lt3686a Linear Technology Corporation, lt3686a Datasheet
lt3686a
Available stocks
Related parts for lt3686a
lt3686a Summary of contents
Page 1
... Soft-start and frequency foldback eliminate input current surge during start-up. An optional internal regulated active load at the output via the BD pin keeps the LT3686A at full switching frequency at light loads, resulting in low, predictable output ripple above the audio and AM bands. ...
Page 2
... For more information on tape and reel specifi cations, go to: 2 SYNC/MODE Voltage ...................................................6V Operating Junction Temperature Range (Note 2) LT3686AE ........................................... –40°C to 125°C LT3686AI............................................ –40°C to 125°C LT3686AH .......................................... –40°C to 150°C Storage Temperature Range ................... –65°C to 150°C Lead Temperature (MSE Package Only, Soldering, 10 Sec) ................................................ 300° ...
Page 3
... The LT3686AI is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3686AH is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes; operating lifetime is derated for junction temperatures greater than 125° ...
Page 4
... LT3686A TYPICAL PERFORMANCE CHARACTERISTICS 3.3V Effi ciency (2MHz) OUT 90 80 MODE/SYNC > 0. MODE/SYNC < 0. 12V 3.3V OUT 6.8μ 2MHz 0 800 0 200 400 600 1000 LOAD CURRENT (mA) 3686A G01 5V Maximum Load Current OUT 2.0 1.8 TYPICAL 1.6 1.4 MINIMUM 1.2 1.0 0.8 0.6 0 OUT L = 10μH 0 2MHz ...
Page 5
... OUT IN Frequency (2MHz) 25 MODE/SYNC > 0.8 20 MODE/SYNC < 0 OUT L = 6.8μ 2MHz 0 100 0 500 1000 LOAD CURRENT (mA) LT3686A T = 25°C unless otherwise noted. A Soft-Start/Track vs Frequency (1MHz) 1200 1000 800 600 400 200 0 0 500 1000 1500 150 SS (mV) 3686A G10 Switch Current Limit vs Temperature 3 ...
Page 6
... LT3686A TYPICAL PERFORMANCE CHARACTERISTICS 3.3V Typical Minimum Input OUT Voltage 7 6 MODE/SYNC < 0 MODE/SYNC > 0 3.3V OUT 15μ 1MHz 100 1000 I (mA) LOAD 3686A G18 Light Load Discontinuous Mode Waveform V SW 2V/DIV I L 200mA/DIV 3686A G21 200ns/DIV V = 10V 3.3V OUT L = 6.8μ ...
Page 7
... The SYNC/MODE Pin also acts as mode select for the BD active load; when it is driven with pulses or tied above 0.8V, the LT3686A will prevent pulse skipping at light loads by regulating an active load on the BD pin. To disable the active load, tie SYNC/MODE to below 0.4V. ...
Page 8
... LT3686A BLOCK DIAGRAM INT REG EN/ULVO UVLO OFF ON OVLO R5 1.27V – ACTIVE LOAD + SLOPE C COMP 0.8V OSC FREQUENCY FOLDBACK RT SYNC/MODE R3 BOOST Q1 C3 DRIVER GND 3686A BD V OUT C1 3686af ...
Page 9
... OPERATION The LT3686A is a current mode step-down regulator. The EN/UVLO pin is used to place the LT3686A in shut- down. The 1.27V threshold on the EN/UVLO pin can be programmed by an external resistor divider (R4, R5) to disable the LT3686A. When the EN/UVLO pin is driven above 1.27V, an internal regulator provides power to the control circuitry ...
Page 10
... R4 also sets the hysteresis voltage for the programmable UVLO: Hysteresis = R4 • 2.4μA Once V drops below the programmed voltage, the IN LT3686A will enter a low quiescent current state (Iq ≈ 15μA). To shutdown the LT3686A completely (Iq < 1μA), reduce EN/UVLO pin voltage to below 0.4V. 10000 1000 100 ...
Page 11
... V Minimum On Time As the input voltage is increased, the LT3686A is required to switch for shorter periods of time. Delays associated with turning off the power switch dictate the minimum on time of the part. The minimum on time for the LT3686A is 100ns (Figure 3 10V/DIV ...
Page 12
... Finite transistor band- width limits the speed at which the power switch can be turned on and off, effectively setting the minimum on-time of the LT3686A. For a given output voltage, the minimum on-time determines the maximum input voltage to remain in continuous mode operation outlined in the Minimum On Time section of the data sheet ...
Page 13
... Input Capacitor Bypass the input of the LT3686A circuit with a 2.2μF or higher value ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and ap- plied voltage and should not be used. A 2.2μF ceramic is adequate to bypass the LT3686A and will easily handle the ripple current ...
Page 14
... EMI. A 2.2μF capacitor is capable of this task, but only placed close to the LT3686A and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maxi- mum input voltage rating of the LT3686A ...
