Datasheet LT3086 (Analog Devices) - 18

LT3086
applicaTions inForMaTion
and output voltage transients increase as the output voltage IN OUT VOUT is set above 0.4V. See Figure 4 and Transient Response LT3086 VIN RSET curves in the Typical Performance Characteristics section. SHDN SET 50 VIMON I TO ADC I I OUT MON/ILIM MON = 1000 0 CSET = 10nF RMON GND VIMON =IMON •RMON TAGE –50 CSET = 100pF 0.8V 3086 F05 ILIMIT =1000• R TION (mV) –100 MON –150 CSET = 1nF OUTPUT VOL DEVIA VIN = 5.5V
Figure 5. Output Current Monitor and External Current Limit
–200 VOUT = 5V CSET = 0 –250 COUT = 10µF rent limit, typically 2.4A, is always active and limits output 3 ∆IL = 210mA TO 2.1A current even if the I 2 LIM pin is grounded. In addition, inter- nal current limit foldback overrides external current limit LOAD 1 CURRENT (A) if the V 0 IN – VOUT differential voltage becomes excessive. 0 10 20 30 40 50 60 70 80 TIME (µs) Note that the output current monitor represents not just 3086 F04
Figure 4. Transient Response vs Feedforward Capacitor
the load current, but the current into the output capacitor as well. During startup and large load transients, the output Start-up time is affected by the use of a CSET feedforward current monitor indicates the current required to charge capacitor. Start-up time is directly proportional to the size the output capacitor in addition to the load current. To of the feedforward capacitor and output voltage. Settling prevent external current limit from engaging prematurely, time to 1% is approximately: set the external current limit above the maximum load current to allow the output capacitor to recover without 4.2• V t OUT •CSET being current limited. SETTLE = 50µA For external current limits set for less than 1A, connect a See the Start-Up Time vs CSET curve in the Typical Perfor- series 1k-10nF RC network from ILIM to ground to ensure mance Characteristics section.
If the LT3086 is configured
current limit loop stability. Adding an RC network from ILIM
for cable drop compensation, LTC does not recommend
to ground also delays the current monitor signal, allowing
using a feedforward capacitor because CSET filters the
output currents higher than the external current limit for a
CDC correction signal and transient response to load
limited duration. This is useful for applications with large
current changes degrades.
output capacitance that would otherwise trigger external current limit during startup and large load transients,
Output Current Monitor and External Current Limit
slowing output voltage recovery. To guarantee external Current out of the IMON pin is typically equal to 1/1000 current limit stability, ensure that the RC network from of the regulator’s output current. The output current ILIM to GND has a capacitor value equal to or greater than monitor maintains accuracy across the full input voltage 10nF and the resistor value is between 0.01• C–0.6 and range, even during dropout. A resistor, RMON, placed from 1k. C is the capacitor value in units of farads. LTC does IMON to ground, sets the voltage scale factor for use with not recommend an RC network other than the 1k-10nF analog-to-digital converters, as shown in Figure 5. For combination if using the cable drop compensation and example, with 442Ω for RMON, VIMON is set for 0.663V paralleling functions. when IOUT = 1.5A. To configure the output current monitor and external cur- External current limit activates if the voltage on the ILIM rent limit correctly, decide on the necessary current limit pin exceeds the typical 0.8V threshold. Tying the IMON and and full-scale monitor output voltage. Voltage is limited to ILIM pins together allows the user to program a desired 0.8V if IMON is tied to ILIM. External current limit is typically current limit based on the output current. An internal cur- set 10% to 20% above maximum load current to allow for 3086fb 18 For more information www.linear.com/LT3086 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts