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LT3686

3686fc

ApplicAtions

37V/1.2A Step-Down

Regulator in 3mm × 3mm DFN

The LT

3686 is a current mode PWM step-down DC/DC

converter with an internal 1.2A power switch, packaged in

10-lead 3mm × 3mm DFN. The wide input range of 3.6V to

37V makes the LT3686 suitable for regulating power from

a wide variety of sources, including 24V industrial supplies

and automotive batteries. Its high maximum frequency al-

lows the use of tiny inductors and capacitors, resulting in

a very small solution. Operating frequency above the AM

band avoids interfering with radio reception, making the

LT3686 particularly suitable for automotive applications.

Cycle-by-cycle current limit and DA current sense provide

protection against fault conditions. 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 LT3686 at full switching frequency

at light loads, resulting in low, predictable output ripple

above the audio and AM bands. Internal compensation and

an internal boost diode reduce external component count.

L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners.

3.3V Step-Down Converter

MODE

6V TO 37V

OUT

3.3V

1.2A

0.22µF

MBRM140

2.2µF

10nF

15.4k

22µF

10k

6.8µH

31.6k

GND

LT3686

3686 TA01a

EN/UVLO

BOOST

6V TO 16V AT 2.1MHz)

12V

Efficiency (2.1MHz)

FeAtures Description

Automotive Systems

Battery-Powered Equipment

Wall Transformer Regulation

Distributed Supply Regulation

Wide Input Range:

Operation from 3.6V to 37V

Overvoltage Lockout Protects Circuit Through

55V Transients

Low Minimum On-Time:

Converts 16V

to 3.3V

OUT

at 2MHz

1.2A Output Current

Adjustable Frequency: 300kHz to 2.5MHz

Constant Switching Frequency at Light Loads

Tracking and Soft-Start

Precision UVLO

Short-Circuit Robust

in Shutdown <1µA

Internally Compensated

10-Lead 3mm × 3mm DFN Package

typicAl ApplicAtion

LOAD CURRENT (mA)

EFFICIENCY (%)

400

800

1000

3686 TA01b

200

600

1200

OUT

3.3V

OUT

Summary of content (28 pages)

