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LTC2922CF-2.5 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2922CF-2.5 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC2921/LTC2922 Series 11 29212fa APPLICATIO S I FOR ATIO Figure 5. Basic Monitor Connection Setting the Supply Monitor Levels The LTC2921 and LTC2922 series both feature low 0.5V monitoring thresholds with tight 1% accuracy. To set a supply monitoring level tightly, design a precision ratio resistive divider to relate the lowest valid supply voltage to the maximum specified monitor threshold voltage. Use resistors with 1% tolerance or better to limit the error due to mismatch. The basic resistive divider connection for supply monitoring is shown in Figure 5. 2921/22 F05 LTC2922 V1 GND GATE VQ1 Q1 CGATE LOAD IMON IA1 RB1 RY1 RZ1 RA1 VL1 VV1 VSRC1 RG1 10 Ω VOUT VFB GND DC/DC CONVERTER +– ±0.1µA First, divide the nominal monitor threshold voltage by an acceptable bias current (IA1), and choose a nearby stan- dard value for resistor RA1 (see Equation 1). Next, calculate the bounds on the value of RB1 that guarantee that the divided minimum supply voltage ex- ceeds the maximum specified monitor threshold voltage, and that the minimum specified overvoltage threshold exceeds the divided maximum supply voltage. Use Equa- tions 2 and 3 to calculate RB1(MAX) and RB1(MIN) from RA1, the resistor tolerance (RTOL), the supply voltage, the monitor threshold and overvoltage specifications, and the monitor pin leakage current specification. When the integrated remote sensing switch is closed, the DC/DC converter will compensate for the IR drop from drain to source of the external N-channel FET (VQ1(ON)) by increasing the supply voltage by the same amount. Calcu- late with VQ1(ON)(MAX) = 0V if the remote sense switch is not used. R V I A A 1 1 0 500 = . (1) R R RTOL RTOL VV VA R B MAX A SRC MIN A 1 1 1 1 1 1 0 505 0 505 0 1 () () • – • –. .. • = + +µ (2) RR RTOL RTOL VV V VA R B MIN A SRC MAX Q ON MAX A 11 11 1 1 1 0 665 0 665 0 1 () () ( )() • – • –. .– . • = + + µ (3) Choose a standard resistor value for RB1 that satisfies the inequality of Equation 4. RB1(MIN) ≤ RB1 ≤ RB1(MAX) (4) When several standard values meet the requirement, choose the value closest to RB1(MAX) to set the tightest monitor threshold. This also allows more headroom for larger VQ1(ON)(MAX). Alternatively, choose the standard value closest to RB1(MIN) to set the tightest overvoltage threshold. All four monitor input voltages must be between the monitor threshold and the overvoltage threshold for the turn-on sequence to begin. Connect unneeded monitor input pins to any of the utilized monitor input pins. Selecting the External N-Channel MOSFETs The GATE pin drives the gate of external N-channel MOSFETs above VCC to connect the supplies to the loads. The GATE drive voltage provided by the LTC2921/LTC2922 series is best suited to logic-level and sublogic-level power MOSFETs. To achieve the lowest switch resistance, the VCC pin must be connected to the highest supply voltage. Consider the application requirements for current, turnoff speed, on-resistance, gate-source voltage specification, etc. Refer to the Electrical Specifications and Typical Performance Curves to determine the GATE voltages for given VCC voltages over the required range of conditions. Calculate the minimum gate drive voltage for each moni- tored supply for use in selecting the FETs. Check the maximum GATE voltage against the FETs’ gate-source |
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