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LTC1705

APPLICATIO S I FOR ATIO

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 transient response with posi-

tive and negative current steps. The narrow 9% 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 advan-

tage the 1-step conversion might have had.

Note that power dissipation in the LTC1705 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 LTC1705 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

LTC1705 section of the system is relatively low, minimiz-

ing the heat near the CPU.

Additionally, with a 1-step converter, the high input battery

voltage requires the MOSFET to operate at high voltage

levels. This imposes stringent requirements on the

MOSFETs selection. Most of the MOSFETs that meet the

high voltage and high current requirements are expensive

and bulky. This makes for an awkward power supply

design, especially in portable applications. The high input

voltage also necessitates higher gate drive, which aggra-

vate switching losses.

Figure 10. 2-Step Conversion Block Diagram

2-STEP CONVERSION OFFERS

• BETTER TRANSIENT RESPONSE

• SMALLER COMPONENT SIZE

• BETTER THERMAL MANAGEMENT

• LOWER VOLTAGE REQUIREMENT FOR MOSFETS

• SMALLER SWITCHING LOSS

• EQUIVALENT EFFICIENCY

LTC1628*

5V/3A LOGIC SUPPLY

1.5V/2A CPU I/O SUPPLY

1.3V/15A CPU CORE SUPPLY

3.3V/5A LOGIC SUPPLY

2.5V/0.15A CPU CLOCK SUPPLY

*OR TWO LTC1625s

I/O

CORE

LTC1705

CPU SUPPLY CONTROLLER

LDO

1705 F10

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