Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - PFC 70 µA MSOP 16 EP

LT3798

3798fa

Isolated PFC Flyback with Minimum Number of

External Components

and V

OUT

Limited Only by External Components

Active Power Factor Correction

Low Harmonic Distortion

No Opto-Coupler Required

Constant-Current and Constant-Voltage Regulation

Accurate Regulated Voltage and Current (±5% Typical)

Energy Star Compliant (<0.5W No Load Operation)

Thermally Enhanced 16-lead MSOP Package

TYPICAL APPLICATION

FEATURES DESCRIPTION

Isolated No Opto-Coupler

Flyback Controller

with Active PFC

The LT

3798 is a constant-voltage/constant-current iso-

lated flyback controller that combines active power factor

correction (PFC) with no opto-coupler required for output

voltage feedback into a single-stage converter. A LT3798

based design can achieve a power factor of greater than 0.97

by actively modulating the input current, allowing compli-

ance with most Harmonic Current Emission requirements.

The LT3798 is well suited for a wide variety of off-line

applications. The input range can be scaled up or down,

depending mainly on the choice of external components.

Efficiencies higher than 86% can be achieved with output

power levels up to 100W. In addition, the LT3798 can easily

be designed into high DC input applications.

Universal Input 24W PFC Bus Converter V

OUT

vs I

OUT

APPLICATIONS

Offline 5W to 100W+ Applications

High DC V

Isolated Applications

Offline Bus Converter (12V, 24V or 48V Outputs)

L, LT, LTC, LTM, 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.

3798 TA01a

IN_SENSE

EN/UVLO

DCM

REF

CTRL2

CTRL1

OVP

GATE

SENSE

INTV

GND

LT3798

COMP

–

560µF

× 2

4.7pF

10µF

2.2nF

4:1:1

2.2µF

0.1µF

20Ω

0.05Ω

499k

95.3k

22pF

100k

4.99k

90.9k

CTRL3

90V

TO 265V

0.1µF

40.2k

100k

16.5k

221k

4.7µF

24V

OUT

(A)

23.50

OUT

(V)

24.25

24.00

23.75

24.50

0.6 0.8 10.2 0.4

3798 TA01b

VAC = 265V

VAC = 220V

VAC = 120V

VAC = 90V

Summary of content (20 pages)

