Datasheet

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LT4430

4430fc

For more information www.linear.com/LT4430

Typical applicaTion

FeaTures DescripTion

Secondary-Side

Opto-Coupler Driver

The LT

4430 drives the opto-coupler that crosses the gal-

vanic barrier in an isolated power supply. The IC contains

precision-trimmed reference, a high bandwidth error

amplifier, an inverting gain of 6 stage to drive the opto-

coupler and unique overshoot control circuitry.

The LT4430’s 600mV reference provides ±0.75% initial

accuracy and ±1.25% tolerance over temperature. A high

bandwidth 9MHz error amplifier permits simple frequency

compensation and negligible phase shift at typical loop

crossover frequencies. The opto-coupler driver provides

10mA of output current and is short-circuit protected.

A unique overshoot control function prevents output

overshoot on start-up and short-circuit recovery with a

single capacitor.

The LT4430 is available in the low profile 6-lead TSOT-23

package.

applicaTions

600mV Reference (1.25% Over Temperature)

Wide Input Supply Range: 3V to 20V

Overshoot Control Function Prevents Output

Overshoot on Start-Up and Short-Circuit Recovery

High Bandwidth Error Amplifier Permits Simple Loop

Frequency Compensation

Ground-Referenced Opto-Coupler Drive

10mA Opto-Coupler Drive with Current Limiting

Low Profile (1mm) ThinSOT

Package

48V Input Isolated DC/DC Converters

Isolated Telecommunication Power Systems

Distributed Power Step-Down Converters

Offline Isolated Power Supplies

Industrial Control Systems

Automotive and Heavy Equipment

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

Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

Simplified Isolated Synchronous Forward Converter

Isolated Flyback Telecom Converter

Start-Up with Overshoot Control

(See Schematic on Back Page)

•

••

4430 TA01

OUT

OPTO

GND COMP

OC FB

LTC3900

SYNC

CG FG

LT4430

ISOLATION

BARRIER

LT1952

50V/DIV

OUT

5V/DIV

OVERSHOOT CONTROL

IMPLEMENTED

t = 5ms/DIV

4430 TA01b

Summary of content (26 pages)