PAGE 1
LT3686 37V/1.2A Step-Down Regulator in 3mm × 3mm DFN Features n n n n n n n n n n n Description Wide Input Range: Operation from 3.6V to 37V Overvoltage Lockout Protects Circuit Through 55V Transients Low Minimum On-Time: Converts 16VIN to 3.3VOUT at 2MHz 1.2A Output Current Adjustable Frequency: 300kHz to 2.
PAGE 2
LT3686 Absolute Maximum Ratings Pin Configuration (Note 1) Input Voltage (VIN).....................................................55V BOOST Voltage..........................................................55V BOOST Pin Above SW Pin..........................................25V FB Voltage....................................................................6V EN/UVLO Voltage.......................................................55V BD Voltage.................................................................
PAGE 3
LT3686 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VEN/UVLO ≥ 1.32V. PARAMETER CONDITIONS Quiescent Current at Shutdown Quiescent Current MIN TYP MAX VEN/UVLO < 0.4V VEN/UVLO = 1V 0.1 10 1 15 µA µA Not Switching, MODE ≤ 0.4V Not Switching, MODE ≥ 0.8V 1.1 1.2 1.3 1.4 mA mA 3.4 3.6 V Internal Undervoltage Lockout Overvoltage Lockout l 37 38 39 V l 0.790 0.
PAGE 4
LT3686 Typical Performance Characteristics 5VOUT Efficiency 3.3VOUT Efficiency (2MHz) 90 30 VIN = 12V VOUT = 3.3V L = 6.8µH f = 2MHz 20 10 0 200 800 1000 600 400 LOAD CURRENT (mA) 50 MODE < 0.4V 40 30 10 0 200 800 1000 600 400 LOAD CURRENT (mA) 3686 G01 10 20 VIN (V) 30 40 3686 G03 Internal Undervoltage Lockout (UVLO) 4.0 700 3.5 600 MINIMUM VSW (mV) IOUT (A) 0 800 1.0 0.8 500 400 3.0 300 0.6 0.
PAGE 5
LT3686 Typical Performance Characteristics Switching Frequency vs Temperature Switching Frequency vs RT 2.20 FREQUENCY (MHz) 250 RT (kΩ) 200 150 100 50 0 Soft-Start/Track vs Frequency (1MHz) 1200 RT = 15.4k 2.15 1000 2.10 800 FREQUENCY (kHz) 300 TA = 25°C unless otherwise noted. 2.05 2.00 0.5 1 1.5 FREQUENCY (MHz) 2 1.90 –50 2.
PAGE 6
LT3686 Typical Performance Characteristics 3.3VOUT Typical Minimum Input Voltage 8 6 7 MODE < 0.4 4 MODE < 0.4 VSW 2V/DIV MODE > 0.8 5 MODE > 0.8 3 IL 200mA/DIV 4 200ns/DIV 3 2 2 VOUT = 3.3V L = 15µH f = 1MHz 1 0 Continuous Mode Waveform 6 VIN (V) VIN (V) 5VOUT Typical Minimum Input Voltage 7 5 TA = 25°C unless otherwise noted. 1 10 100 1000 VOUT = 5V L = 22µH f = 1MHz 1 0 1 ILOAD (mA) 10 100 VIN = 10V VOUT = 3.3V L = 6.
PAGE 7
LT3686 Pin Functions VIN (Pin 1): The VIN pin supplies current to the LT3686’s internal regulator and to the internal power switch. This pin must be locally bypassed. BD (Pin 2): The BD pin is used to provide current to the internal Boost Schottky diode. Tie this pin to output whenever possible. When the MODE pin is greater than 0.8V, the LT3686 will prevent pulse-skipping at light loads by regulating an active load on the BD pin; see the Applications Information section Fixed Frequency at Light Load.
PAGE 8
LT3686 Block Diagram BOOST VIN VIN R4 C2 EN/ULVO OFF ON R5 INT REG UVLO OVLO DRIVER 1.27V C3 Q1 SW L1 R1 FB SS R2 VOUT C1 BD – + + D1 gm ACTIVE LOAD VC SLOPE COMP C4 R Q S Q 0.
PAGE 9
LT3686 Operation The LT3686 is a current mode step-down regulator. The EN/UVLO pin is used to place the LT3686 in shutdown. The 1.27V threshold on the EN/UVLO pin can be programmed by an external resistor divider (R4, R5) to disable the LT3686. When the EN/UVLO pin is driven above 1.27V, an internal regulator provides power to the control circuitry. This regulator includes both overvoltage and undervoltage lockout to prevent switching when VIN is more than 37V or less than 3.6V.
PAGE 10
LT3686 Applications Information FB Resistor Network 45 The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: 40 EN/UVLO (µA) 35 ⎛V ⎞ R1= R2 ⎜ OUT – 1⎟ ⎝ 0.8V ⎠ 30 25 20 15 10 R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. 5 0 0 10 Programmable Undervoltage Lockout  V  R4 =R5  IN – 1  1.
PAGE 11
LT3686 Applications Information The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins. For fixed frequency operation, the maximum input voltage is determined by the minimum duty cycle DCMIN: VIN(MAX) = VOUT + VD – VD + VSW DCMIN DCMIN can be adjusted with frequency. Note that this is a restriction on the operating input voltage for fixed frequency operation; the circuit will tolerate transient inputs up to the absolute maximum ratings of the VIN and BOOST pins.
PAGE 12
LT3686 Applications Information inductor current is therefore peak current plus minimum switch delay: Table 1. RT vs Frequency FREQUENCY (MHz) RT (kΩ) 2.5 9.53 2.3 12.1 2.1 15.4 1.7A + (VIN – VOUT)/L • 100ns VSW 10V/DIV IL 500mA/DIV VOUT 100mA/DIV AC 2µs/DIV 3686 F05 VIN = 35V VOUT = 3.3V L = 6.8µH COUT = 22µF IOUT = 1.2A Figure 5. Pulse Skip with Large Load Current Will Be Limited by the DA Valley Current Limit. Notice the Flat Inductor Valley Current and Reduced Switching Frequency 1.
PAGE 13
LT3686 Applications Information The MODE pin serves as mode select for the BD active load circuit. The active load is enabled when MODE is tied high and disabled when MODE is tied low. See Fixed Frequency at Light Load section. Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L= 4(VOUT + VD) f Catch Diode A low capacitance 1-2A Schottky diode is recommended for the catch diode, D1.
PAGE 14
