Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - voltage regulators - DC DC switch controllers MSOP 16

LT3748

3748fb

For more information www.linear.com/LT3748

Features

applications

Description

100V Isolated

Flyback Controller

The LT

3748 is a switching regulator controller specifically

designed for the isolated flyback topology and capable of

high power. It drives a low side external N-channel power

MOSFET from an internally regulated 7V supply. No third

winding or opto-isolator is required for regulation as the

part senses the isolated output voltage directly from the

primary-side flyback waveform.

The LT3748 utilizes boundary mode to provide a small

magnetic solution without compromising load regulation.

Operating frequency is set by load current and transformer

magnetizing inductance. The gate drive of the LT3748

combined with a suitable external MOSFET allow it to

deliver load power up to several tens of watts from input

voltages as high as 100V.

The LT3748 is available in a high voltage 16-lead MSOP

package with four leads removed.

25W, 12V Output, Isolated Telecom Supply

5V to 100V Input Voltage Range

1.9A Average Gate Drive Source and Sink Current

Boundary Mode Operation

No Transformer Third Winding or Opto-Isolator

Required for Regulation

Primary-Side Winding Feedback Load Regulation

OUT

Set with Two External Resistors

INTV

Pin for Control of Gate Driver Voltage

Programmable Soft Start

Programmable Undervoltage Lockout

Available in MSOP Package

Isolated Telecom Converters

High Power Automotive Supplies

Isolated Industrial Power Supplies

Military and High Temperature Applications

Output Load and Line Regulation

typical application

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

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

Protected by U.S. Patents, including 5438499 and 7471522.

EN/UVLO

REF

GND INTV

LT3748

3748 TA01a

56.2k

2nF

36V TO 72V

OUT

12V

OUT

–

4:1

412k

15.4k

µF

3.8

µH60.8µH

100

µF

GATE

SENSE

4700pF

4.7µF

6.04k

243k

0.033Ω

10k

LOAD CURRENT (A)

11.4

OUT

(V)

11.6

11.8

12.0

12.2

12.4

12.6

0.5 1.0 1.5 2.0

3748 TA01b

= 72V

= 48V

= 36V

Downloaded from Arrow.com.

Summary of content (34 pages)

