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LT1614 Datasheet(PDF) 11 Page - Linear Technology |
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LT1614 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1614 APPLICATIONS INFORMATION Shutdown Pin The LT1614 has a Shutdown pin (SHDN) that must be grounded to shut the device down or tied to a voltage equal or greater than VIN to operate. The shutdown circuit is shown in Figure 13. Note that allowing SHDN to float turns on both the start- up current (Q2) and the shutdown current (Q3) for VIN > 2VBE. The LT1614 doesn’t know what to do in this situation and behaves erratically. SHDN voltage above VIN is al- lowed. This merely reverse-biases Q3’s base emitter junc- tion, a benign condition. The low-battery detector is dis- abled when SHDN is low. Figure 13. Shutdown Circuit VIN Q3 SHUTDOWN CURRENT R2 400k 200k Q2 1614 F13 Q1 START-UP CURRENT SHDN Low-Battery Detector The LT1614’s low-battery detector is a simple PNP input gain stage with an open collector NPN output. The nega- tive input of the gain stage is tied internally to a 200mV reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 14 details hookup. R1 and R2 need only be low enough in value so that the bias current of the LBI pin doesn’t cause large errors. For R2, 100k is adequate. The 200mV refer- ence can also be accessed as shown in Figure 15. The low- battery detect is not operative when the device is shut down. Coupled Inductors The applications shown in this data sheet use two un- coupled inductors because the Murata units specified are small and inexpensive. This topology can also be used with a coupled inductor as shown in Figure 16. Be sure to get the phasing right. LBO LBI TO PROCESSOR R1 1M R2 100k VIN LT1614 1614 F14 3.3V GND 200mV INTERNAL REFERENCE – + R1 = VLB – 200mV 2 µA Figure 14. Setting Low-Battery Detector Trip Point VIN LT1614 LBI LBO 200k 10 µF GND 10k 1614 F15 2N3906 VREF 200mV + Figure 15. Accessing 200mV Reference VIN SW LT1614 L1A 10 µH VIN 5V D1 NFB SHDN 69.8k 24.9k 100k 1nF VOUT –5V 200mA C1, C2: AVX TAJB336M010 C3: AVX 1206CY106 D1: MBR0520 L1: COILTRONICS CTX10-1 1614 F16 C1 33 µF C2 33 µF VC GND L1B 10 µH C3 1 µF •• + Figure 16. 5V to – 5V Converter with Coupled Inductor |
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