PAGE 1
LT4430 Secondary-Side Opto-Coupler Driver Features n n n n n n n Description 600mV Reference (1.
PAGE 2
LT4430 Absolute Maximum Ratings Pin Configuration (Note 1) Supply Voltage VIN.........................................................................20V FB Voltage..................................................... –0.3V to 6V OPTO Short-Circuit Duration............................. Indefinite Operating Junction Temperature Range (Note 2) E-, I-Grades........................................ –40°C to 125°C H-Grade.............................................. –40°C to 150°C MP-Grade....................
PAGE 3
LT4430 Electrical Characteristics The ● denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, FB = VFB, COMP = 1V, unless otherwise noted (Note 3). SYMBOL PARAMETER CONDITIONS IOC Overshoot Control Charging Current VOC = 0V (E-, I-Grades) VOC = 0V (H-Grade) VOC = 0V (MP-Grade) ● ● ● MIN TYP MAX –15 –17 –17 –8.5 –8.5 –8.5 –5 –5 –4 OC Clamp Voltage AVOL 0.93 OC Amplifier Offset Voltage FB = 0.
PAGE 4
LT4430 Typical Performance Characteristics Undervoltage Lockout Threshold vs Temperature Quiescent Current vs Temperature 4.0 Feedback Reference Voltage vs Temperature 0.606 3.0 0.605 0.604 0.603 2.5 3.0 2.5 VIN = 3V 2.0 VFB (V) 0.602 VIN = 20V VUVLO (V) QUIESCENT CURRENT (mA) 3.5 2.0 0.601 0.600 0.599 0.598 0.597 1.5 1.5 0.596 0.595 1.
PAGE 5
LT4430 Typical Performance Characteristics Error Amplifier Output Swing Low vs Temperature 70 60 135 PHASE 40 90 30 GAIN 20 45 PHASE (°) 10 0 0 –10 1M 100k FREQUENCY (Hz) 10M –45 50M 0.4 0.3 0.2 0.1 0 –75 –50 –25 4430 G10 0 25 50 75 100 125 150 TEMPERATURE (°C) 1000 900 800 700 600 500 400 300 200 100 1.3 1.2 1.1 1.
PAGE 6
LT4430 Typical Performance Characteristics Opto Driver Output Swing Low vs Temperature Opto Driver Output Swing High vs Temperature 1.5 8.0 0.8 1.3 0.7 1.2 0.6 0.5 0.4 1.1 1.0 0.9 0.3 0.8 0.2 0.7 0.1 0.6 0 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) OPTO DRIVER OUTPUT SWING HIGH (V) VIN = 3V 1.4 IOPTO = 10mA 0.9 VIN – VOPTO (V) OPTO DRIVER OUTPUT SWING LOW (V) 1.0 Opto Driver Output Swing High vs Temperature 0.
PAGE 7
LT4430 Pin Functions VIN (Pin 1): This is the input supply that powers all internal circuitry. The input supply range is 3V minimum to 20V maximum and the typical input quiescent current is 1.9mA. Connect a 1µF bypass capacitor directly from VIN to GND. GND (Pin 2): Analog Ground Pin. It is also the negative sense terminal for the internal 0.6V reference. Connect the external feedback divider network that terminates to ground directly to this pin for best regulation and performance.
PAGE 8
LT4430 Block Diagram VIN + I1 12.5µA VIN STARTUP BIAS AND REFERENCE GENERATOR OPTO DRIVER COMP R3 15k – + 1.1V Q2 0.6V R4 90k ERROR AMP Q3 OUT – VIN I2 12.5µA Q7 VIN Q4 R2 2k DFB Q5 + – IOC 8.5µA OC AMP Q6 S1 NORMALLY OPEN V1 0.2V OC V2 0.6V + R1 2k + – LOGIC AND DELAY – UVLO FB GND Q1 + – VOS 48mV 4430 BD01 8 4430fc For more information www.linear.
PAGE 9
LT4430 Applications Information Block Diagram Operation A precision voltage reference, a high-bandwidth error amplifier, an inverting opto-coupler driver and an overshoot control amplifier comprise the LT4430. Referring to the block diagram, a start-up circuit establishes all internal current and voltage biasing for the IC. A precision-trimmed bandgap generates the 600mV reference voltage and a 1.1V bias voltage for the opto-coupler driver. Room temperature reference voltage accuracy is specified at ±0.
PAGE 10
LT4430 Applications Information If the fault condition ceases, the output voltage increases. In response, the error amplifier COMP pin’s voltage decreases. This action opens switch S1, deactivates the overshoot control amplifier and allows the OC pin capacitor to charge. The FB pin voltage increases quickly until the FB pin voltage exceeds the OC pin voltage. The feedback loop increases the drive to the opto-coupler until the FB pin follows and regulates to the OC pin voltage.
PAGE 11
LT4430 Applications Information T1 VIN T1 D1 VOUT • VIN VOUT • COUT • • 1:N 1:N 4430 F01a 4430 F01b Figure 1b. Equivalent Flyback Converter Connection Figure 1a. Typical Flyback Converter Connection T1 Tx1 VIN COUT D1 VIN VOUT • • Q1 VOUT • COUT • 1:N 1:N SYNC 4430 F01c *OPTIONAL SEE TEXT COUT D1 D2 R1* LT4430 VBIAS C1 4430 F01d Figure 1c. Synchronous Flyback Converter Connection Figure 1d.
PAGE 12
LT4430 Applications Information The second solution is to make a preregulator as shown in Figure 2b. In this example, the bias supply equals (VZ1 – VBE). Select R2 to bias Zener diode Z1 and to supply base current to QBS. Resistor R3 (on the order of a few hundred ohms), in series with Q5’s base, suppresses possible high frequency oscillations depending on QBS’s selection. The preregulator circuit has additional value for fully synchronous converters.
PAGE 13
LT4430 Applications Information Generate a bias supply for a forward converter using similar techniques to that of the flyback converter. Figure 3a to 3c detail the three common bias circuits for the synchronous single-switch forward converter. In the flyback converter of Figure 1d, the bias supply is proportional to VIN and VOUT. However, in the forward converter, L1’s presence decouples the bias supply from VOUT. In Figure 3a, the bias supply equals (VIN • N – VF).
