LTC1702ACGN#TR-SMI Linear Technology, LTC1702ACGN#TR-SMI Datasheet - Page 26

• Manufacturer Part Number: LTC1702ACGN#TR-SMI
• Manufacturer: Linear Technology

Specifications of LTC1702ACGN#TR-SMI

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS
LTC1702A
OPTIMIZING PERFORMANCE
2-Step Conversion
The LTC1702A is ideally suited for use in 2-step conver-
sion systems. 2-step systems use a primary regulator to
convert the input power source (batteries or AC line
voltage) to an intermediate supply voltage, often 5V. The
LTC1702A then converts the intermediate voltage to the
low voltage, high current supplies required by the system.
Compared 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/0.5A, 1.8V/2A
and 1.5V/10A to power the CPU. A typical 2-step conver-
sion system would use a step-down switcher (perhaps an
LTC1628 or two LTC1625s) to convert 15V to 5V and
another to convert 15V to 3.3V (Figure 14). One channel of
the LTC1702A would generate the 1.5V supply using the
3.3V supply as the input and the other channel would gen-
erate 1.8V using the 5V supply as the input. The corre-
sponding 1-step system would use four similar step-down
switchers, each using 15V as the input supply and gener-
ating one of the four output voltages. Since the 2.5V sup-
ply represents a small fraction of the total output power,
26
V
15V
Figure 14. 2-Step Conversion Block Diagram
BAT
*OR TWO LTC1625s
LTC1628*
U
INFORMATION
U
LTC1702A
LDO
W
5V/3A
1.8V/2A
1.5V/10A
3.3V/5A
2.5V/0.5A
1702A F14
U
either system can generate it from the 3.3V output using
an LDO linear regulator, without the 75% linear efficiency
making much of an impact on total system efficiency.
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 1.8V and 1.5V supplies are
generated. For example, the 2-step system converts 3.3V
to 1.5V with a 45% duty cycle. During the QT on-time, the
voltage across the inductor is 1.8V and during the QB on-
time, the voltage is 1.5V, giving roughly symmetrical
transient response to positive and negative load steps. The
1.8V maximum voltage across the inductor allows the use
of a small 0.47µH inductor while keeping ripple current
under 4A (40% of the 10A maximum load). By contrast,
the 1-step converter is converting 15V to 1.5V, requiring
just a 10% duty cycle. Inductor voltages are now 13.5V
when QT is on and 1.5V when QB is on, giving vastly
different di/dt values and correspondingly skewed tran-
sient response with positive and negative current steps.
The narrow 10% duty cycle usually requires a lower
switching frequency, which in turn requires a higher value
inductor and larger output capacitor. Parasitic losses due
to the large voltage swing at the source of QT cost
efficiency, eliminating any advantage the 1-step conver-
sion might have had.
Note that power dissipation in the LTC1702A portion of a
2-step circuit is lower than it would be in a typical 1-step
converter, even in cases where the 1-step converter has
higher total efficiency than the 2-step system. In a typical
microprocessor core supply regulator, for example, the
regulator is usually located right next to the CPU. In a
1-step design, all of the power dissipated by the core
regulator is right there next to the hot CPU, aggravating
thermal management. In a 2-step LTC1702A design, a
significant percentage of the power lost in the core regu-
lation system happens in the 5V or 3.3V supply, which is
usually away from the CPU. The power lost to heat in the
LTC1702A section of the system is relatively low, minimiz-
ing the heat near the CPU.
1702afa