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LTC1159CS 数据表(PDF) 13 Page - Linear Technology |
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LTC1159CS 数据表(HTML) 13 Page - Linear Technology |
13 / 20 page 13 LTC1159 LTC1159-3.3/LTC1159-5 the same process as in conventional applications, using either the internal divider (LTC1159-3.3, LTC1159-5) or an external divider with the adjustable version. Figure 15 in the Typical Applications shows a synchronous 12V to –12V converter that can supply up to 1A with better than 85% efficiency. By grounding the EXTVCC pin in the Figure 15 circuit, the entire 12V output voltage is placed across the driver and control circuits since the LTC1159 ground pins are at –12V. During start-up or short-circuit conditions, operating power is supplied by the internal 4.5V regulator. The shutdown signal is level-shifted to the negative output rail by Q3, and Q4 ensures that Q1 and Q2 remain off during the entire shutdown sequence. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100 – (L1 + L2 + L3 + ...) where L1, L2, etc., are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1159 circuits: 1) LTC1159 VIN current, 2) LTC1159 VCC current, 3) I 2R losses and 4) P-channel transition losses. 1. LTC1159 VIN current is the DC supply current given in the electrical characteristics which excludes MOSFET driver and control currents. VIN current results in a small (< 1%) loss which increases with VIN. 2. LTC1159 VCC current is the sum of the MOSFET driver and control circuit currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VCC to ground. The resulting dQ/dt is a current out of VCC which is typically much larger than the control circuit current. In continuous mode, IGATECHG ≈ f (QP + QN), where QP and QN are the gate charges of the two MOSFETs. By powering EXTVCC from an output-derived source, the additional VIN current resulting from the driver and control currents will be scaled by a factor of (Duty Cycle)/(Effi- ciency). For example in a 20V to 5V application, 10mA of VCC current results in approximately 3mA of VIN current. This reduces the mid-current loss from 10% or more (if the driver was powered directly from VIN) to only a few percent. 3. I 2R losses are easily predicted from the DC resistances of the MOSFET, inductor and current shunt. In continuous mode all of the output current flows through L and RSENSE, but is “chopped” between the P-channel and N-channel MOSFETs. If the two MOSFETs have approxi- mately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L and RSENSE to obtain I2R losses. For example, if each RDS(ON) = 0.1Ω, RL = 0.15Ω, and RSENSE = 0.05Ω, then the total resistance is 0.3 Ω. This results in losses ranging from 3% to 12% as the output current increases from 0.5A to 2A. I2R losses cause the efficiency to roll-off at high output currents. 4. Transition losses apply only to the P-channel MOSFET, and only when operating at high input voltages (typically 20V or greater). Transition losses can be estimated from: Transition Loss ≈ 5(VIN)2(IMAX)(CRSS)(f) Other losses including CINandCOUT ESRdissipativelosses, Schottky conduction losses during dead time, and inductor core losses, generally account for less than 2% total additional loss. Auxiliary Windings—Suppressing Burst Mode Operation The LTC1159 synchronous switch removes the normal limitation that power must be drawn from the inductor primary winding in order to extract power from auxiliary windings. With synchronous switching, auxiliary out- puts may be loaded without regard to the primary output load, providing that the loop remains in continuous mode operation. Burst Mode operation can be suppressed at low output currents with a simple external network that cancels the 0.025V minimum current comparator threshold. This tech- nique is also useful for eliminating audible noise from APPLICATIO S I FOR ATIO |
类似零件编号 - LTC1159CS |
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类似说明 - LTC1159CS |
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