Datasheet ADP151 (Analog Devices) - 13

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Data Sheet ADP151 ENABLE FEATURE 3.5
The ADP151 uses the EN pin to enable and disable the VOUT
3.0
pin under normal operating conditions. As shown in Figure 30, when a rising voltage on EN crosses the active threshold,
2.5 GE
VOUT turns on. When a falling voltage on EN crosses the
2.0
inactive threshold, VOUT turns off.
OLTA V LE 1.5 3.0 B A N E 1.0 2.5 ENABLE 0.5 3.3V 2.8V 2.0 1.1V 0 0 50 100 150 200 250 300 350 400 450
030
UT 1.5 O TIME (µs) V
08627- Figure 32. Typical Start-Up Behavior
1.0 ADJUSTABLE OUTPUT VOLTAGE OPERATION 0.5
The unique architecture of the ADP151 makes an adjustable version difficult to implement in silicon. However, it is possible
0 0 0.5 1.0 1.5 2.0 2.5
to create an adjustable regulator at the expense of increasing the 028
ENABLE VOLTAGE
quiescent current of the regulator circuit. 08627- Figure 30. ADP151 Typical EN Pin Operation The ADP151, and similar LDOs, are designed to regulate the As shown in Figure 30, the EN pin has hysteresis built in to output voltage (VOUT) appearing at the VOUT pin with respect prevent on and off oscil ations that can occur due to noise on to the GND pin. If the GND pin is at a potential other than 0 V the EN pin as this pin passes through the threshold points. (for example, at the offset voltage (VOFFSET)), the ADP151 The EN pin active and inactive thresholds are derived from the output voltage is VOUT + VOFFSET. By taking advantage of this VIN voltage. Therefore, these thresholds vary with changing behavior, it is possible to create an adjustable ADP151 circuit input voltage. Figure 31 shows typical EN active and inactive that retains most of the desirable characteristics of the ADP151. thresholds when the input voltage varies from 2.2 V to 5.5 V.
V V IN OUT 1 VIN VOUT 5 1200 C1 U1 C2 2 GND 1000 3 EN NC 4 V V EN RISE OFFSET 800 GE R1 R2 C3
131
VEN FALL OLTA VOUT = VLDO × (1 + R2/R1) 600 V
08627-
LE
Figure 33. Adjustable LDO Using the ADP151
B A N 400 E
The circuit shown in Figure 33 is an example of an adjustable LDO using the ADP151. A stable VOFFSET voltage is created by
200
passing a known current through R2. The current through R2 is determined by the voltage across R1. Because the voltage across
0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
029 R1 is set by the voltage between VOUT and GND, the current
INPUT VOLTAGE
08627- passing through R2 is fixed, and VOFFSET is stable. Figure 31. Typical EN Pin Thresholds vs. Input Voltage To minimize the effect variation of the ADP151 ground current The ADP151 uses an internal soft start to limit the inrush (IGND) with load, it is best to keep R1 as small as possible. It is current when the output is enabled. The start-up time for the also best to size the current passing through R2 to at least 20× 3.3 V option is approximately 160 µs from the time the EN greater than the maximum expected ground current. active threshold is crossed to when the output reaches 90% of its final value. As shown in Figure 32, the start-up time is dependent on the output voltage setting. Rev. H | Page 13 of 23 Document Outline FEATURES APPLICATIONS TYPICAL APPLICATION CIRCUITS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS INPUT AND OUTPUT CAPACITOR, RECOMMENDED SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL DATA THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS INFORMATION CAPACITOR SELECTION Output Capacitor Input Bypass Capacitor Input and Output Capacitor Properties ENABLE FEATURE ADJUSTABLE OUTPUT VOLTAGE OPERATION CURRENT-LIMIT AND THERMAL OVERLOAD PROTECTION THERMAL CONSIDERATIONS PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE AUTOMOTIVE PRODUCTS