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LT3724

3724fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

High Voltage, Current Mode

Switching Regulator Controller

The LT

3724 is a DC/DC controller used for medium power,

low part count, low cost, high efﬁciency supplies. It of-

fers a wide 4V-60V input range (7.5V minimum startup

voltage) and can implement step-down, step-up, inverting

and SEPIC topologies.

The LT3724 includes Burst Mode operation, which re-

duces quiescent current below 100µA and maintains

high efﬁciency at light loads. An internal high voltage bias

regulator allows for simple biasing and can be back driven

to increase efﬁciency.

Additional features include ﬁxed frequency current mode

control for fast line and load transient response; a gate driver

capable of driving large N-channel MOSFETs; a precision

undervoltage lockout function; 10µA shutdown current;

short-circuit protection; and a programmable soft-start

function that directly controls output voltage slew rates at

startup which limits inrush current, minimizes overshoot

and facilitates supply sequencing.

High Voltage Step-Down Regulator

Wide Input Range: 4V to 60V

Output Voltages up to 36V (Step-Down)

Burst Mode

Operation: <100µA Supply Current

10µA Shutdown Supply Current

±1.3% Reference Accuracy

200kHz Fixed Frequency

Drives N-Channel MOSFET

Programmable Soft-Start

Programmable Undervoltage Lockout

Internal High Voltage Regulator for Gate Drive

Thermal Shutdown

Current Limit Unaffected by Duty Cycle

16-Pin Thermally Enhanced TSSOP Package

Industrial Power Distribution

12V and 42V Automotive and Heavy Equipment

High Voltage Single Board Systems

Distributed Power Systems

Avionics

Telecom Power

Efﬁciency and Power Loss

vs Load Current

SHDN

Burst_EN

SGND

BOOST

PGND

SENSE

–

30V TO

60V

OUT

24V

75W

3724 TA01a

LT3724

68.1k

1000pF

4.99k 93.1k

10Ω

200k

0.22µF

OUT

330µF

68µF

1µF

47µH

0.025Ω

Si7852

SS3H9

120pF680pF

40.2k

LOAD CURRENT (A)

0.1

EFFICIENCY (%)

POWER LOSS (W)

1 10

3724 TA01b

= 48V

LOSS

EFFICIENCY

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

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

of their respective owners. Protected by U.S. Patents including 5731694, 6498466, 6611131.

Summary of content (26 pages)

