Datasheet

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LT3686A

3686afa

ApplicAtions

37V/1.2A Step-Down

Regulator in 3mm × 3mm DFN

and MSE

The LT

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

converter with an internal power switch. The wide input

range of 3.6V to 37V makes the LT3686A suitable for

regulating power from a wide variety of sources, including

24V industrial supplies and automotive batteries. Its high

maximum frequency allows 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 LT3686A particularly suitable for

automotive applications.

Cycle-by-cycle current limit, thermal shutdown 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 LT3686A 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.

The LT3686A offers external synchronization capability,

whereas the LT3686 does not.

3.3V Step-Down Converter

SYNC/MODE

6V TO 37V

OUT

3.3V

1.2A

0.22µF

MBRM140

2.2µF

2MHz

10nF

31.6k

22µF

10k

6.8µH

31.6k

GND

LT3686A

3686A TA01a

EN/UVLO

BOOST

6V TO 16V AT 2MHz)

12V

Efﬁciency (2MHz)

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

60V 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

■

Can Be Synchronized to External Clock

■

Tracking and Soft-Start

■

Precision UVLO

■

Short-Circuit Robust

■

in Shutdown <1µA

■

Internally Compensated

■

Thermally Enhanced MSOP and 3mm × 3mm DFN

Packages

typicAl ApplicAtion

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

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

owners.

LOAD CURRENT (mA)

EFFICIENCY (%)

400

800

1000

3686A TA01b

200

600

1200

OUT

3.3V

OUT

Summary of content (30 pages)

