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LTC1702A

1702afa

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,

APPLICATIONS INFORMATION

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 0.47

µH 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.

Figure 14. 2-Step Conversion Block Diagram

VBAT

15V

LTC1628*

*OR TWO LTC1625s

LTC1702A

LDO

5V/3A

1.8V/2A

1.5V/10A

3.3V/5A

2.5V/0.5A

1702A F14

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