Datasheet

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

LTC1698

1698f

APPLICATIO S

DESCRIPTIO

FEATURES

TYPICAL APPLICATIO

The LTC

1698 is a precision secondary-side forward

converter controller that synchronously drives external

N-channel MOSFETs. It is designed for use with the

3781 primary-side synchronous forward converter

controller to create a completely isolated power supply.

The LT3781 synchronizes the LTC1698 through a small

pulse transformer and the LTC1698 drives a feedback

optocoupler to close the feedback loop. Output accuracy

of ±0.8% and high efficiency over a wide range of load

currents are obtained.

The LTC1698 provides accurate secondary-side current

limit using an external current sense resistor. The input

voltage at the MARGIN pin provides ±5% output voltage

adjustment. A power good flag and overvoltage input are

provided to ensure proper power supply conditions. An

auxiliary 3.3V logic supply is included that supplies up to

10mA of output current.

Isolated Secondary

Synchronous Rectifier Controller

■

High Efficiency Over Wide Load Current Range

■

±0.8% Output Voltage Accuracy

■

Dual N-Channel MOSFET Synchronous Drivers

■

Pulse Transformer Synchronization

■

Optocoupler Feedback Driver

■

Programmable Current Limit Protection

■

±5% Margin Output Voltage Adjustment

■

Adjustable Overvoltage Fault Protection

■

Power Good Flag

■

Auxiliary 3.3V Logic Supply

■

Available in 16-Lead SSOP and SO Packages

■

48V Input Isolated DC/DC Converters

■

Isolated Telecommunication Power Systems

■

Distributed Power Step-Down Converters

■

Industrial Control Systems

■

Automotive and Heavy Equipment

Figure 1. Simplified 2-Transistor Isolated Forward Converter

, LTC and LT are registered trademarks of Linear Technology Corporation.

PWRGD

LTC1698

PGND GND

CILM

OUT

CILM

OUT

110

36V to 72V

•

PRISEN

SNS

SECSEN

SNSGND

SYNC

COMP

AUX

COMP

OVPIN

MARGIN V

MARGIN

O.1µF

1681 F01

AUX

3.3V

10mA

OPTODRV

PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS

SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC

SYNC

ISOLATION

BOUNDARY

REF

TG BG

LT3781

SYNC

BIAS

–

•

Summary of content (24 pages)

