linear low-dropout
regulators — and all have built-in current limiting