PAGE 1
LT3748 100V Isolated Flyback Controller Features Description 5V to 100V Input Voltage Range nn 1.
PAGE 2
LT3748 Absolute Maximum Ratings (Note 1) Pin Configuration VIN, RFB....................................................................100V VIN to RFB...................................................................±5V EN/UVLO.......................................................–0.3V, 100V INTVCC.....................................................VIN + 0.3V, 20V SS, VC, TC, RREF..........................................................6V SENSE..........................................................
PAGE 3
LT3748 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, unless otherwise noted. PARAMETER CONDITIONS Maximum SENSE Current Limit Threshold VC = 2.2V l Minimum SENSE Current Limit Threshold MIN TYP MAX UNITS 95 90 100 100 105 110 mV mV VC = 0V 15 Maximum to Minimum SENSE Threshold Ratio l SENSE Overcurrent Threshold VC = 2.
PAGE 4
LT3748 Typical Performance Characteristics TA = 25°C, unless otherwise noted. 1.6 1.7 VSS = 0V 1.6 INTVCC = OPEN VOUT (V) 15.2 15.0 14.8 VIN = 72V 1.5 1.4 VIN = 36V 1.3 VIN = 12V 1.2 1.1 VIN = 6V 1.0 14.6 0 0.8 –55 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 7.5 7.4 IINTVCC = 10mA 5.5 5.0 6.7 4.5 6.6 4.0 25 50 75 100 125 150 TEMPERATURE (°C) 2.5 4 6 8 10 20 40 60 0 0 10 20 30 INTVCC CURRENT (mA) 40 3748 G07 4 80 Downloaded from Arrow.com. FALLING THRESHOLD 3.5 3.
PAGE 5
LT3748 Typical Performance Characteristics EN/UVLO Threshold vs Temperature EN/UVLO Current vs Temperature 2.0 EN/UVLO THRESHOLD (V) VEN/UVLO = 0.9V 1.5 1.0 0.5 0.9 1.35 0.8 0.7 1.30 1.25 1.20 1.15 1.10 0 –55 –25 VEN/UVLO = 1.3V 0 0 60 190 50 160 SENSE THRESHOLD (mV) 160 150 140 10 0 –10 –20 130 –30 120 150°C 100°C 25°C –50°C –40 VIN = 100V VIN = 6V –50 –60 25 50 75 100 125 150 TEMPERATURE (°C) 0.5 0 1.5 1.0 VREF (V) 2.
PAGE 6
LT3748 Pin Functions VIN (Pin 1) Input Voltage. This pin supplies current to the internal start-up circuitry and is the reference voltage for the feedback circuitry connected to the RFB pin. This pin must be locally bypassed with a capacitor. EN/UVLO (Pin 3): Enable/Undervoltage Lockout. A resistor divider connected to VIN is tied to this pin to program the minimum input voltage at which the LT3748 will operate. At a voltage below ~0.5V, the part draws less than 1µA quiescent current. When below 1.
PAGE 7
LT3748 block diagram T1 DOUT NPS:1 VIN CIN LPRI LSEC RFB 1 Q1 12 VOUT – RFB BOUNDARY MODE DETECT Q2 TC 1.223V 20µA INTVCC CBIAS ERROR AMP 1.223V RREF 50µs MAX OFF TIMER + gm – S R VARIABLE DELAY TIMER 3 1.223V EN/UVLO R2 2.4µA + A3 – 5 6.
PAGE 8
LT3748 Operation The LT3748 is a current mode switching regulator controller designed specifically for the isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation. Historically, this has been done with optoisolators or extra transformer windings.
PAGE 9
LT3748 Applications Information Pseudo-DC Theory of Operation The RREF and RFB resistors as depicted in the Block Diagram are external resistors used to program the output voltage. The LT3748 operates much the same way as traditional current mode switchers with the exception of the unique error amplifier which derives its feedback information from the flyback pulse. Operation is as follows: when the NMOS output switch turns off, its drain voltage rises above VIN. The amplitude of this flyback pulse (i.e.
PAGE 10
