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LTC1702A

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2-Step Conversion

“2-step” architectures use a primary regulator to convert

the input power source (batteries or AC line voltage) to an

intermediate supply voltage, often 5V. This intermediate

voltage is then converted to the low voltage, high current

supplies required by the system using a secondary regu-

lator— the LTC1702A. 2-step conversion eliminates the

need for a single converter that converts a high input

voltage to a very low output voltage, often an awkward

design challenge. It also fits naturally into systems that

continue to use the 5V supply to power portions of their

circuitry, or have excess 5V capacity available as newer

circuit designs shift the current load to lower voltage

supplies.

Each regulator in a typical 2-step system maintains a

relatively low step-down ratio (5:1 or less), running at high

efficiency while maintaining a reasonable duty cycle. In

contrast, a regulator taking a single step from a high input

voltage to a 1.xV or 2.xV output must run at a very narrow

duty cycle, mandating trade-offs in external component

values and compromising efficiency and transient

response. The efficiency loss can exceed that of using a

2-step solution (see the 2-Step Efficiency Calculation

section and Figure 14). Further complicating the calcula-

tion is the fact that many systems draw a significant

fraction of their total power off the intermediate 5V supply,

bypassing the low voltage supply. 2-step solutions using

the LTC1702A usually match or exceed the total system

efficiency of single-step solutions, and provide the addi-

tional benefits of improved transient response, reduced

PCB area and simplified power trace routing.

2-step regulation can buy advantages in thermal manage-

ment as well. 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 ex-

ample, 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, aggravat-

ing thermal management. In a 2-step LTC1702A design,

a significant percentage of the power lost in the core

APPLICATIONS INFORMATION

regulation system happens in the 5V supply, which is

usually located away from the CPU. The power lost to heat

in the LTC1702A section of the system is relatively low,

minimizing the added heat near the CPU.

See the Optimizing Performance section for a detailed

explanation of how to calculate system efficiency.

2-Phase Operation

The LTC1702A dual switching regulator controller also

features the considerable benefits of 2-phase operation.

Notebook computers, hand-held terminals and automo-

tive electronics all benefit from the lower input filtering

requirement, reduced electromagnetic interference (EMI)

and increased efficiency associated with 2-phase

operation.

Why the need for 2-phase operation? Up until the

LTC1702A, constant-frequency dual switching regulators

operated both channels in phase (i.e., single-phase opera-

tion). This means that both topside MOSFETs turned on at

the same time, causing current pulses of up to twice the

amplitude of those for one regulator to be drawn from the

input capacitor. These large amplitude current pulses

increased the total RMS current flowing from the input

capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and input power supply.

With 2-phase operation, the two channels of the LTC1702A

are operated 180 degrees out of phase. This effectively

interleaves the current pulses coming from the switches,

greatly reducing the overlap time where they add together.

The result is a significant reduction in total RMS input

current, which in turn allows less expensive input capaci-

tors to be used, reduces shielding requirements for EMI

and improves real world operating efficiency.

Figure 7 shows example waveforms for a single switching

regulator channel versus a 2-phase LTC1702A system

with both sides switching. A single-phase dual regulator

with both sides operating would exhibit double the single

side numbers. In this example, 2-phase operation reduced

the RMS input current from 9.3ARMS (2 × 4.66ARMS) to

4.8ARMS. While this is an impressive reduction in itself,

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