PAGE 1
LT3686A 37V/1.2A Step-Down Regulator in 3mm × 3mm DFN and MSE Description Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Range: Operation from 3.6V to 37V Overvoltage Lockout Protects Circuit Through 60V Transients Low Minimum On-Time: Converts 16VIN to 3.3VOUT at 2MHz 1.2A Output Current Adjustable Frequency: 300kHz to 2.
PAGE 2
LT3686A Absolute Maximum Ratings (Note 1) Input Voltage (VIN) (Note 7).......................................60V BOOST Voltage..........................................................55V BOOST Pin Above SW Pin..........................................25V FB Voltage....................................................................6V EN/UVLO Voltage (Note 7).........................................60V BD Voltage.................................................................25V RT Voltage................
PAGE 3
LT3686A Electrical Characteristics The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VEN/UVLO ≥ 1.34V. 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, SYNC/MODE ≤ 0.4V Not Switching, SYNC/MODE ≥ 0.8V 1.1 1.2 1.3 1.4 mA mA Internal Undervoltage Lockout Overvoltage Lockout Feedback Voltage VIN = 3.
PAGE 4
LT3686A 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/SYNC < 0.4V 40 30 10 0 200 800 1000 600 400 LOAD CURRENT (mA) 3686A G01 10 20 VIN (V) 30 40 3686A 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
LT3686A 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 2.05 2.00 0 0.5 1 1.5 FREQUENCY (MHz) 2 1.90 –50 2.5 0 50 100 TEMPERATURE (°C) 600 30 400 300 20 15 5 0 0 1000 1200 0 10 20 30 EN/UVLO (V) 40 3.0 1.0 –50 50 SWITCH PEAK MODE/SYNC > 0.8 30 25 MODE/SYNC < 0.
PAGE 6
LT3686A Typical Performance Characteristics 3.3VOUT Typical Minimum Input Voltage 8 6 7 VIN (V) 3 IL 200mA/DIV 4 2 VOUT = 3.3V L = 15µH f = 1MHz 1 1 10 100 1000 VOUT = 5V L = 22µH f = 1MHz 1 0 3686A G20 200ns/DIV 3 2 0 VSW 2V/DIV MODE/SYNC > 0.8 5 MODE/SYNC > 0.8 Continuous Mode Waveform MODE/SYNC < 0.4 6 MODE/SYNC < 0.4 4 VIN (V) 5VOUT Typical Minimum Input Voltage 7 5 TA = 25°C unless otherwise noted. 10 1 ILOAD (mA) 100 1000 VIN = 10V VOUT = 3.3V L = 6.
PAGE 7
LT3686A Pin FunCtions (DFN/MSE) VIN (Pin 1/2): The VIN pin supplies current to the LT3686A’s internal regulator and to the internal power switch. This pin must be locally bypassed. BD (Pin 2/3): When the SYNC/MODE Pin is driven with clock pulses or tied greater than 0.8V, the LT3686A will prevent pulse-skipping at light loads by regulating an active load on the BD pin; see Applications Information section Fixed Frequency at Light Load. FB (Pin 3/4): The LT3686A regulates its feedback pin to 0.8V.
PAGE 8
LT3686A 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
LT3686A Operation The LT3686A is a current mode step-down regulator. The EN/UVLO pin is used to place the LT3686A in shutdown. The 1.28V 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 1.28V, 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
LT3686A 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) ⎛V ⎞ R1= R2 ⎜ OUT – 1⎟ ⎝ 0.8V ⎠ 35 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
LT3686A 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
LT3686A Applications Information to keep the inductor valley current to 1.7A (Figure 5). Peak inductor current is therefore peak current plus minimum switch delay: 1.7A + (VIN – VOUT )/L • 100ns VSW 10V/DIV IL 500mA/DIV VOUT 100mA/DIV AC 2µs/DIV 3686A F05 VIN = 35V VOUT = 3.3V L = 6.8µH COUT = 22µF IOUT = 1.2A Table 1. RT vs Frequency FREQUENCY (MHz) RT (kΩ) MIN SYNC FREQUENCY (MHz) 2.5 9.53 N/A 2.3 12.1 N/A 2.1 15.4 N/A 1.9 20.0 N/A 1.8 22.6 2.50 1.7 25.5 2.30 1.5 31.6 1.99 1.
PAGE 13
LT3686A Applications Information The SYNC/MODE pin is used to synchronize the internal oscillator with an external square wave. The synchronizing clock signal to the LT3686A should be below 2.5MHz with pulse width of at least 200ns on and off time, a low state below 0.4V and a high state above 0.8V. The SYNC frequency must be higher than the RT programmed frequency; see Table 1.
PAGE 14
LT3686A Applications Information necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3686A and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 2.
PAGE 15
LT3686A Applications Information Figure 7 shows the transient response of the LT3686A 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
LT3686A 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
LT3686A 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. This minimum load will depend on the input and output voltages, and on the arrangement of the boost circuit.
PAGE 18
LT3686A 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
LT3686A Applications Information Soft-Start The SS pin is used to soft-start the LT3686A, 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
LT3686A Applications Information Short and Reverse Protection If the inductor is chosen so that it won’t saturate excessively, the LT3686A will tolerate a shorted output. When operating in short-circuit condition, the 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.
PAGE 21
LT3686A 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 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.
PAGE 22
LT3686A Applications Information Frequency Compensation The LT3686A uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3686A 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 LT3686A control loop. The error amp is a transconductance amplifier with finite output impedance.
PAGE 23
LT3686A Applications Information PCB Layout High Temperature Considerations For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 17 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3686A’s VIN and SW pins, the catch diode (D1) and the input capacitor (C2). The loop formed by these components should be as small as possible and tied to system ground in only one place.
PAGE 24
LT3686A Applications Information Outputs Greater Than 19V Note that for outputs above 19V, the input voltage range will be limited by the maximum rating of the BOOST pin. The sum of input and output voltages cannot exceed the BOOST pin’s 55V rating. The 25V circuit (Figure 18) shows how to overcome this limitation using an additional Zener diode.
PAGE 25
LT3686A Typical Applications 0.8V Step-Down Converter VIN 3.6V TO 25V BD VIN 2.2µF BOOST EN/UVLO LT3686A SYNC/MODE SS 1nF 0.22µF 2.2µH VOUT 0.8V 1.2A SW DA RT GND FB 61.9k 100µF 3686A TA02a 3.3V Step-Down Converter VIN 5V TO 37V BD VIN 2.2µF BOOST EN/UVLO LT3686A SYNC/MODE SS 1nF 0.22µF 6.8µH VOUT 3.3V 1.2A SW DA RT GND 31.6k FB 61.9k 22µF 10k 3686A TA02b 1.8V Step-Down Converter VIN 3.6V TO 25V BD VIN 2.2µF BOOST EN/UVLO LT3686A SYNC/MODE SS 1nF 3.3µH VOUT 1.8V 1.
PAGE 26
LT3686A Typical Applications 3.3V Step-Down Converter with Programmed UVLO VIN 7.5V TO 37V BD VIN 500k 2.2µF EN/UVLO LT3686A 0.22µF 6.8µH VOUT 3.3V 1.2A SW 100k SYNC/MODE SS 1nF BOOST RT DA 31.6k FB 22µF 10k GND 61.9k 3686A TA02d 2.5V Step-Down Converter VIN 3.6V TO 25V BD VIN 2.2µF BOOST EN/UVLO LT3686A SYNC/MODE SS 1nF 0.22µF 4.7µH VOUT 2.5V 1.2A SW DA RT GND 21.5k FB 61.9k 33µF 10k 3686A TA02e 5V Step-Down Converter VIN 7V TO 37V BD VIN 2.
PAGE 27
LT3686A Package Description MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev D) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ± 0.102 (.112 ± .004) 5.23 (.206) MIN 2.845 ± 0.102 (.112 ± .004) 0.889 ± 0.127 (.035 ± .005) 6 1 1.651 ± 0.102 3.20 – 3.45 (.065 ± .004) (.126 – .136) 0.12 REF 12 0.65 0.42 ± 0.038 (.0256) (.0165 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 4.039 ± 0.102 (.159 ± .
PAGE 28
LT3686A Package Description DD Package DD Package 10-Lead Plastic DFN× (3mm 10-Lead Plastic DFN (3mm 3mm) × 3mm) (Reference DWG # 05-08-1699 (Reference LTC DWGLTC # 05-08-1699 Rev C) 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.
PAGE 29
LT3686A Revision History REV DATE DESCRIPTION PAGE NUMBER A 5/11 Revised MSOP in Features, Pin Configuration, Order Information and Package Description sections 1, 2, 27 Updated Electrical Characteristics section 3 Updated value for EN/UVLO pin in Pin Functions section 7 Updated values in Operation section Revised equation in Programmable Undervoltage Lockout section, Table 1 and minor text edit to Minimum On Time in Applications Information section 9 10, 12, 13 3686afa Information furnished
PAGE 30
LT3686A Typical Application 5V Step-Down Converter VIN 7V TO 37V BD VIN 2.2µF BOOST EN/UVLO LT3686A SYNC/MODE SS 1nF 0.22µF 10µH VOUT 5V 1.2A SW DA RT GND 52.3k FB 61.9k 15µF 10k 3686A TA02f related Parts PART NUMBER DESCRIPTION COMMENTS LT3686 37V, 55V with Transient Protection 1.2A, 2.5MHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 37V, Transient to 55V, VOUT(MIN) = 0.8V, IQ = 1.1mA, ISD <1µA, 10-Pin 3mm × 3mm DFN Package LT3689 36V, 60V Transient Protection, 800mA, 2.