LT3748 Applications Information Selecting Actual RREF , RFB and RTC Resistor Values The preceding equations define how the LT3748 would regulate the output voltage if the system had no time delays and no error sources. However, there are a number of repeatable delays and parasitics in each application which will affect the output voltage and force a re-evaluation of the RFB and RTC component values. The following approach is the best method for selecting the correct values.
PAGE 11
LT3748 Applications Information Minimum Primary Inductance Requirements Output Power The LT3748 obtains output voltage information from the external MOSFET drain voltage when the secondary winding conducts current. The sampling circuitry needs a minimum of 400ns to settle and sample the output voltage while the MOSFET switch is off.
PAGE 12
LT3748 Applications Information The current limitation on output power delivery is generally constrained by transformer saturation current in higher power applications, although the MOSFET switch and output diode will need to be rated for the desired currents, as well.
PAGE 13
LT3748 Applications Information Table 1. Pre-Designed Transformers—Typical Specifications Unless Otherwise Noted TRANSFORMER PART NUMBER LLEAK (nH) NPS (NP:NS) ISAT (A) RPRI (mΩ) RSEC (mΩ) TARGET APPLICATION† Size (W x L x H) mm LPRI (µH) MANUFACTURER INPUT (V) OUTPUT 750311424 17.7 × 14.0 × 12.7 100 844 3:1 3 180 29 Würth Electronics 40 to 75 12V/1A 750311456* 17.7 × 14.0 × 12.7 100 900 3:1 2.4 225 31 Würth Electronics 40 to 75 12V/1A 750311439 17.7 × 14.0 × 12.
PAGE 14
LT3748 Applications Information ( VOUT + VF(DODE) ) • NPS VIN + ( VOUT + VF(DIODE) ) • NPS 2 ILIM • NPS ) • (1– D) ( = 5 0 20 0 60 40 INPUT VOLTAGE (V) 80 100 3748 F03 Figure 3. Maximum Output Power at 12V Out Using Three Transformers with Equal Peak Output Current and Secondary Inductance VIN = 12V 95 3 DOUT 90 85 80 fSW • QG + IQ FET RDS(ON) 75 TRANSFORMER I • R + LEAKAGE 70 0.2A MIN 2A MAX IOUT (A) 3748 F03 Figure 4.
PAGE 15
LT3748 Applications Information Saturation Current As discussed earlier in the Maximum Output Power section, because the core of the transformer is being used for energy storage in a flyback, the current in the transformer windings should not exceed their rated saturation current as energy injected once the core is saturated will not be transferred to the secondary and will instead be dissipated in the core.
PAGE 16
LT3748 Applications Information Although it typically does not decrease efficiency, leakage inductance energy that would normally have been dissipated in the switch or transformer is also dissipated in the RC snubber resistor and can be calculated as: PSNUBBER = fSW • LLEAK • ILIM2/2 An RCD clamp, shown in Figure 7, also prevents the leakage inductance spike from exceeding the breakdown voltage of the MOSFET switch. In most applications, there will be a very fast voltage spike caused by a slow clamp diode.
PAGE 17
LT3748 Applications Information (over manufacturing variations), this can be accommodated by adjusting the RFB /RREF resistor ratio. Winding Resistance Effects Resistance in either the primary or secondary will reduce overall efficiency (POUT /PIN). Good output voltage regulation will be maintained independent of winding resistance due to the boundary mode operation of the LT3748. Bifilar Winding A bifilar, or similar winding technique, is a good way to minimize troublesome leakage inductances.
PAGE 18
