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LT3682

3682f

TYPICAL APPLICATION

DESCRIPTION

1A Micropower Step-Down

Switching Regulator

The LT

3682 is an adjustable frequency (250kHz to

2.2MHz) monolithic buck switching regulator that accepts

input voltages up to 36V. A high efﬁ ciency 0.5 switch is

included on the device along with a boost diode and the

necessary oscillator, control, and logic circuitry. Current

mode topology is used for fast transient response and good

loop stability. A SYNC pin allows the user to synchronize the

part to an external clock, and to choose between Low Ripple

Burst Mode operation and standard PWM operation.

The Low Ripple Burst Mode maintains high efﬁ ciency at

low output currents while keeping output ripple below

15mV in typical applications. Shutdown reduces input

supply current to less than 1µA while a resistor and

capacitor on the RUN/SS pin provide a controlled output

voltage ramp (soft-start). A power good ﬂ ag signals when

OUT

reaches 90% of the programmed output voltage.

Protection circuitry senses the current in the power switch

and external Schottky catch diode to protect the LT3682

against short-circuit conditions. Frequency foldback and

thermal shutdown provide additional protection.

The LT3682 is available in a 12-Pin 3mm × 3mm DFN with

exposed pad for low thermal resistance.

5V Step-Down Converter

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode

is a trademark of Linear Technology Corporation. All other trademarks are the property of their

respective owners.

FEATURES

APPLICATIONS

Automotive Battery Regulation

Automotive Entertainment Systems

Industrial Supplies

Power for Portable Products

Distributed Supply Regulation

Wide Input Range:

Operation from 3.6V to 36V

Overvoltage Lockout Protects Circuits through

60V Transients

1A Output Current

Low Ripple Burst Mode

Operation

= 75μA at 12V

to 3.3V

OUT

Output Ripple < 15mV

P-P

Adjustable Switching Frequency: 250kHz to 2.2MHz

Short-Circuit Protected

Synchronizable Between 300kHz and 2.2MHz

0.8V Feedback Reference Voltage

Output Voltage 0.8V to 20V

Soft-Start Capability

Power Good Flag

Small 12-Pin Thermally Enhanced 3mm × 3mm

DFN Package

Efﬁ ciency

6.9V TO 36V

TRANSIENT TO 60V

OUT

0.9A, V

r 6.9V

1A, V

r 12V

2.2μF

16.2k

40.2k

470pF

102k 10μF

536k

0.22μF

10μH

3682 TA01

OFFON

GND

PGND

LT3682

RUN/SS

SYNC

BOOST

f = 800kHz

LOAD CURRENT (A)

EFFICIENCY (%)

100

0.2 0.4 0.6 0.8 1

OUT

= 5V

OUT

= 3.3V

= 12V

L = 10µH

f = 800 kHz

3682 TA01b

Summary of content (24 pages)

