Datasheet LTC1174, LTC1174-3.3, LTC1174-5 (Analog Devices) - 10

LTC1174 LTC1174-3.3/LTC1174-5
U U W U APPLICATIO S I FOR ATIO
VOUT LTC1174-5 regulator and also to one or more loads in 6.8nF** R2 parallel with the the regulator’s VIN. If the battery is dis- 1 connected while the LTC1174/LTC1174-3.3/LTC1174-5 LTC1174 VFB regulator is supplying a light load and one of the parallel 100pF* R1 circuits is a heavy load, the input capacitor of the LTC1174/ LTC1174-3.3/LTC1174-5 regulator could be pulled down * ADJUSTABLE APPLICATIONS 1174 F04 ** LOW NOISE APPLICATIONS faster than the output capacitor, causing the absolute maximum ratings to be exceeded. The result is often a
Figure 4. LTC1174 Adjustable Configuration
latchup which can be destructive if VIN is reapplied. Bat- tery disconnect is possible as a result of mechanical stress, INPUT VOLTAGE 4V TO 12V bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. The user needs to assess 6 + 47µF* his/her application to determine whether this situation 0.1µF 16V VIN × 3 2 8 could occur. If so, additional protection is necessary. LBIN SHUTDOWN 2 1 LBOUT VOUT Prevention against latchup can be accomplished by sim- 7 5 I SW ply connecting a Schottky diode across the SW and V PGM IN 50µH** LTC1174HV-5 + 47µF* pins as shown in Figure 6. The diode will normally be GND MBRS140T3 16V ×2 reverse biased unless VIN is pulled below VOUT at which 4 VOUT –5V time the diode will clamp the (VOUT – VIN) potential to less 45mA * AVX TPSD476K016 than the 0.6V required for latchup. Note that a low leakage 1174 F05 ** COILTRONICS CTX50-4 Schottky should be used to minimize the effect on no-load
Figure 5. Positive-to-Negative 5V Converter
supply current. Schottky diodes such as MBR0530, BAS85 and BAT84 work well. Another more serious effect of the output voltage is now taken off the GND pin. Therefore, protection diode leakage is that at no load with nothing to the maximum input voltage is now determined by the provide a sink for this leakage current, the output voltage difference between the absolute maximum voltage rating can potentially float above the maximum allowable toler- and the output voltage. A maximum of 12V is specified in ance. To prevent this from occuring, a resistor must be Figure 5, giving the circuit a 1.5V of headroom for VIN. Note connected between VOUT and ground with a value low that the circuit can operate from a minimum of 4V, making enough to sink the maximum possible leakage current. it ideal for a 4 NiCad cell application. For a higher output current circuit, please refer to the Typical Applications LATCHUP PROTECTION section. SCHOTTKY
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW (Pin 5) can VIN SW VOUT + never exceed V LTC1174 IN (Pin 6) by more than 0.3V. Normally this LTC1174-3.3 situation should never occur. It could, however, if the LTC1174-5 1174 F06 output is held up while the supply is pulled down. A con- dition where this could potentially occur is when a battery
Figure 6. Preventing Absolute Maximum
is supplying power to an LTC1174/LTC1174-3.3/
Ratings from Being Exceeded
1174fe 10