PAGE 1
LT3724 High Voltage, Current Mode Switching Regulator Controller FEATURES n n n n n n n n n n n n n DESCRIPTION Wide Input Range: 4V to 60V Output Voltages up to 36V (Step-Down) Burst Mode® Operation: <100µA Supply Current 10µA Shutdown Supply Current ±1.
PAGE 2
LT3724 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Input Supply Voltage (VIN).......................... 65V to –0.3V Boosted Supply Voltage (BOOST)............... 80V to –0.3V Switch Voltage (SW)(Note 8)......................... 65V to –1V Differential Boost Voltage (BOOST to SW)....................................... 24V to –0.3V Bias Supply Voltage (VCC)........................... 24V to –0.3V SENSE+ and SENSE– Voltages.................... 40V to –0.3V (SENSE+ to SENSE–)...............
PAGE 3
LT3724 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, SENSE – = SENSE+ = 10V, SGND = PGND = SW = 0V, unless otherwise noted.
PAGE 4
LT3724 ELECTRICAL CHARACTERISTICS temperature range. The LT3724MP is 100% tested and guaranteed over the –55°C to 125°C operating junction temperature range. Note 4: VIN voltages below the start-up threshold (7.5V) are only supported when the VCC is externally driven above 6.5V. Note 5: Operating range is dictated by MOSFET absolute maximum VGS. Note 6: Supply current specification does not include switch drive currents. Actual supply currents will be higher.
PAGE 5
LT3724 TYPICAL PERFORMANCE CHARACTERISTICS Error Amp Transconductance vs Temperature ICC vs VCC (SHDN = 0V) 25 6.4 20 6.3 15 6.2 6.1 TA = 25°C 10 5 6.0 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 0 125 0 2 4 6 8 I(SENSE+ + SENSE–) vs VSENSE (CM) 100 0 –100 320 –50 –25 1.234 220 1.233 210 200 190 180 1.231 1.230 1.229 1.228 50 25 75 0 TEMPERATURE (°C) 100 125 1.227 –50 –25 152 150 148 146 144 142 100 125 3724 G13 100 3724 G12 3.86 4.52 3.84 4.50 3.82 4.48 4.46 3.80 4.
PAGE 6
LT3724 PIN FUNCTIONS VIN (Pin 1): The VIN pin is the main supply pin and should be decoupled to SGND with a low ESR capacitor located close to the pin. NC (Pin 2): No Connection. SHDN (Pin 3): The SHDN pin has a precision IC enable threshold of 1.35V (rising) with 120mV of hysteresis. It is used to implement an undervoltage lockout (UVLO) circuit. See Application Information section for implementing a UVLO function. When the SHDN pin is pulled below a transistor VBE (0.
PAGE 7
LT3724 PIN FUNCTIONS VCC (Pin 12): The VCC pin is the internal bias supply decoupling node. Use a low ESR 1µF ceramic capacitor to decouple this node to PGND. Most internal IC functions are powered from this bias supply. An external diode connected from VCC to the BOOST pin charges the bootstrapped capacitor during the off-time of the main power switch.
PAGE 8
LT3724 FUNCTIONAL DIAGRAM VIN UVLO (<4V) 8V VCC REGULATOR VIN VCC UVLO (<6V) 1 VIN CIN CBOOST 3.8V REGULATOR RA INTERNAL SUPPLY RAIL + RB – + gm ERROR AMP VOUT COUT D1 D3 (OPTIONAL) OSCILLATOR Q S R SLOPE COMP GENERATOR CURRENT SENSE COMPARATOR + + CC1 RSENSE PGND 11 – RC SOFT-START DISABLE/BURST ENABLE D2 CVCC DRIVE CONTROL 0.
PAGE 9
LT3724 (Refer to Functional Diagram) The LT3724 is a PWM controller with a constant frequency, current mode control architecture. It is designed for low to medium power, switching regulator applications. Its high operating voltage capability allows it to step-up or down input voltages up to 60V without the need for a transformer. The LT3724 is used in nonsynchronous applications, meaning that a freewheeling rectifier diode (D1 of Function Diagram) is used instead of a bottom side MOSFET.
PAGE 10
LT3724 OPERATIONS (Refer to Functional Diagram) either the start of the next cycle or until the bootstrapped capacitor is fully charged. MOSFET Driver The LT3724 contains a high speed boosted driver to turn on and off an external N-channel MOSFET switch. The MOSFET driver derives its power from the boost capacitor which is referenced to the SW pin and the source of the MOSFET. The driver provides a large pulse of current to turn on the MOSFET fast and minimize transition times.
PAGE 11
LT3724 OPERATIONS (Refer to Functional Diagram) Soft-Start The soft-start function controls the slew rate of the power supply output voltage during start-up. A controlled output voltage ramp minimizes output voltage overshoot, reduces inrush current from the VIN supply, and facilitates supply sequencing. A capacitor, CSS, connected between VOUT of the supply and the CSS pin of the IC, programs the slew rate.
PAGE 12
LT3724 APPLICATIONS INFORMATION The basic LT3724 step-down (buck) application, shown in the Typical Application on the front page, converts a larger positive input voltage to a lower positive or negative output voltage. This Application Information section assists selection of external components for the requirements of the power supply. RSENSE Selection The current sense resistor, RSENSE, monitors the inductor current of the supply (See Typical Application on front page).
PAGE 13
LT3724 APPLICATIONS INFORMATION For maximum efficiency, minimize RDS(ON) and CRSS. Low RDS(ON) minimizes conduction losses while low CRSS minimizes transition losses. The problem is that RDS(ON) is inversely related to CRSS. Balancing the transition losses with the conduction losses is a good idea in sizing the MOSFET. Select the MOSFET to balance the two losses.