LT3686 Applications Information a high quality (underdamped) tank circuit. If the LT3686 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3686’s voltage rating. This situation is easily avoided; see the Hot-Plugging Safely section. response. Transient performance can be improved with a high value capacitor, but a phase lead capacitor across the feedback resistor, R1, may be required to get the full benefit (see the Compensation section).
PAGE 15
LT3686 Applications Information Figure 7 shows the transient response of the LT3686 with several output capacitor choices. The output is 3.3V. The load current is stepped from 0.25A to 1A and back to 0.25A, and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The second photo shows the improved response (less voltage drop) resulting from a larger output capacitor and a phase lead capacitor.
PAGE 16
LT3686 Applications Information BAT54) in parallel with the internal diode to reduce VD. The following equations can be used to calculate and minimize boost capacitance in μF: CBOOST = the optimal boost capacitor for the chosen BD voltage. The absence of BD voltage during startup will increase minimum voltage to start and reduce efficiency. You must also be sure that the maximum voltage rating of BOOST pin is not exceeded.
PAGE 17
LT3686 Applications Information the LT3686 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. This minimum load will depend on the input and output voltages, and on the arrangement of the boost circuit.
PAGE 18
LT3686 Applications Information below the minimum voltage to sustain boosted operation (2.2V across the boost capacitor), the output voltage will fall suddenly to: VOUT = (VIN – 2.2) • DCMAX Figure 9 shows the minimum VIN necessary to sustain boosted operation during dropout. Once VIN drops below the sustain voltage, VIN will need to reach the start voltage again to refresh the boost capacitor.
PAGE 19
LT3686 Applications Information Soft-Start The SS pin is used to soft-start the LT3686, eliminating input current surge during start-up. It can also be used to track another voltage in the system (Figure 11). An internal 2µA current source charges an external softstart capacitor to generate a voltage ramp. FB voltage is regulated to the voltage at the SS pin until it exceeds 0.8V, FB is then regulated to the reference 0.8V.
PAGE 20
LT3686 Applications Information Short and Reverse Protection If the inductor is chosen so that it won’t saturate excessively, the LT3686 will tolerate a shorted output. When operating in short-circuit condition, the LT3686 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 LT3686 is absent.
PAGE 21
LT3686 Applications Information supply, the input network should be designed to prevent this overshoot. Figure 15 shows the waveforms that result when an LT3686 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2μF ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit.
PAGE 22
LT3686 Applications Information Frequency Compensation The LT3686 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3686 does not require the ESR of the output capacitor 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 LT3686 control loop. The error amp is a transconductance amplifier with finite output impedance.
PAGE 23
LT3686 Applications Information ideally at the ground terminal of the output capacitor C1. The SW and 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 LT3686 to help remove heat from the LT3686 to the ground plane. PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout.
PAGE 24
LT3686 Applications Information junction temperature. The die temperature is calculated by multiplying the LT3686 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3686 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss. The resulting temperature rise at full load is nearly independent of input voltage.
PAGE 25
LT3686 Typical Applications 0.8V Step-Down Converter VIN 3.6V TO 25V 3.3V Step-Down Converter VIN 5V TO 37V 2.2µF BOOST EN/UVLO SS 1nF 4.7µH VOUT 0.8V 1.2A SS 1nF GND GND 31.6k FB 61.9k 61.9k VOUT 3.3V 1.2A DA RT FB 15µH SW MODE DA RT 0.22µF LT3686 SW MODE BOOST EN/UVLO 0.22µF LT3686 BD VIN 2.2µF BD VIN 22µF 10k 100µF 3686 TA02b 3686 TA02a 1.8V Step-Down Converter VIN 3.6V TO 25V 3.3V Step-Down Converter with Programmed UVLO VIN 7.5V TO 37V BD VIN 2.
PAGE 26
LT3686 Package Description DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 6 0.40 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.00 – 0.05 5 1 (DD) DFN REV C 0310 0.25 ± 0.05 0.50 BSC 2.
PAGE 27
LT3686 Revision History REV DATE DESCRIPTION PAGE NUMBER A 1/10 Revised Features section B 6/10 C 5/11 Updated Electrical Characteristics (Feedback Voltage) Added H-grade information Revised Electrical Characteristics and updated Note 6 Revised Programmable Undervoltage Lockout section and updated Table 1 and minor value update in Outputs Greater Than 19V section in Applications Information section Revised unit to µH on 0.
PAGE 28
LT3686 Related Parts PART NUMBER DESCRIPTION COMMENTS LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz, High Efficiency MicroPower Step-Down DC/DC Converter with POR Reset and Watchdog Timer VIN: 3.6V to 36V Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA, 16-Pin 3mm 3mm QFN Package LT3682 36V, 60VMAX, 1A, 2.2MHz, High Efficiency MicroPower Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA, 12-Pin 3mm 3mm DFN Package LT3970 40V, 350mA (IOUT), 2.