LTC1703IG Datasheet, PDF (26/36 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID

LTC1703
APPLICATIO S I FOR ATIO
OPTIMIZING PERFORMANCE
2-Step Conversion
The LTC1703 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 LTC1703
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 management 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,
1.5V/2A and 1.3V/10A 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 14). One
channel of the LTC1703 would generate the 1.3V supply
using the 3.3V supply as the input and the other channel
would generate 1.5V using the 5V supply as the input. The
corresponding 1-step system would use four similar step-
down switchers, each using 15V as the input supply and
generating one of the four output voltages. Since the 2.5V
supply represents a small fraction of the total output
power, 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.
V_BAT
15V
LTC1628*
*OR TWO LTC1625s
LTC1703
LDO
5V/3A
1.5V/2A
1.3V/10A
3.3V/5A
2.5V/0.5A
1703 F14
Figure 14. 2-Step Conversion Block Diagram
26
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
under 4A (40% of the 10A maximum load). By contrast,
the 1-step converter is converting 15V to 1.3V, requiring
just a 9% duty cycle. Inductor voltages are now 13.7V
when QT is on and 1.3V when QB is on, giving vastly
different di/dt values and correspondingly skewed tran-
sient response with positive and negative current steps.
The narrow 9% duty cycle usually requires a lower switch-
ing 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 LTC1703 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 LTC1703 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
LTC1703 section of the system is relatively low, minimiz-
ing the heat near the CPU.
2-Step Efficiency Calculation
Calculating the efficiency of a 2-step converter system
involves some subtleties. Simply multiplying the effi-
ciency of the primary 5V or 3.3V supply by the efficiency
of the 1.5V or 1.3V supply underestimates the actual
1703fa