PAGE 1
LT3682 1A Micropower Step-Down Switching Regulator FEATURES DESCRIPTION n The LT®3682 is an adjustable frequency (250kHz to 2.2MHz) monolithic buck switching regulator that accepts input voltages up to 36V. A high efﬁciency 0.5Ω switch is included on the device along with a boost diode and the necessary oscillator, control, and logic circuitry. Current mode topology is used for fast transient response and good loop stability.
PAGE 2
LT3682 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN, RUN/SS Voltage (Note 2) …….……………. ....60V BOOST Pin Voltage …………………………..…….50V BOOST Pin Above SW Pin …………………..……..30V FB, RT, VC Voltage ......................................................5V SYNC …………………………….………….……. 20V BD and PG Voltage ..…….………………..……….. 30V Operating Junction Temperature Range (Notes 3 and 6) LT3682E ……………….………….. . −40°C to 125°C LT3682I ……………………………. −40°C to 125°C Storage Temperature Range …………..
PAGE 3
LT3682 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, VIN = 10V, VRUN/SS = 10V, VBD = 3.3V unless otherwise noted. (Note 3) PARAMETER CONDITIONS MIN TYP MAX UNITS VC Source Current 50 μA VC Sink Current 50 μA 1.25 A/V VC Pin to Switch Current Gain VC Switching Threshold 0.5 0.6 1.98 0.9 225 VC Clamp Voltage Switching Frequency 0.7 V 2.2 1 250 2.42 1.
PAGE 4
LT3682 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency (VOUT = 5V, SYNC = 0V) Efﬁciency (VOUT = 3.3V, SYNC = 0V) VIN = 12V 80 90 80 EFFICIENCY (%) 60 VIN = 34V 50 40 VIN = 24V EFFICIENCY (%) 70 70 60 50 VIN = 34V 40 VIN = 24V 30 30 10 0.1 1 10 100 LOAD CURRENT (mA) 1000 70 0.1 60 50 0.01 40 VIN = 12V VOUT = 3.3V L = 10μH f = 800kHz 30 L = 10μH f = 800kHz 20 L = 10μH f = 800kHz 20 1 10 0.1 1 10 100 LOAD CURRENT (mA) 20 10 0.
PAGE 5
LT3682 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit (SYNC Pin Grounded) 1.9 1.9 1.7 1.7 1.5 1.3 SYNC > 0.8V OR CLOCKED 0.9 0.7 0.5 Switch Voltage Drop 700 DC = 10% 600 1.5 1.3 VOLTAGE DROP (mV) SYNC < 0.3V SWITCH CURRENT LIMIT (A) SWITCH CURRENT LIMIT (A) Switch Current Limit 1.1 TA = 25°C, unless otherwise noted. DC = 90% 1.1 0.9 40 60 DUTY CYCLE (%) 20 80 0.5 100 50 25 0 25 50 75 TEMPERATURE (°C) 3682 G10 Boost Pin Current 100 0 Feedback Voltage 15 10 1.
PAGE 6
LT3682 TYPICAL PERFORMANCE CHARACTERISTICS RUN/SS Pin Current TA = 25°C, unless otherwise noted. Boost Diode Forward Voltage 12 1.4 10 1.2 Error Ampliﬁer Output Current 60 8 6 4 2 40 VC PIN CURRENT (μA) BOOST DIODE Vf (V) RUN/SS PIN CURRENT (μA) 50 1 0.8 0.6 0.4 0 0 5 10 15 20 25 30 RUN/SS PIN VOLTAGE (V) 35 0.25 0.5 0.75 BOOST DIODE CURRENT (A) 3682 G19 Minimum Input Voltage 30 200 1 100 0 100 FB PIN ERROR VOLTAGE (mV) 4.5 6 200 3682 G21 Minimum Input Voltage 6.
PAGE 7
LT3682 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Switching Waveforms; Burst Mode Power Good Threshold THRESTHOLD VOLTAGE (%) 95 VSW 5V/DIV 90 IL 0.2A/DIV VOUT 20mV/DIV 85 80 5μs/DIV 70 3682 G29 VIN = 12V; FRONT PAGE APPLICATION ILOAD = 5mA 50 25 0 25 50 75 TEMPERATURE(°C) 100 125 3682 G28 Switching Waveforms; Full Frequency Continuous Operation Switching Waveforms; Transition from Burst Mode to Full Frequency VSW 5V/DIV VSW 5V/DIV IL 0.5A/DIV IL 0.
PAGE 8
LT3682 PIN FUNCTIONS VC (Pin 1): The VC pin is the output of the internal error ampliﬁer. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop. FB (Pin 2): The LT3682 regulates the FB pin to 0.8V. Connect the feedback resistor divider tap to this pin. PG (Pin 3): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 10% of the ﬁnal regulation voltage.
PAGE 9
LT3682 BLOCK DIAGRAM VIN 12 VIN C1 – + OVLO INTERNAL 0.8V REF THERMAL SHUTDOWN BD SLOPE COMP BOOST R 10 OUT RT OSCILLATOR 250kHz-2.2MHz OUTB RT 11 9 3 SYNC RUN/SS SYNC PG ERROR AMP C3 SW L1 8 VOUT D1 VC CLAMP DA 7 + – VC 1 CC RC 4 GND 2 R2 C2 DISABLE + – 0.
PAGE 10
LT3682 OPERATION The LT3682 is a constant frequency, current mode step-down regulator. An oscillator, with frequency set by RT, enables an RS ﬂip-ﬂop, turning on the internal power switch. An ampliﬁer and comparator monitor the current ﬂowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error ampliﬁer measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin.
PAGE 11
LT3682 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: ⎛V ⎞ R1= R2 ⎜ OUT − 1⎟ ⎝ 0.8 V ⎠ operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efﬁciency, lower maximum input voltage, and higher dropout voltage.
