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LTC1702AIGN

Dual 550kHz Synchronous 2-Phase Switching Regulator Controller

Linear Technology 

LTC1702A

APPLICATIONS INFORMATION

remember that the power losses are proportional to IRMS2,

meaning that the actual power wasted is reduced by a

factor of 3.75. The reduced input ripple voltage also means

less power is lost in the input power path, which could

include batteries, switches, trace/connector resistances

and protection circuitry. Improvements in both conducted

and radiated EMI also directly accrue as a result of the

reduced RMS input current and voltage.

Small Footprint

The LTC1702A operates at a 550kHz switching frequency,

allowing it to use low value inductors without generating

excessive ripple currents. Because the inductor stores

less energy per cycle, the physical size of the inductor can

be reduced without risking core saturation, saving PCB

board space. The high operating frequency also means

less energy is stored in the output capacitors between

cycles, minimizing their required value and size. The

remaining components, including the 150mil SSOP-24

LTC1702A, are tiny, allowing an entire dual-output

LTC1702A circuit to be constructed in 1.5in2 of PCB

space. Further, this space is generally located right next to

the microprocessor or in some similarly congested area,

where PCB real estate is at a premium. The fact that the

LTC1702A runs off the 5V supply, often available from a

power plane, is an added benefit in portable systems —it

does not require a dedicated supply line running from the

battery.

Fast Transient Response

The LTC1702A uses a fast 25MHz GBW op amp as an error

amplifier. This allows the compensation network to be

designed with several poles and zeros in a more flexible

configuration than with a typical gm feedback amplifier.

The high bandwidth of the amplifier, coupled with the high

switching frequency and the low values of the external

inductor and output capacitor, allow very high loop cross-

over frequencies. The low inductor value is the other half

of the equation—with a typical value on the order of 1µH,

the inductor allows very fast di/dt slew rates. The result is

superior transient response compared with conventional

solutions.

High Efficiency

The LTC1702A uses a synchronous step-down (buck)

architecture, with two external N-channel MOSFETs per

output. A floating topside driver and a simple external

charge pump provide full gate drive to the upper MOSFET.

The voltage mode feedback loop and MOSFET VDS current

limit sensing remove the need for an external current

sense resistor, eliminating an external component and a

source of power loss in the high current path. Properly

designed circuits using low gate charge MOSFETs are

capable of efficiencies exceeding 90% over a wide range

of output voltages.

ARCHITECTURE DETAILS

The LTC1702A dual switching regulator controller in-

cludes two identical, independent regulator channels. The

two sides of the chip and their corresponding external

components act independently of each other with the

exception of the common input bypass capacitor and the

FCB and FAULT pins, which affect both channels. In the

following discussions, when a pin is referred to without

mentioning which side is involved, that discussion applies

equally to both sides.

Switching Architecture

Each half of the LTC1702A is designed to operate as a

synchronous buck converter (Figure 1). Each channel

includes two high power MOSFET gate drivers to control

external N-channel MOSFETs QT and QB. These drivers

have 0.5Ω output impedances and can carry well over an

TG

LTC1702A

SW

PGND BG

VIN

QT LEXT

QB

+

CIN

VOUT

+

COUT

1702A F01

Figure 1. Synchronous Buck Architecture

1702afa

9

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