PAGE 1
LTC1698 Isolated Secondary Synchronous Rectifier Controller U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC ®1698 is a precision secondary-side forward converter controller that synchronously drives external N-channel MOSFETs. It is designed for use with the LT®3781 primary-side synchronous forward converter controller to create a completely isolated power supply.
PAGE 2
LTC1698 W U U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) VDD, PWRGD ....................................................... 13.2V Input Voltage MARGIN, VFB, OVPIN, ISNSGND, ISNS ... – 0.3V to 5.3V SYNC ..................................................... – 14V to 14V Output Voltage VCOMP, ICOMP (Note 2) ......................... – 0.3V to 5.3V Power Dissipation .............................................. 500mW Operating Temperature Range LTC1698E (Note 3) .............
PAGE 3
LTC1698 ELECTRICAL CHARACTERISTICS The ● indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 8V, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 3.135 3.320 3.465 V 0.05 0.05 1 1 µA µA VAUX VAUX Auxiliary Supply Voltage CVAUX = 0.1µF, ILOAD = 0mA to 10mA, VDD = 7V to 12.
PAGE 4
LTC1698 U W TYPICAL PERFOR A CE CHARACTERISTICS VFB vs VDD VFB vs Temperature 1.248 1.248 VDD = 8V VFB vs VMARGIN 1.295 TA = 25°C 1.282 1.242 1.242 1.230 1.230 1.224 1.224 1.218 –50 –25 1.218 0 6 5 25 50 75 100 125 150 TEMPERATURE (°C) 7 8 9 10 VDD (V) 11 12 ISNS Threshold vs Temperature –22.5 VDD = 8V 0 1.221 –1 1.208 –2 1.196 –3 1.184 –4 13 1.171 14 0 0.33 0.66 0.99 1.32 1.65 1.98 2.31 2.64 2.97 3.
PAGE 5
LTC1698 U W TYPICAL PERFOR A CE CHARACTERISTICS VAUX vs Temperature VAUX vs Line Voltage 3.465 3.465 3.383 3.383 3.383 3.341 3.341 3.341 VDD = 8V 3.424 ILOAD = 0mA 3.300 VDD = 8V 3.424 TA = 25°C VAUX (V) VAUX (V) VDD = 8V 3.424 ILOAD = 0mA 3.300 3.300 3.259 3.259 3.259 3.218 3.218 3.218 3.176 3.176 3.176 3.135 3.
PAGE 6
LTC1698 U W TYPICAL PERFOR A CE CHARACTERISTICS SYNC Positive Threshold vs Temperature IVDD vs SYNC Frequency CFG = CCG = 4700pF CFG = CCG = 3300pF 35 CFG = CCG = 2200pF 30 25 20 15 10 CFG = CCG = 1000pF 5 0 2.00 1.75 1.50 1.
PAGE 7
LTC1698 U U U PI FU CTIO S VDD (Pin 1): Power Supply Input. For isolated applications, a simple rectifier from the power transformer is used to power the chip. This pin powers the opto driver, the VAUX supply and the FG and CG drivers. An internal 5V regulator powers the remaining circuitry. VDD requires an external 4.7µF bypass capacitor. CG (Pin 2): Catch Gate Driver. If SYNC slews positive, CG pulls high to drive an external N-channel MOSFET.
PAGE 8
LTC1698 W BLOCK DIAGRA 1 VDD 14 15 VAUX AUX GEN VCC GEN FG VCC SYNC CG 7 16 SYNC IN 2 RMARGIN MARGIN I-TO-V CONVERTER BANDGAP ±5% VREF VREF + 5 OPTODRV OPTO – + 20k ERR VFB – 100k VCOMP 10 PWRGD MPWRGD ISNSGND + MILIM ILIM – 25mV – RILIM 3k + ICOMP PWRGD + ROVP 3k VFB ISNS 0.
PAGE 9
LTC1698 U OPERATIO (Refer to Block Diagram) catch MOSFETs Q3 and Q4 are off. As soon as transistors Q1 and Q2 turn on, the flux in the power transformer T1 forces the body diodes of Q3 and Q4 to conduct, and the whole circuit starts like a conventional forward converter. At the same time, the LTC1698 VDD potential ramps up quickly through the VDD bias circuitry. Once the VDD voltage exceeds 4.0V, the LTC1698 enables its drivers and enters synchronous operation.
PAGE 10
LTC1698 U W U U APPLICATIO S I FOR ATIO Undervoltage Lockout In UVLO (low VDD voltage) the drivers FG and CG are shut off and the pins OPTODRV, VAUX, PWRGD and ICOMP are forced low. The LTC1698 allows the bandgap and the internal bias currents to reach their steady-state values before releasing UVLO. Typically, this happens when VDD reaches approximately 4.0V. Beyond this threshold, the drivers start switching.
PAGE 11
LTC1698 U W U U APPLICATIO S I FOR ATIO drivers TG and BG go high. The pulse transformer T2 generates a negative slew at the SYNC pin and forces the secondary MOSFET driver FG to go high and CG to go low. When TG and BG go low, SG goes high and the secondary controller forces CG high and FG low. For a given pulse transformer, a bigger capacitor CSG generates a higher and wider SYNC pulse. The peak of this pulse should be much higher than the SYNC threshold.
PAGE 12
LTC1698 U W U U APPLICATIO S I FOR ATIO MARGIN Adjustment The MARGIN input is used for adjusting the programmed output voltage linearly by varying the current flowing into and out of the pin. Forcing 100µA into the pin moves the output voltage 5% higher. Forcing 100µA out of the pin moves the output voltage 5% lower. With the MARGIN pin floating, the VFB pin is regulated to the bandgap voltage of 1.233V. The MARGIN pin is a high impedance input.
PAGE 13
LTC1698 U W U U APPLICATIO S I FOR ATIO supply must cross the isolation boundary. Coupling this signal requires an element that will withstand the isolation potentials and still transfer the loop error signal. Optocouplers are widely used for this function due to their ability to couple DC signals. To properly apply them, a number of factors must be considered.