LT3748 Applications Information Soft-Start Minimum Load Requirement The LT3748 contains an optional soft-start function that is enabled by connecting an explicit external capacitor between the SS pin and ground. Internal circuitry prevents the control voltage at the VC pin from exceeding that on the SS pin. The LT3748 recovers output voltage information using the flyback pulse that occurs once the external MOSFET turns off and the secondary winding conducts current.
PAGE 19
LT3748 Applications Information LT3748 VIN 3.0 5V TO 100V VIN = 5V 2.5 INTVCC DROPOUT (V) LDO (VIN – DROPOUT) TO 7V INTVCC LT3748 VIN 2.0 INTVCC UVLO = 3.6V 1.5 1.0 IINTVCC = 20mA 0.5 5V TO 20V 0 –50 –25 LDO 0 25 50 75 100 125 150 TEMPERATURE (°C) 3748 F12 INTVCC LT3748 VIN LDO Figure 12. INTVCC Current at Low VIN Can Cause the LT3748 to Stop Switching Due to INTVCC Undervoltage Lockout OPTIONAL temperature, but when the dropout for the same current exceeds 1.
PAGE 20
LT3748 Applications Information Synchronous Secondary Applications 100 PRIMARY SIDE DRAIN VOLTAGE 80 VOLTAGE (V) Using a synchronous secondary controller such as the LT8309 with the LT3748 is an excellent method to boost converter efficiency and minimize heat, especially for lower output voltages and higher output currents. However, there are some important details to understand when designing a synchronous application.
PAGE 21
LT3748 Applications Information DESIGN EXAMPLE: 12VIN to 5V, 2A OUT The first example is an automotive application shown on the back page of this data sheet—a nominal 12VIN, 5VOUT at 2A with an operating input voltage range of 6V to 45V with a design focus of maximizing efficiency. 1.
PAGE 22
LT3748 Applications Information LPRI ≤ VIN(MIN) • (VOUT + VF(DIODE)) • NPS/(fSW(MIN) • ILIM • (VOUT + VF(DIODE)) • NPS + VIN(MIN))) LPRI ≥ (VOUT + VF(DIODE)) • RSENSE • 400ns • NPS/15mV LPRI ≥ VIN(MAX) • RSENSE • 200ns/15mV For this application, the primary inductance with a 2:1 transformer and a 0.016Ω sense resistor for an 6.25A current limit is bounded by the minimum desired switching frequency and the minimum off time requirement to be between 9.6µH and 11.5µH.
PAGE 23
LT3748 Applications Information 6. Select the Feedback Resistor for Proper Output Voltage Using the iterative process laid out earlier in the Applications Information section, select the feedback resistor RFB and program the output voltage to 5V. Adjust the RTC resistor for temperature compensation of the output voltage. RREF is selected as 6.04k. 7.
PAGE 24
LT3748 Applications Information DESIGN EXAMPLE: 48VIN to 12V, 2A OUT The second example is a telecom application shown on the front page of the datasheet. The focus of this application is a cheap, small and simple solution. Table 3 shows the results of the initial step for selecting the turns ratio. In this example, the output diode is a much smaller efficiency loss due to the smaller voltage drop across it in ratio to VOUT so minimizing output diode current is not as important.
PAGE 25
LT3748 Applications Information Figure 14. Demo Board Topside Silkscreen Figure 15. Demo Board Topside Metal 3748fb For more information www.linear.com/LT3748 Downloaded from Arrow.com.
PAGE 26
LT3748 typical Applications T1 1:1:1:1:1 VIN 12V TYP 10µF 1µF 825k EN/UVLO 6µH VIN RFB RREF 150k 71.5k D2 15V 300mA LT3748 GATE SS SENSE VC 133k 2nF GND M1 C2 D3 15V 300mA 10k C3 IGBT DRIVER Z1 4.7µF 4700pF IGBT DRIVER Z1 VO3 INTVCC 0.0125Ω 320V IGBT DRIVER Z1 C1 VO2 6.04k TC D1 15V 300mA VO1 VO4 D4 15V 300mA C1-C4: 22µH 25V X7R ×2 D1-D4: DIODES INC. PDS3100 M1: VISHAY Si7898DP T1: COILTRONICS VERSA-PAC VP4-0075-R Z1: DIODES INC. DFLZ18-7 C4 49.
