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LT1991 Datasheet(PDF) 11 Page - Linear Technology |
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LT1991 Datasheet(HTML) 11 Page - Linear Technology |
11 / 22 page LTC6101/LTC6101HV 11 6101fh APPLICATIONS INFORMATION 6101 F03b – + – + – + R5 7.5k VIN 301 301 VOUT ILOAD LTC6101 RSENSE LO 100m M1 Si4465 10k CMPZ4697 7.5k VIN 1.74M 4.7k Q1 CMPT5551 40.2k 3 4 5 6 1 2 8 7 619k HIGH RANGE INDICATOR (ILOAD > 1.2A) VLOGIC (3.3V TO 5V) LOW CURRENT RANGE OUT 2.5V/A (V LOGIC +5V ) ≤ V IN ≤ 60V 0 ≤ ILOAD ≤ 10A HIGH CURRENT RANGE OUT 250mV/A 301 301 LTC6101 RSENSE HI 10m VLOGIC BAT54C LTC1540 V+ V– OUT –IN +IN V+ V– OUT –IN +IN Selection of External Input Resistor, RIN The external input resistor, RIN, controls the transconduc- tance of the current sense circuit. Since IOUT = VSENSE/RIN, transconductance gm = 1/RIN. For example, if RIN = 100, then IOUT = VSENSE/100 or IOUT = 1mA for VSENSE = 100mV. RIN should be chosen to allow the required resolution while limiting the output current. At low supply voltage, IOUT may be as much as 1mA. By setting RIN such that the largest expected sense voltage gives IOUT = 1mA, then the maximum output dynamic range is available. Output dynamic range is limited by both the maximum allowed output current and the maximum allowed output voltage, as well as the minimum practical output signal. If less dynamic range is required, then RIN can be increased accordingly, reducing the max output current and power dissipation. If low sense currents must be resolved accurately in a system that has very wide dynamic range, a smaller RIN than the max current spec allows may be used if the max current is limited in another way, such as with a Schottky diode across RSENSE (Figure 3a). This will reduce the high current measurement accuracy by limiting the result, while increasing the low current measurement resolution. Figure 3b. Dual LTC6101s Allow High-Low Current Ranging V+ LOAD DSENSE 6101 F03a RSENSE Figure 3a. Shunt Diode Limits Maximum Input Voltage to Allow Better Low Input Resolution Without Overranging This approach can be helpful in cases where occasional large burst currents may be ignored. It can also be used in a multirange configuration where a low current circuit is added to a high current circuit (Figure 3b). Note that a comparator (LTC1540) is used to select the range, and transistor M1 limits the voltage across RSENSE LO. Care should be taken when designing the board layout for RIN, especially for small RIN values. All trace and inter- connect impedances will increase the effective RIN value, causing a gain error. In addition, internal device resistance will add approximately 0.2Ω to RIN. |
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