Datasheet ADA4800 (Analog Devices) - 10

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ADA4800 THEORY OF OPERATION
The ADA4800 is a buffer integrated with an active load. Each Figure 22 illustrates an ADA4800 application configuration for element (the active load and the buffer) operates independently, using this power save feature. as described in the following sections. An external resistor connected between the ISF and the
SETTING ACTIVE LOAD CURRENT WITH PIN 6 (ISF)
microcontroller GPO pin determines the amount of current that flows into the input pin. This current can be calculated The ISF pin is used to establish the value of the active current by using Equation 1 and Equation 2. load (IAL). Set the ISF current using Equation 1. V −1.55 V
SETTING BANDWIDTH WITH PIN 4 (IDRV)
I = ISF (1) ISF R + 3 kΩ The IDRV pin establishes the buffer’s ICC quiescent current. ISF As ICC is increased, power dissipation and bandwidth both where: increase. Set the current using Equation 3. VISF is referenced to Pin 2. VISF can be an external voltage source, V − V . 0 8 V CC, or a GPO output as explained in the following paragraphs. I = IDRV (3) IDRV R R + 28 ISF is the external resistor between the ISF pin and VISF. kΩ IDRV The active load current (into the IN pin) is directly proportional where: to IISF and can be calculated by Equation 2. VIDRV is referenced to Pin 2. VIDRV can be an external voltage source or VCC. IAL = IISF × 27 (2) RIDRV is the external resistor between the IDRV pin and VIDRV. The ADA4800 allows for additional power savings by reducing the active load current. The active load current can be logically The ICC current is directly proportional to IIDRV and can be controlled by connecting the ISF pin to any general-purpose calculated by Equation 4. output (GPO) pin of a system microcontroller through an ICC = IIDRV × 26 (4) external resistor. A GPO logic high enables the flow of the Applying –VS to the IDRV pin shuts down the buffer. active load current. Appling –VS or connecting a high-Z to the ISF pin places the ADA4800 into power save mode by shutting down the active load current. Rev. A | Page 10 of 16 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS BUFFER ELECTRICAL CHARACTERISTICS ACTIVE CURRENT LOAD ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUIT THEORY OF OPERATION SETTING ACTIVE LOAD CURRENT WITH PIN 6 (ISF) SETTING BANDWIDTH WITH PIN 4 (IDRV) APPLICATIONS INFORMATION OPEN SOURCE CCD OUTPUT BUFFER POWER SAVE MODE POWER SUPPLY BYPASSING POWER SEQUENCING OUTLINE DIMENSIONS ORDERING GUIDE