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LTC1703 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1703 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 36 page  10 LTC1703 of the equation—with a typical value on the order of 1 µH, the inductor allows very fast di/dt slew rates. The result is superior transient response compared with conventional solutions. High Efficiency The LTC1703 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. VID Programming The LTC1703 includes an onboard feedback network that programs the output voltage at side 1 in accordance with the Intel Mobile VID specification (Table 1). The network includes a 10k resistor (R11) connected from SENSE to FB1, and a variable value resistor (RB1) from FB1 to SGND, with the value set by the digital code present at the VID0:4 pins. SENSE should be connected to VOUT1 to allow the network to monitor the output voltage. No additional feedback components are required to set the output volt- age at controller 1, although loop compensation compo- nents are still required. Each VID n pin includes an internal 40k pull-up resistor, allowing it to float high if left uncon- nected. The pull-up resistors are connected to VCC through diodes (see Block Diagram), allowing the VID n pins to be pulled above VCC without damage. Note that codes 01111 and 11111, defined by Intel to indicate “no CPU present,” do generate output voltages at VOUT1 (1.25V and 0.9V, respectively). Note also that controller 2 on the LTC1703 is not connected to the VID circuitry, and works independently from controller 1. ARCHITECTURE DETAILS The LTC1703 dual switching regulator controller includes two 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, the VID circuitry at side 1, and the FCB and FAULT pins, which affect both chan- nels. In the following discussions, when a pin is referred to without mentioning which side is involved, that discus- sion applies equally to both sides. Switching Architecture Each half of the LTC1703 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 0.5 Ω output impedances and can carry well over an amp of continuous current with peak currents up to 5A to slew large MOSFET gates quickly. The external MOSFETs are connected with the drain of QT attached to the input supply and the source of QT at the switching node SW. QB is the synchronous rectifier with its drain at SW and its source at PGND. SW is connected to one end of the inductor, with the other end connected to VOUT. The output capacitor is connected from VOUT to PGND. When a switching cycle begins, QB is turned off and QT is turned on. SW rises almost immediately to VIN and the inductor current begins to increase. When the PWM pulse finishes, QT turns off and one nonoverlap interval later, QB turns on. Now SW drops to PGND and the inductor current decreases. The cycle repeats with the next tick of the master clock. The percentage of time spent in each mode is controlled by the duty cycle of the PWM signal, which in turn is controlled by the feedback amplifier. The master clock runs at a 550kHz rate and turns QT once every 1.8 µs. In a typical application with a 5V input and a 1.5V output, the duty cycle will be set at 1.5/5 × 100% or 30% by the feedback loop. This will give roughly a 540ns on-time for QT and a 1.26 µs on-time for QB. Figure 1. Synchronous Buck Architecture + TG LTC1703 BG SW PGND COUT 1703 F01 + CIN QT QB VOUT VIN LEXT APPLICATIO S I FOR ATIO |
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