LTC1705 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1705
Linear Technology
'LTC1705' PDF : 28 Pages View PDF
LTC1705
APPLICATIO S I FOR ATIO
Accuracy Trade-Offs
The VDS sensing scheme used in the LTC1705 is not
particularly accurate, primarily due to uncertainty in the
RDS(ON) from MOSFET to MOSFET. A second error term
arises from the ringing present at the SW pin, which
causes the VDS to look larger than (ILOAD)(RDS(ON)) at the
beginning of QB’s on-time. These inaccuracies do not
prevent the LTC1705 current limit circuit from protecting
itself and the load from damaging overcurrent conditions,
but they do prevent the user from setting the current limit
to a tight tolerance if more than one copy of the circuit is
being built. The 50% factor in the current setting equation
above reflects the margin necessary to ensure that the
circuit will stay out of current limit at the maximum normal
load, even with a hot MOSFET that is running quite a bit
higher than its RDS(ON) spec.
VCLK LINEAR REGULATOR
The LTC1705 monolithic LDO linear regulator is easy to
use. Input and output supply bypass capacitors are the
only two external components required for this LDO. The
VINCLK pin powers up the regulator and an internal P-channel
MOS transistor sources at least 150mA of current at a
fixed output voltage of 2.5V. This device is short-circuit
protected and includes thermal shutdown to turn off all
three regulator outputs should the junction temperature
exceed about 155°C.
The circuit design in the LTC1705 requires the use of an
output capacitor as part of the frequency compensation. A
minimum output capacitor of 2.2µF and ESR larger than
100mΩ is recommended to prevent oscillations. Larger
values of output capacitance decrease the peak deviations
and provide improved transient response for large load
current changes. Many different types of capacitors are
available and have widely varying characteristics. These
capacitors differ in capacitor tolerance (sometimes rang-
ing up to ±100%), equivalent series resistance, equivalent
series inductance and capacitance temperature coeffi-
cient. In a typical operating condition, a 10µF solid tanta-
lum at the VOUTCLK pin ensures stability. The AVX
TPSD106M035R0300 or equivalent works well in this
application.
OPTIMIZING PERFORMANCE
2-Step Conversion
The LTC1705 is ideally suited for use in 2-step conversion
systems. 2-step systems use a primary regulator to con-
vert the input power source (batteries or AC line voltage)
to an intermediate supply voltage, often 5V. The LTC1705
then converts the intermediate voltage to the lower volt-
age, high current supplies required by the system. Com-
pared to a 1-step converter that converts a high input
voltage directly to a very low output voltage, the 2-step
converter exhibits superior transient response, smaller
component size and equivalent efficiency. Thermal man-
agement and layout complexity are also improved with a
2-step approach.
A typical notebook computer supply might use a 4-cell Li-
Ion battery pack as an input supply with a 15V nominal
terminal voltage. The logic circuits require 5V/3A and
3.3V/5A to power system board logic and 2.5V/0.15A,
1.5V/2A and 1.3V/15A to power the CPU. A typical 2-step
conversion system would use a step-down switcher (per-
haps an LTC1628 or two LTC1625s) to convert 15V to 5V
and another to convert 15V to 3.3V (Figure 10). The 3.3V
input supply can power the 1.3V output at the LTC1705
core channel and the 2.5V LDO. The 5V input supply can
power the 1.5V I/O channel. The corresponding 1-step
system would use four similar step-down switchers plus
an LDO, each switcher using 15V as the input supply and
generating one of the four output voltages.
Clearly, the 5V and 3.3V sections of the two schemes are
equivalent. The 2-step system draws additional power
from the 5V and 3.3V outputs, but the regulation tech-
niques and trade-offs at these outputs are similar. The
difference lies in the way the 1.5V and 1.3V supplies are
generated. For example, the 2-step system converts 3.3V
to 1.3V with a 39% duty cycle. During the QT on-time, the
voltage across the inductor is 2V and during the QB on-
time, the voltage is 1.3V, giving roughly symmetrical
transient response to positive and negative load steps. The
2V maximum voltage across the inductor allows the use of
a small 0.47µH inductor while keeping ripple current to 3A
(20% of the 15A maximum load). By contrast, the 1-step
22
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