Page 15
... V SANYO OUT 50mV/DIV 4TPB100M AC 3686A F07g Figure 7. Transient Load Response of the LT3686A with Different Output Capacitors as the Load Current Is Stepped from 0.25A to 1A 12V 3.3V 6.8μH , Frequency = 2MHz IN OUT BOOST and BD Pin Considerations Capacitor C3 and the internal boost diode are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μ ...
Page 16
... The BD pin can also be tied to V (Figure 8c) but V load circuit is automatically disabled. is the CATCH The minimum operating voltage of an LT3686A applica- tion is limited by the undervoltage lockout (3.6V) and by . BOOST the maximum duty cycle as outlined above. For proper start-up, the minimum input voltage is also limited by the boost circuit ...
Page 17
... APPLICATIONS INFORMATION the LT3686A is turned on with its EN/UVLO pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly ...
Page 18
... The LT3686A achieves this goal by commanding the minimum load necessary to keep itself at full switching frequency, hence the circuitry is called an active load. As the LT3686A approaches minimum on time in dis- continuous mode, its power switch transitions smoothly 18 into a fi xed on time, fi xed frequency, open loop current source. Instead of controlling switch current, the internal error amplifi ...
Page 19
... FB voltage is Figure 11. LT3686A Confi gured to Track Voltage on SS Pin 1.2nF Figure 12. To Soft Start the LT3686A, Add a Capacitor to the SS Pin regulated to the voltage at the SS pin until it exceeds 0.8V then regulated to the reference 0.8V. Soft-start also reduces the oscillator frequency to avoid hitting current limit during start-up ...
Page 20
... LT3686A will reduce its frequency until the valley current is 1.7A (Figure 13). There is another situation to consider in systems where the output will be held high when the input to the LT3686A is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3686A’ ...
Page 21
... APPLICATIONS INFORMATION the LT3686A into an energized supply, the input network should be designed to prevent this overshoot. Figure 15 shows the waveforms that result when an LT3686A circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The fi rst plot is the response with a 2.2μ ...
Page 22
... The LT3686A uses current mode control to regulate the output. This simplifi es loop compensation. In particular, the LT3686A does not require the ESR of the output ca- pacitor for stability allowing the use of ceramic capacitors to achieve low output ripple and small circuit size. Figure 16 shows an equivalent circuit for the LT3686A control loop. The error amp is a transconductance amplifi ...
Page 23
... BOOST nodes should be as small as possible. Finally, keep the FB node small so that the ground pin and ground traces will shield it from the SW and BOOST nodes. Include vias near the exposed GND pad of the LT3686A to help remove heat from the LT3686A to the ground plane. High Temperature Considerations The die temperature of the LT3686A must be lower than the maximum rating of 125° ...
Page 24
... LT3686A BD IN BOOST EN/UVLO 0.22μF SYNC/MODE SW 100μ 301k FB GND 61.9k 10k Figure 18. 25V Step-Down Converter 3.3V Step-Down Converter 2.2μF EN/UVLO BOOST 0.22μF LT3686A 6.8μH SW SYNC/MODE SS DA 31.6k 1nF RT FB GND 61.9k 10k 0.22μF 3.3μH V OUT 1.8V 1.2A 12.4k 10k 47μF 3686A TA02c 0.22μF V OUT 500mA 10μ ...
Page 25
... BOOST LT3686A SYNC/MODE 1nF RT FB GND 61.9k 5V Step-Down Converter 2.2μF EN/UVLO BOOST LT3686A SYNC/MODE 1nF RT FB GND 61.9k LT3686A 0.22μF 6.8μH V OUT 3.3V 1.2A 31.6k 22μF 10k 3686A TA02d 0.22μF 4.7μH V OUT 2.5V 1.2A 21.5k 10k 33μF 3686A TA02e 0.22μF 10μH V OUT 5V 1.2A 52.3k 10k 15μF 3686A TA02f ...
Page 26
... LT3686A PACKAGE DESCRIPTION 2.845 0.102 (.112 .004) 5.23 (.206) MIN 0.42 0.038 (.0165 .0015) TYP RECOMMENDED SOLDER PAD LAYOUT DETAIL “A” 0.254 (.010) GAUGE PLANE DETAIL “A” 0.18 (.007) NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. ...
Page 27
... ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE LT3686A R = 0.125 0.40 0.10 TYP 6 10 PIN 1 NOTCH ...
Page 28
... Linear Technology Corporation 28 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 ● 5V Step-Down Converter 2.2μF EN/UVLO BOOST 0.22μF LT3686A SYNC/MODE 1nF RT FB GND 61.9k 10k COMMENTS V : 3.6V to 37V, Transient to 55V 10-Pin 3mm × 3mm DFN Package ...