PAGE 1
LT3798 Isolated No Opto-Coupler Flyback Controller with Active PFC DESCRIPTION FEATURES n n n n n n n n n Isolated PFC Flyback with Minimum Number of External Components VIN and VOUT Limited Only by External Components Active Power Factor Correction Low Harmonic Distortion No Opto-Coupler Required Constant-Current and Constant-Voltage Regulation Accurate Regulated Voltage and Current (±5% Typical) Energy Star Compliant (<0.
PAGE 2
LT3798 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) EN/UVLO....................................................................30V VIN.............................................................................42V INTVCC.......................................................................12V CTRL1, CTRL2, CTRL3.................................................4V FB, VREF, COMP +.........................................................3V VC, OVP, COMP–.............................................
PAGE 3
LT3798 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 18V, INTVCC = 11V, unless otherwise noted.
PAGE 4
LT3798 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Threshold vs Temperature EN/UVLO Hysteresis Current vs Temperature RISING 1.26 1.24 FALLING 1.22 1.2 –50 –25 0 11.5 11 60 50 40 30 10.5 20 10 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 VREF vs Temperature 2.05 SENSE Current Limit Threshold vs Temperature VREF vs VIN 120 2.04 CURRENT LIMIT (mA) VIN = 24V WITH NO LOAD 2.000 1.975 VIN = 24V WITH 200µA LOAD VREF (V) 2.02 2.025 2.01 NO LOAD 2 1.99 200µA LOAD 1.98 1.
PAGE 5
LT3798 TYPICAL PERFORMANCE CHARACTERISTICS 10.5 INTVCC vs Temperature 10.2 NO LOAD 10mA LOAD 25mA LOAD TA = 25°C, unless otherwise noted. INTVCC vs VIN 42 10 VIN SHUNT VOLTAGE (V) INTVCC (V) INTVCC (V) 9.8 9.6 9.4 9.2 0 9 25 50 75 100 125 150 TEMPERATURE (°C) 5 10 25 20 VIN (V) 30 35 9 8 7 6 25 50 75 100 125 150 TEMPERATURE (°C) 3798 G11 25 1.8 1.6 VOUT vs Temperature PAGE 17 SCHEMATIC: UNIVERSAL 24.5 1.4 1.2 1 0.8 VAC = 120V 24 VAC = 220V 0.6 23.5 0.4 0.
PAGE 6
LT3798 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Power Factor vs Input Voltage Efficiency vs Input Voltage 100 1.00 0.99 90 0.97 EFFICIENCY (%) POWER FACTOR 0.98 PAGE 17 SCHEMATIC: UNIVERSAL 0.96 0.95 0.94 0.93 80 70 0.92 0.91 PAGE 17 SCHEMATIC: UNIVERSAL 0.
PAGE 7
LT3798 PIN FUNCTIONS GATE (Pin 14): N-Channel FET Gate Driver Output. Switches between INTVCC and GND. Driven to GND during shutdown state and stays high during low voltage states. VIN_SENSE (Pin 16): Line Voltage Sense Pin. The pin is used for sensing the AC line voltage to perform power factor correction. Connect a resistor in series with the line voltage to this pin. If no PFC is needed, connect this pin to INTVCC with a 25k resistor. SENSE (Pin 15): The Current Sense Input for the Control Loop.
PAGE 8
LT3798 OPERATION The LT3798 is a current mode switching controller IC designed specifically for generating a constant current/ constant voltage supply in an isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage and current 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 an opto-isolator.
PAGE 9
LT3798 OPERATION Primary Side Current Control Loop The CTRL1/CTRL2/CTRL3 pins control the output current of the flyback controller. To simplify the loop, let’s assume the VIN_SENSE pin is held at a constant voltage above 1V eliminating the multiplier from the control loop. The error amplifier, A5, is configured as integrator with the external capacitor C6. The COMP+ node voltage is converted to a current into the multiplier with the V/I converter, A6.
PAGE 10
LT3798 OPERATION fundamental frequency of the supply voltage is 120Hz so the control loop unity gain frequency needs to be set less than approximately 12Hz. Without a large amount of energy storage on the secondary side, the output current will be affected by the supply voltage changes, but the DC component of the output current will be accurate. For DC input or non-PFC AC input applications, connect a 25k resistor from VIN_SENSE to INTVCC instead of the AC line voltage.
PAGE 11
LT3798 OPERATION Programming Output Current The maximum output current depends on the supply voltage and the output voltage in a flyback topology.
PAGE 12
LT3798 OPERATION to the input capacitor. This is a second-order network composed of the parasitic capacitance on the switch node and the magnetizing inductance of the primary winding of the transformer. The minimum voltage of the switch node during this discontinuous ring is VIN – VOUT • NPS. The LT3798 turns the switch back on at this time, during the discontinuous switch waveform, by sensing when the slope of the switch waveform goes from negative to positive using the dv/dt detector.
PAGE 13
LT3798 OPERATION Switch Voltage Clamp Requirement Leakage inductance of an offline transformer is high due to the extra isolation requirement. The leakage inductance energy is not coupled to the secondary but goes into the drain node of the MOSFET. This is problematic since 400V and higher rated MOSFETs cannot always handle this energy by avalanching. Therefore the MOSFET needs protection.
PAGE 14
LT3798 OPERATION voltage and the MOSFET switching frequency to determine the expected power dissipation: PSNUBBER = fSW • CSNUBBER • VDRAIN2/2 Decreasing the value of the capacitor will reduce the dissipated power in the snubber at the expense of increased peak voltage on the MOSFET drain, while increasing the value of the capacitance will decrease the overshoot. Transformer Design Considerations Transformer specification and design is a critical part of successfully applying the LT3798.
PAGE 15
LT3798 OPERATION MOSFET and Diode Selection Power Factor Correction/Harmonic Content With a strong 1.9A gate driver, the LT3798 can effectively drive most high voltage MOSFETs. A low Qg MOSFET is recommended to maximize efficiency. In most applications, the RDS(ON) should be chosen to limit the temperature rise of the MOSFET. The drain of the MOSFET is stressed to VOUT • NPS + VIN during the time the MOSFET is off and the secondary diode is conducting current.
PAGE 16
LT3798 OPERATION Protection from Shorted Output Conditions Usage with DC Input Voltage During a shorted output condition as shown in Figure 6, the LT3798 operates at the minimum operating frequency. In normal operation, the third winding provides power to the IC, but the third winding voltage is zero during a shorted condition. This causes the part’s VIN UVLO to shutdown switching. The part starts switching again when VIN has reached its turn-on voltage.
PAGE 17
LT3798 TYPICAL APPLICATIONS Universal Input 24W PFC Bus Converter L2 800µH L1 33mH BR1 C1 0.068µF 90V TO 265V AC C2 0.1µF R3 499k R4 499k R5 1M R7 100k R17 D2 20Ω R8 100k C4 C5 4.7pF 10µF D3 4:1:1 R13 2k VIN DCM VIN_SENSE FB R14 90.9k Z1 R15 4.99k EN/UVLO R6 95.3k GATE VREF BR1: DIODES, INC. HD06 C8: VISHAY 440LD22-R D1: CENTRAL SEMICONDUCTOR CMR1U-06M D2, D3: DIODES INC.
PAGE 18
LT3798 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ± 0.102 (.112 ± .004) 5.23 (.206) MIN 2.845 ± 0.102 (.112 ± .004) 0.889 ± 0.127 (.035 ± .005) 8 1 1.651 ± 0.102 (.065 ± .004) 1.651 ± 0.102 3.20 – 3.45 (.065 ± .004) (.126 – .136) 0.305 ± 0.038 (.0120 ± .0015) TYP 16 0.50 (.0197) BSC 4.039 ± 0.
PAGE 19
LT3798 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/12 Added H- and MP-grade parts 2, 3 3798fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 20
LT3798 TYPICAL APPLICATION 112W Wide DC Input Industrial Power Supply VIN 20V TO 60V R17 D2 20Ω R7 6.8k C2 10µF TO INTVCC R8 6.8k R4 24k R5 402k C4 15pF C5 10µF D3 R13 15k VIN DCM FB VIN_SENSE EN/UVLO R6 51.1k 1:1:0.8 R14 100k R15 5.9k LT3798 D4 C6 22pF Z1 Z3 D1 D5 C10 10µF ×4 28V 4A VREF R11 100k R9 40.2k CTRL3 GATE CTRL2 SENSE CTRL1 INTVCC OVP R12 221k R10 34.8k VC M1 C9 4.7µF GND COMP+ C3 0.1µF R3 16.2k COMP– RS 0.004Ω C8 2.