PAGE 14
LT4430 Applications Information Figures 4a to 4d demonstrate bias supply circuits for the fully-synchronous push-pull topology. Biasing for fullbridge schemes is identical to the push-pull circuits with the obvious difference in the primary-side drive. In Figure 4a, the bias supply equals (VIN • N – VF). In Figure 4b and 4d, the bias supply equals (2 • VIN • N – VF). In Figure 4c and 4e, the bias supply equals (VZ1 – VF).
PAGE 15
LT4430 Applications Information D1 D1 R1* R2 T1 R3* T1 Q2 LT4430 VBIAS C2 Z1 L2 ME • VOUT VIN COUT • Q2 • L1 • • ME LT4430 VBIAS C1 • QBS C1 R1* 1:N L1 VOUT VIN Q1 MF *OPTIONAL SEE TEXT 4430 F04d COUT • • Figure 4d. Typical Synchronous Push-Pull Current-Doubler Converter with Bias Generator Q1 1:N MF *OPTIONAL SEE TEXT 4430 F04c Figure 4c.
PAGE 16
LT4430 Applications Information Setting Output Voltage Figure 5 shows how to program the power supply output voltage with a resistor divider feedback network. Connect the top of R1 to VOUT, the tap point of R1/R2 to FB and the bottom of R2 directly to GND of the LT4430. The FB pin regulates to 600mV and has a typical input pin bias current of 75nA flowing out of the pin. circuitry resides on the primary-side.
PAGE 17
LT4430 Applications Information In this example, the error amplifier is typically a transconductance amplifier with high output impedance and RC dominates the impedance at the VC node. Frequency compensation for this feedback loop is directly affected by the output transistor’s collector-to-base capacitance as it introduces a pole into the feedback loop. This pole varies considerably with the transistor’s operating conditions. In many cases, this pole limits the achievable loop bandwidth.
PAGE 18
LT4430 Applications Information In this example, the error amplifier is typically a voltage error amplifier configured as a transimpedance amplifier. The opto-coupler transistor’s emitter provides feedback information directly to the FB pin and the resistor RE from FB to GND sets the DC bias condition for the opto-coupler.
PAGE 19
LT4430 Applications Information Setting Overshoot Control Time Figure 7 shows how to calculate the overshoot time by connecting a capacitor from the OC pin to GND. The overshoot control time, tOC, is set by the formula: tOC = (COC • 0.6V)/8.5µA The OC pin requires a minimum capacitor of 100pF due to stability requirements with the overshoot control amplifier. This yields a minimum time of 7µs which is generally on the order of a few cycles of the switching regulator.
PAGE 20
LT4430 Applications Information Next, increase COC’s value. Either use a capacitor substitution box or solder each new value into the circuit. Monitor the start-up and short-circuit recovery waveforms. Note any changes. Figures 8b to 8e illustrate what happens as COC increases. In general, overshoot decreases as COC increases. COC = 0.0168µF in Figure 8b begins to affect loop dynamics, but start-up still exhibits about 1.5V of overshoot. Short-circuit recovery is considerably more damped. COC = 0.
PAGE 21
LT4430 Applications Information START-UP VOUT 5V/DIV START-UP VOUT 5V/DIV SHORT-CIRCUIT RECOVERY VOUT 5V/DIV SHORT-CIRCUIT RECOVERY VOUT 5V/DIV t = 5ms/DIV 4430 F08a COC = 100pF Figure 8a. Start-Up and Short-Circuit Recovery Waveforms t = 5ms/DIV COC = 0.0168µF = 0.01µF + 6.8nF Figure 8b.
PAGE 22
C6 0.1µF R7 33k C5 0.47µF R5 114k R8 33k R6 33k R4 13.2k 36V TO 72V VIN For more information www.linear.com/LT4430 R11 1.2k R10 22k NC R9 33k R3 370k 4 2 1 6 5 9 3 7 GND BLANK SYNC FB COMP SOUT ISENSE OC LT1952 DELAY VREF SS_MAXDC PGND VIN SOUT D1 12V ROSC SD_VSEC C1 2.2µF 100V R1 82k R12 39k C3 2.2µF VU1 D2 18V VU1 Q1 BCX55 C7 220pF D4 BAT760 C2 1µF R26 10Ω R14 0.
PAGE 23
+VIN 93 94 95 96 97 –VIN 6 8 For more information www.linear.com/LT4430 66.5k 1.5nF 30k 1/4W 4 1µF 15 5 A 6 A DRVA 13 7 B 4 8 1.5k 2 10k 33k 12 14 68nF 4 1 VREF SDRB 2 ISNS 150k 9 SPRG RLEB SS DPRG 16 D3 B ISNS 11 330pF 0.47µF 243k L4 1mH 22nF 6 0.1µF D6 D5 3 4 2 6 1 5 8 5 C4 2.2nF 250V MOC207 1.1k 5 2 1 7 SYNC 4.7nF 5 6 OPTO 14 15 6 CSE+ 5 1 VIN 2 GND 10 OC FB 3 4 470pF 1.
PAGE 24
LT4430 Package Description S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4.
PAGE 25
LT4430 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION B 5/11 H-Grade and MP-Grade parts added. Reflected throughout the data sheet. PAGE NUMBER C 10/13 1-26 Correct Q2 in Block Diagram from NPN to PNP 8 Changed R24 from 26.1k to 261k 22 4430fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 26
LT4430 Typical Application 5V, 2A Isolated Flyback Telecom Converter Start-Up Waveforms with Overshoot Control Implemented ISOLATION BARRIER 36V TO 72V VIN C1 1µF 100V –VIN ITH/SHDN R1 220k D1 PDZ-9.1B 9.1V ITH/RUN GND R2 100k Q2 MMBTA42 CTX-02-15242 T1 8.5V 2 D2 BAS516 4 Q1 FDC2512 NGATE LTC3803 FB C2 1µF 10V 9, 10 CO1 100µF 6.3V D4 UPS840 • 11, 12 R4 220Ω CO2 100µF 6.3V RCS 0.068Ω D5 MBR0530 R7 11k 1% 8.5V R5 6.8k D3 BAS516 R10 680Ω C5 1µF VIN C6 0.033µF GND C8 0.