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LTC3443 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3443 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC3532 7 3532fc OPERATIO The LTC3532 provides high efficiency, low noise power for applications such as portable instrumentation, digital cameras, and MP3 players. The LTC proprietary topology allows input voltages above, below or equal to the output voltage by properly phasing the output switches. The error amp output voltage on VC determines the output duty cycle of the switches. Since VC is a filtered signal, it provides rejection of frequencies well below the switching frequency. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across the synchronous switch D and synchronous switch B are not required, but provide a lower voltage drop during the break-before-make time (typically 15ns). Schottky diodes will improve peak efficiency by typically 1% to 2%. High efficiency is achieved at light loads when Burst Mode operation is entered and the IC’s quiescent current drops to a low 35μA. LOW NOISE FIXED FREQUENCY OPERATION Oscillator The frequency of operation is programmed by an external resistor from RT to ground, according to the following equation: f(kHz) = 48,000 RT(kΩ) Error Amp The error amplifier is a voltage mode amplifier. The loop compensation components are configured around the amplifier (from FB to VC) to obtain stability of the converter. For improved bandwidth, an additional RC feedforward network can be placed across the upper feedback divider resistor. The voltage on SHDN/SS clamps the error amp output, VC, to provide a soft-start function. Internal Current Limit There are two different current limit circuits in the LTC3532. They have internally fixed thresholds which vary inversely with VIN. The first circuit is a high speed peak current limit comparator that will shut off switch A if the current exceeds 1.1A typical. The delay to output of this amplifier is typi- cally 50ns. A second amplifier will begin to source current into the FB pin to drop the output voltage once the peak input current exceeds 1A typical. This method provides a closed loop means of clamping the input current. During conditions where VOUT is near ground, such as during a short-circuit or during startup, this threshold is cut in half providing a fold back feature. For this current limit feature to be most effective, the Thevenin resistance from FB to ground should be greater than 100k. Reverse Current Limit During fixed frequency operation, the LTC3532 operates in forced continuous conduction mode. The reverse current limit amplifier monitors the inductor current from the out- put through switch D. Once the negative inductor current exceeds 340mA typical, the IC will shut off switch D. 4-Switch Control Figure 1 shows a simplified diagram of how the four internal switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3532 as a function of the internal control voltage, VCI. Depending on the control voltage, the IC will operate in either buck, buck/boost or boost mode. The VCI voltage is a level shifted voltage from the output of the error amp (VC) (see Figure 5). The four power switches are properly phased so the transfer between operating modes is continuous, smooth and transparent to the user. When VIN approaches VOUT the buck/boost region is reached where the conduction time of the 4-switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation will now be described. 3 SW1 4 SW2 PMOS A NMOS B 7 VIN PMOS D NMOS C 3532 F01 6 VOUT Figure 1. Simplified Diagram of Output Switches |
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