PAGE 27
LT3748 typical Applications DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VIN 7V TO 15V T1 1:10:10 C1 10µF C2 1µF C8 0.22µF 50V R1 357k R2 93.1k VIN EN/UVLO RFB R3 140k R5 10k C5 TC GATE VOUT+ 300V 8mA VOUT– D2 C6 R4 6.04k LT3748 R7 600k D3 RREF SS D1 M1 R8 600k VOUT+ 300V 8mA VOUT– SENSE C7 0.1µF VC GND INTVCC 50mΩ 3748 F18 C9 100pF R6 24.9k C4 2.2nF C3 4.7µF C5, C6: 0.
PAGE 28
LT3748 typical Applications VIN 48V TYP T1 1:1 4.7µF 0.22µF 66Ω 825k EN/UVLO VIN D1 4.7µF 100V ×3 44.1µH 150pF VOUT– 226k RFB 49.9k VOUT+ 48V 0.5A RREF 6.04k LT3748 GATE TC M1 SENSE SS VC GND INTVCC 0.030Ω 10k 2nF 3748 F19 4.7µF 4700pF D1: CENTRAL SEMICONDUCTOR CMR5U-02-LTC M1: VISHAY Si7464DP T1: COILTRONICS VERSA-PAC VP4-0060-R Figure 19. 48V, 0.5A Supply from 24V to 96V Input 100 95 VIN = 24V EFFICIENCY (%) 90 85 VIN = 48V 80 VIN = 96V 75 70 65 60 0 0.1 0.2 0.4 0.
PAGE 29
LT3748 typical Applications PA1735NL 5.33:1:2.67 VIN 36V TO 72V 62µF D1 1.2M EN/UVLO 51k VIN LT3748 • 100Ω D2 120pF VOUT+ 5V, 8A • 910µF D4 3Ω VCC 147k RFB RREF 1µF DRAIN TC GATE 28k D1: SMBJ85A-13-F D2: CMMRIU-02 D3: BAV20W-7-F D4: BAV20W-7-F D5: CMZ5919B D6: CMHZ5258B M1: BSC320N20NS3G M2: BSC028N06NS SS SENSE GND INTVCC VC 0.22µF M1 470pF INTVCC GND 4.7µF VOUT– 68Ω 12.1k 3784 TA21 4.7nF • 4.7µF 15nF GATE M2 0.012Ω D3 LT8309 D5 2.15k 6.04k D6 Figure 21.
PAGE 30
LT3748 typical Applications Figure 23 Thermal Image of the Supply in Figure 21 Using a PDS760 Instead of the LT8309 and Synchronous Switch at 5V/5A Output Figure 24 Thermal Image of the Supply in Figure 21 with Synchronous Secondary-Side at 5V/5A Output with Much Lower Temperatures 30 Downloaded from Arrow.com. 3748fb For more information www.linear.
PAGE 31
LT3748 typical Applications PA1477NL 8:1.4 VIN 36V TO 72V 62µF D1 1.2M EN/UVLO 51k D1: SMBJ85A-13-F D2: CMMRIU-02 D3: BAV20W-7-F D4: BAV20W-7-F D5: CMZ5914 B D6: CMHZ5258B M1: BSC320N20NS3G M2: BSC016N04LS 19.1k 0.22µF VIN LT3748 120pF TC GATE SS VC SENSE GND INTVCC 1500µF D4 3Ω VCC 2k M1 M2 D6 LT8309 D5 DRAIN GATE INTVCC GND 4.7µF 0.015Ω • 4.7µF 1µF 6.04k 68Ω 15k 22nF • 158k RFB RREF D3 470pF 100Ω D2 VOUT+ 3.3V, 10A • VOUT– 3748 F25 4.7nF Figure 25. 3.
PAGE 32
LT3748 Package Description MS Package Varitation: MS16 (12) 16-Lead Plastic MSOP with 4 Pins Removed (Reference LTC DWG # 05-08-1847 Rev A) 1.0 (.0394) BSC 5.23 (.206) MIN 0.889 ± 0.127 (.035 ± .005) 3.20 – 3.45 (.126 – .136) 4.039 ± 0.102 (.159 ± .004) (NOTE 3) 16 14 121110 9 0.50 (.0197) BSC 0.305 ± 0.038 (.0120 ± .0015) TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.280 ± 0.076 (.011 ± .003) REF 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .
PAGE 33
LT3748 Revision History REV DATE DESCRIPTION A 10/10 Added H-grade information to Absolute Maximum Ratings, Pin Configuration, Order Information, and Electrical Characteristics sections. PAGE NUMBER Revised text and Table 2 in the Applications Information section. Revised Figures 10 and 17 in the Applications Information section. B 2/15 2, 3 15, 16, 20, 22 26, 27 Revised Typical Application drawing. 30 Added MP-grade device.
PAGE 34
LT3748 Typical Application 5V, 2A Output from Automotive Input with Continuous Operation from 6V to 45V D1 T1 2:1 VIN 12V TYP 10µF 18.2Ω 825k 330pF VIN EN/UVLO 8.3µH D2 VOUT– 48.7k RFB 215k 100µF 10V VOUT+ 5V, 2A RREF 6.04k LT3748 TC GATE SS SENSE VC 86.6k 47nF GND M1 INTVCC 0.016Ω 24.7k 2.2nF D1: DIODES INC. SBR8U60P5 D2: DIODES INC. BZT52C5V6 M1: Si7738DP T1: PULSE PA3177NL 3748 TA02 4.