PAGE 12
LT3682 APPLICATIONS INFORMATION Frequency Tradeoffs section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD −V +V 1− fSW tOFF(MIN) D SW where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (210ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used.
PAGE 13
LT3682 APPLICATIONS INFORMATION where fSW is the switching frequency in MHz, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and L is the inductor value in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. To keep the efﬁciency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications.
PAGE 14
LT3682 APPLICATIONS INFORMATION For SYNC pin tied to 0.8V or more, or tied to a clock source for synchronization: necessary. This can be provided with a lower performance electrolytic capacitor. ΔIL ΔI = 1.18 A •(1− 0.29 • DC) − L 2 2 Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times.
PAGE 15
LT3682 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408)749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP Murata (408)436-1300 www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum www.taiyo-yuden.
PAGE 16
LT3682 APPLICATIONS INFORMATION Table 3. Schottky Diodes VR (V) IAVE (A) VF at 1A (mV) VF at 2A (mV) On Semiconducor MBR0520L 20 0.5 MBR0540 40 0.5 620 MBRM120E 20 1 530 MBRM140 40 1 550 595 LT3682 Diodes Inc. 30 0.5 B0540W 40 0.5 620 B120 20 1 500 B130 30 1 500 CURRENT MODE POWER STAGE gm = 1.25S SW FB – B0530W OUTPUT R1 CPL gm = 420μS 3Meg B140 40 1 500 B150 50 1 700 B220 20 2 500 B230 30 2 500 B140HB 40 1 DFLS240L 40 2 DFLS140 40 1.
PAGE 17
LT3682 APPLICATIONS INFORMATION capacitor on the VC pin (CC) integrates the error ampliﬁer output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency.
PAGE 18
LT3682 APPLICATIONS INFORMATION BOOST and BD Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μF capacitor will work well. Figure 5 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efﬁciency. For outputs of between 3V and 8V, the standard circuit (Figure 5a) is best. For outputs between 2.
PAGE 19
LT3682 APPLICATIONS INFORMATION 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 input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started.
PAGE 20
LT3682 APPLICATIONS INFORMATION Synchronization To select Low Ripple Burst Mode operation, tie the SYNC pin below 0.3V (this can be ground or a logic output). Synchronizing the LT3682 oscillator to an external frequency can be done by connecting a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.3V and peaks that are above 0.8V (up to 6V).
PAGE 21
LT3682 APPLICATIONS INFORMATION PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 10 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents ﬂow in the LT3682’s VIN, SW and PGND pins, the catch diode and the input capacitor (CIN). The loop formed by these components should be as small as possible.
PAGE 22
LT3682 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 6.9V TO 36V TRANSIENT TO 60V BD VIN ON OFF VOUT 5V 0.9A, VIN 6.9V 1A, VIN 12V RUN/SS BOOST L 10μH 0.22μF VC 2.2μF SW LT3682 RT 16.2k DA D1 B150 PG 40.2k 536k SYNC GND FB PGND 102k 470pF 10μF 3682 TA02 f = 800kHz 3.3V Step-Down Converter VIN 4.8V TO 28.5V TRANSIENT TO 60V BD VIN ON OFF VOUT 3.3V 0.9A, VIN 4.8V 1A, VIN 6.5V RUN/SS BOOST L 10μH 0.22μF VC 2.2μF SW LT3682 RT 14k DA D1 B150 PG 40.
PAGE 23
LT3682 PACKAGE DESCRIPTION DD Package 12-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1725 Rev A) 0.70 p 0.05 3.50 p 0.05 2.10 p 0.05 2.38 p 0.05 1.65 p 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.45 BSC 2.25 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 p 0.10 (4 SIDES) R = 0.115 TYP 7 0.40 p 0.10 12 2.38 p 0.10 1.65 p 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 s 45o CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 6 0.200 REF 1 0.23 p 0.05 0.45 BSC 0.75 p 0.
PAGE 24
LT3682 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VIN 10V TO 16.5V TRANSIENT TO 36V BD VIN ON OFF VOUT 5V 0.9A RUN/SS BOOST 0.22μF VC 2.2μF LT3682 RT 13.3k L 4.7μH SW DA D1 B140 PG 9.76k SYNC GND 536k FB PGND 680pF 102k f = 2MHz 10μF 3682 TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Efﬁciency Step-Down DC/DC Converter VIN = 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25μA, TSSOP16E Package LT1767 25V, 1.2A (IOUT), 1.