PAGE 14
LT3724 APPLICATIONS INFORMATION Step-Down Converter: Input Capacitor Selection Step-Down Converter: Output Capacitor Selection A local input bypass capacitor is required for buck converters because the input current is pulsed with fast rise and fall times. The input capacitor selection criteria are based on the bulk capacitance and RMS current capability. The bulk capacitance will determine the supply input ripple voltage. The RMS current capability is used to keep from overheating the capacitor.
PAGE 15
LT3724 APPLICATIONS INFORMATION VSUPPLY L1 VOUT R2 RA COUT SHDN PIN VFB PIN RB R1 3724 F03 3724 F02 Figure 2. Output Voltage Feedback Divider Figure 3. Undervoltage Lockout Circuit The VFB pin input bias current is typically 25nA, so use of extremely high value feedback resistors could cause a converter output that is slightly higher than expected.
PAGE 16
LT3724 APPLICATIONS INFORMATION The desired soft-start rise time (tSS) is programmed via a programming capacitor CSS1, using a value that corresponds to 2µA average current during the soft-start interval. This capacitor value follows the relation: CSS1 = VOUT 2 • 10 –6 • t SS VOUT VOUT(SS) V(VC) RSS is typically set to 200k for most applications.
PAGE 17
LT3724 APPLICATIONS INFORMATION Inductor current typically does not reach IMAX in the few cycles that occur before soft-start becomes active, but can with high input voltages or small inductors, so the above relation is useful as a worst-case scenario. This energy transfer increase in output voltage is typically small, but for some low voltage applications with relatively small output capacitors, it can become significant.
PAGE 18
LT3724 APPLICATIONS INFORMATION 2. I2R losses are calculated from the DC resistances of the MOSFET, the inductor, the sense resistor, and the input and output capacitors. In continuous conduction mode the average output current flows through the inductor and RSENSE but is chopped between the MOSFET and the Schottky diode.
PAGE 19
LT3724 APPLICATIONS INFORMATION VIN+ RA 1 RB 3 4 RCSS CSS 5 6 7 R2 RC R1 CC1 8 VIN BOOST LT3724 SHDN CSS SW 15 CIN M1 VIN– L1 14 RSENSE + D2 17 BURST_EN VFB TG CBOOST 16 VCC PGND VC SENSE+ SGND SENSE– 12 11 COUT CVCC 10 D3 VOUT D1 9 – CC2 3724 F06 Figure 9. LT3724 Layout Diagram (See PCB Layout Checklist).
PAGE 20
LT3724 TYPICAL APPLICATIONS 12V to 24V/50W Boost (Step-Up) Converter D1 BAV99 1 VIN 8V TO16V CIN 33µF ×2 25V C1 1500pF 0.1µF 25V R3 4.7M 3 RCSS 200k 4 5 R2 187k 6 7 R1 10k C2 120pF R6 40.2k 8 VIN BOOST LT3724 SHDN TG SW RSENSE 0.015Ω 16 L1 10µH D2 SBM540 15 14 VOUT 24V AT 50W CSS BURST_EN VFB VCC PGND VC SENSE+ SGND SENSE– M1 12 C4 1µF 25V 11 10 9 C3 4700pF 3724 TA02 COUT1 330µF 35V COUT2 2.
PAGE 21
LT3724 TYPICAL APPLICATIONS High Voltage LED Driver with Dimmer Control LED L1 300µH VIN 8V TO 60V C1 (OPTIONAL) CIN 22µF 1 OPTIONAL DIMMER CONTROL 1kHz R1 4.7M M2 2N7002 3 4 5 6 C1 = OPTIONAL TO REDUCE LED RIPPLE CURRENT CIN = TDK, C4532X7R2A225K D1 = DIODESINC.
PAGE 22
LT3724 TYPICAL APPLICATIONS 4.5V to 20V Input to 12V at 25W Output SEPIC Converter with 60V Input Transient Capability VIN 4.5V TO 20V TO 60V TRANSIENT CIN1 22µF 2x 25V CIN2 25V 1µF RA 100k 1 3 C1 390pF RB 49.9k R3 200k 4 5 R2 130k 6 7 R1 14.7k R4 47k C2 120pF R5 40.2k C3 680pF 8 VIN BOOST TG SHDN CSS LT3724 BURST_EN VFB SW VCC PGND VC SENSE+ SGND SENSE– D1B GSD2004 16 L1 20µH C7 0.
PAGE 23
LT3724 TYPICAL APPLICATIONS 12V Step-Down with VCC Back Driven from VOUT and Ceramic Capacitor in Output Filter VIN 15V TO 60V CIN 100µF 100V + 2.2µF x2 100V R3 49.9k C1 3300pF R2 499k 3 RCSS 200k R4 130k 1 4 5 6 7 R5 14.7k C2 120pF R6 15k 8 VIN BOOST TG SHDN LT3724 SW C6 0.1µF 16V 16 15 R7 20Ω 14 CSS BURST_EN VFB VCC PGND M1 Si7852DP D2A BAV99 12 11 VC 10 SENSE+ SGND SENSE– C4 1µF 16V D2B BAV99 L1 47µH VOUT 12V AT 50W RSENSE 0.
PAGE 24
LT3724 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev I) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 16 1514 13 12 11 6.60 ±0.10 4.50 ±0.10 0.48 (.019) REF 3.58 (.141) 2.94 (.116) 10 9 DETAIL B 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.51 (.020) REF DETAIL B IS THE PART OF THE LEAD FRAME FEATURE FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.
PAGE 25
LT3724 REVISION HISTORY (Revision history begins at Rev D) REV DATE DESCRIPTION PAGE NUMBER D 3/11 Deleted last paragraph of Description 1 Minor text edits made to SW and BOOST pin descriptions in Pin Functions section 7 Minor text edits made to Main Control Loop and Current Limit/Short Circuit sections in Operations 9 Revised High Voltage LED Driver with Dimmer Control in Typical Applications 21 Revised Typical Application drawing and Related Parts list 24 3724fd 25
PAGE 26
LT3724 TYPICAL APPLICATION Inverting –12V 1.5A Converter VIN 18V TO 36V 1 VIN 0.1µF LT3724 3 4 R1 88.7k CSS 1000pF RCSS 200k CC2 680pF CC1 120pF SHDN TG SW CSS VCC 6 R6, 40.2k R2 10.2k BOOST 7 8 VFB VC GND PGND SENSE+ 0.1µF 16V 16 15 M1 14 12 11 + D1A D1B CIN1 220µF 50V L1 47µH D2 1µF 16V 10 9 SENSE– D1 = BAV99 D2 = ON SEMI, MBRD350 L1 = COEV, DU1311-470M M1 = VISHAY, Si7370DP CIN1 = SANYO, 50CV220KX COUT1 = SANYO, 16SVP330M VOUT –12V 1.5A RSENSE 0.