Datasheet LTC1702A (Analog Devices) - 9

LTC1702A
U U W U APPLICATIONS INFORMATION
remember that the power losses are proportional to I 2 RMS ,
High Efficiency
meaning that the actual power wasted is reduced by a The LTC1702A uses a synchronous step-down (buck) factor of 3.75. The reduced input ripple voltage also means architecture, with two external N-channel MOSFETs per less power is lost in the input power path, which could output. A floating topside driver and a simple external include batteries, switches, trace/connector resistances charge pump provide full gate drive to the upper MOSFET. and protection circuitry. Improvements in both conducted The voltage mode feedback loop and MOSFET V and radiated EMI also directly accrue as a result of the DS current limit sensing remove the need for an external current reduced RMS input current and voltage. sense resistor, eliminating an external component and a source of power loss in the high current path. Properly
Small Footprint
designed circuits using low gate charge MOSFETs are The LTC1702A operates at a 550kHz switching frequency, capable of efficiencies exceeding 90% over a wide range allowing it to use low value inductors without generating of output voltages. excessive ripple currents. Because the inductor stores less energy per cycle, the physical size of the inductor can
ARCHITECTURE DETAILS
be reduced without risking core saturation, saving PCB board space. The high operating frequency also means The LTC1702A dual switching regulator controller in- less energy is stored in the output capacitors between cludes two identical, independent regulator channels. The cycles, minimizing their required value and size. The two sides of the chip and their corresponding external remaining components, including the 150mil SSOP-24 components act independently of each other with the LTC1702A, are tiny, allowing an entire dual-output exception of the common input bypass capacitor and the LTC1702A circuit to be constructed in 1.5in2 of PCB FCB and FAULT pins, which affect both channels. In the space. Further, this space is generally located right next to following discussions, when a pin is referred to without the microprocessor or in some similarly congested area, mentioning which side is involved, that discussion applies where PCB real estate is at a premium. The fact that the equally to both sides. LTC1702A runs off the 5V supply, often available from a power plane, is an added benefit in portable systems —it
Switching Architecture
does not require a dedicated supply line running from the Each half of the LTC1702A is designed to operate as a battery. synchronous buck converter (Figure 1). Each channel includes two high power MOSFET gate drivers to control
Fast Transient Response
external N-channel MOSFETs QT and QB. These drivers The LTC1702A uses a fast 25MHz GBW op amp as an error have 0.5Ω output impedances and can carry well over an amplifier. This allows the compensation network to be designed with several poles and zeros in a more flexible VIN configuration than with a typical gm feedback amplifier. + The high bandwidth of the amplifier, coupled with the high CIN switching frequency and the low values of the external TG QT L LTC1702A EXT inductor and output capacitor, allow very high loop cross- SW VOUT + over frequencies. The low inductor value is the other half BG PGND QB COUT of the equation—with a typical value on the order of 1µH, 1702A F01 the inductor allows very fast di/dt slew rates. The result is superior transient response compared with conventional
Figure 1. Synchronous Buck Architecture
solutions. 1702afa 9