PAGE 14
LTC1698 U W U U APPLICATIO S I FOR ATIO A series RC network can be added in parallel with R2 (Figure 7) to provide a zero for the feedback loop frequency compensation. The opto driver will drive a capacitive load up to 0.1µF. For optocouplers with a base pin, switching signal noise can get into this high impedance node. Connect a large resistor, 1M or 2M between the base and the emitter. This increases the diode current and the overall feedback bandwidth slightly, and decreases the optocoupler gain.
PAGE 15
LTC1698 U W U U APPLICATIO S I FOR ATIO If the application generates a bigger current sense voltage, a potential divider can be easily obtained by adding a resistor across C2. With this additional resistor, the voltage sensed by the current comparator becomes: RDIV • VRSENSE RDIV + (2 • R6) An RC network formed by RCILM and CCILM between ICOMP and VOUT can be used to stabilize the current limit loop.
PAGE 16
LTC1698 U W U U APPLICATIO S I FOR ATIO Power Transformer Selection The forward transformer provides DC isolation and delivers energy from the primary to the secondary. Unlike the flyback topology, the transformer in the forward converter is not an energy storage device. As such, ungapped ferrite material is typically used. Select a power material rated with low loss at the switching frequency. Many core manufacturers have selection guides and application notes for transformer design.
PAGE 17
LTC1698 U W U U APPLICATIO S I FOR ATIO Once the value of the inductor has been determined, an inductor with sufficient DC current rating is selected. Core saturation must be avoided under all operating conditions. Under start-up conditions, the converter sees a short circuit while charging the output capacitor. If the inductor saturates, the peak current will dramatically increase. The current will be limited only by the primary controller minimum on time and the circuit impedances.
PAGE 18
LTC1698 U W U U APPLICATIO S I FOR ATIO L1 VOUT+ • • T1 + 1Ω Q3 COUT Q4 RSECSEN VOUT – D3 1 C4 2 • • T2 4.7µF R7 FG LTC1698 CG SYNC 3 R2 VAUX PGND 4 GND 5 6 RK MOC207 R1 CK VDD RC CC 7 CFB 8 ICOMP ISNS OPTODRV ISENSGND VCOMP MARGIN PWRGD VFB OVPIN 16 CCILM 15 RCILM 14 R5 13 1k 12 0.1µF 11 10 1k 0.1µF 9 0.1µF BOLD LINES INDICATE HIGH CURRENT PATHS R4 1698 F09 Figure 9. LTC1698 Layout Diagram VOUT1 3.
PAGE 19
4.7µF 16V 1000pF 10k VIN 5241B 11V 20k 270k 0.25W VIN MMBD4148 VCC 1µF ON/OFF VIN– 0.56µH DO1813P-561HC VCC 0.1µF 100V 100Ω FMMT718 FMMT619 VCC B2100 10k 0.047µF 330pF BAT54 1.24k 1% 1µF 5VREF 52.3k 1% 5 RT1 100k 82pF 6 0Ω 7 0.1µF 8 4 10Ω 10 3300pF 3k 1k 5VREF 5 6 7 1• 4 MOC207 8 4 3 220pF 0Ω 2 1 •8 7 T2 PULSE ENG P2033 2200pF 250VAC 3.3Ω 2200pF 4700pF VOUT 1µF 16V 1 VAUX FG 16 GND 4 LTC1698 ISNSGND 11 0.22µF 50V PGND 3 ISNS 12 2k 0.
PAGE 20
0.1µF 20V 270k 1/4W 47k 100Ω 1/4W 0.82µF 100V ×2 3300pF 10k RIN (OPTIONAL) VIN 20 18 19 17 16 15 10Ω MMBT3906 11 10k BAT54 BAT54 0.1µF 12 VCC 330pF ZETEX ZVN3310F 1mH DO1608C-105 COILCRAFT 1.24k 1% 52.3k 1% 5 RT1 100k 82pF 6 0.01µF 2.43k 1% 3 10k 7 4 4700pF 8 3k 1k 3300pF 10Ω 10 5 8 4 3 MOC207 8 100Ω 2 1 5 •4 4700pF 1k 8 470Ω 10V MMBZ5240B 2k PULSE ENG PA0184 1• 3.
PAGE 21
1.5µF 100V + 68µF 25V 1000pF 18V MMBZ5248BLT1 0.33µF 50V 0.1µF 50V 270k 0.25W 20k 10k 15V MMBZ5245BLT1 VCC PINS 9-10 5T BIFILAR 33AWG PINS 2-5 12T BIFILAR 33AWG PINS 4-3 7T QUADFILAR 26AWG PINS 7,8-11,12 12T BIFILAR 24AWG PINS 6-4 8T QUADFILAR 26AWG VIN– VIN+ 1.24k 1% 24k 56k 73.2k 1% VIN 0.1µF 100V VTOP 2M POLYESTER FILM 1.5µF 100V 100pF 200Ω 10Ω VCC 330pF ZVN3310F VTOP BAT54 10k BAT54 1mH DO1608C-105 COILCRAFT VCC 1µF 25V 5VREF 52.
PAGE 22
LTC1698 U PACKAGE DESCRIPTIO GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) .045 ±.005 16 15 14 13 12 11 10 9 .254 MIN .009 (0.229) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 TYP RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) .053 – .068 (1.351 – 1.727) 2 3 4 5 6 7 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.
PAGE 23
LTC1698 U PACKAGE DESCRIPTIO S Package 16-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .386 – .394 (9.804 – 10.008) NOTE 3 .045 ±.005 .050 BSC 16 N 15 14 13 12 11 10 9 N .245 MIN .160 ±.005 .150 – .157 (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) 1 .030 ±.005 TYP 2 3 N/2 N/2 RECOMMENDED SOLDER PAD LAYOUT 1 .010 – .020 × 45° (0.254 – 0.508) 2 3 4 5 6 .053 – .069 (1.346 – 1.752) .008 – .010 (0.203 – 0.254) NOTE: 1. DIMENSIONS IN .014 – .
PAGE 24
LTC1698 U TYPICAL APPLICATIO S LT3781/LTC1698 Isolated 5V/30A Converter Efficiency vs Load Current LT3781/LTC1698 Isolated 3.3V/15A Converter 95 VIN = 36V 90 EFFICIENCY (%) VIN = 48V 85 80 VIN = 72V 75 70 65 TOP 0 10 15 20 LOAD CURRENT (A) 5 25 30 1698 TA01 LT3781/LTC1698 Isolated 3.3V/15A Converter LT3781/LTC1698 Isolated 3.