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LTC3803-3

38033fd

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

and ThinSOT and No R

SENSE

are trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Constant Frequency

Current Mode Flyback

DC/DC Controller in ThinSOT

The LTC

3803-3 is a constant frequency current mode

ﬂ yback controller optimized for driving N-channel MOSFETs

in high input voltage applications. Constant frequency

operation is maintained down to very light loads, resulting

in less low frequency noise generation over a wide range

of load currents. Slope compensation can be programmed

with an external resistor.

The LTC3803-3 provides ±1.5% output voltage accuracy

and consumes only 240μA of quiescent current. Ground-

referenced current sensing allows LTC3803-3-based con-

verters to accept input supplies beyond the LTC3803-3’s

absolute maximum V

. A micropower hysteretic start-up

feature allows efﬁ cient operation at high input voltages. For

simplicity, the LTC3803-3 can also be powered from a high

through a resistor, due to its internal shunt regulator. An

internal undervoltage lockout shuts down the LTC3803-3

when the input voltage is too low to provide sufﬁ cient gate

drive to the external MOSFET.

The LTC3803-3 is available in a low proﬁ le (1mm) 6-lead

SOT-23 (ThinSOT

™

) package.

5V Output Nonisolated Telecom Housekeeping Power Supply

and V

OUT

Limited Only by External Components

Adjustable Slope Compensation

Internal Soft-Start

–55°C to 150°C Operating Temperature Range

Constant Frequency 300kHz Operation

±1.5% Reference Accuracy

Current Mode Operation for Excellent Line and Load

Transient Response

No Minimum Load Requirement

Low Quiescent Current: 240μA

Low Proﬁ le (1mm) SOT-23 Package

Telecom Power Supplies

42V and 12V Automotive Power Supplies

Auxiliary/Housekeeping Power Supplies

Power over Ethernet Powered Devices

/RUN

LTC3803-3

GND

NGATE

SENSE

UPS840

300μF*

6.3V

X5R

1μF

100V

X5R

150pF

200V

1μF

10V

X5R

OUT

2A MAX

38033 TA01

68mΩ 220Ω

20k

10k

82k

470pF

105k

4.7k

FDC2512

T1: COOPER CTX02-15242

*THREE 100μF UNITS IN PARALLEL

•

36V TO 72V

LOAD CURRENT (mA)

250

EFFICIENCY (%)

100

750

1250

1500

38033 TA02

500

1000

1750

2000

= 36V

= 48V

= 60V

= 72V

OUT

= 5V

Efﬁ ciency vs Load Current

Summary of content (16 pages)

PAGE 1
LTC3803-3 Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT FEATURES DESCRIPTION n The LTC®3803-3 is a constant frequency current mode ﬂyback controller optimized for driving N-channel MOSFETs in high input voltage applications. Constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. Slope compensation can be programmed with an external resistor.
PAGE 2
LTC3803-3 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) VCC to GND Low Impedance Source .......................... –0.3V to 8V Current Fed ....................................... 25mA into VCC* NGATE Voltage .......................................... –0.3V to VCC VFB, ITH/RUN Voltages............................... –0.3V to 3.5V SENSE Voltage ............................................ –0.3V to 1V NGATE Peak Output Current (<10μs) .........................
PAGE 3
LTC3803-3 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VCC = 8V, unless otherwise noted.
PAGE 4
LTC3803-3 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Reference Voltage vs Supply Voltage Reference Voltage vs Temperature 820 801.0 VCC = 8V 804 VCC b VCLAMP1mA 800.8 815 Reference Voltage vs VCC Shunt Regulator Current 803 800.6 805 800 795 790 802 800.4 VFB VOLTAGE (mV) VFB VOLTAGE (mV) VFB VOLTAGE (mV) 810 800.2 800.0 799.8 799.6 799.0 –30 0 30 60 90 TEMPERATURE (°C) 120 150 6 6.5 8 7.5 8.
PAGE 5
LTC3803-3 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Start-Up ICC Supply Current vs Temperature 800 70 60 50 40 30 20 ITH/RUN PIN CURRENT SOURCE (nA) SHUTDOWN THRESHOLD (mV) 80 400 350 300 250 200 150 10 –30 0 30 60 90 TEMPERATURE (°C) 120 100 –60 150 –30 0 30 60 120 90 Peak Current Sense Voltage vs Temperature 600 500 400 300 200 100 0 –60 150 –30 0 30 60 90 TEMPERATURE (°C) 120 150 38033 G12 Soft-Start Time vs Temperature 4.0 VCC = 8V 115 3.
PAGE 6
LTC3803-3 PIN FUNCTIONS SENSE (Pin 4): This pin performs two functions. It monitors switch current by reading the voltage across an external current sense resistor to ground. It also injects a current ramp that develops slope compensation voltage across an optional external programming resistor. ITH/RUN (Pin 1): This pin performs two functions. It serves as the error ampliﬁer compensation point as well as the run/shutdown control input. Nominal voltage range is 0.7V to 1.9V.
PAGE 7
LTC3803-3 OPERATION The LTC3803-3 is a constant frequency current mode controller for ﬂyback and DC/DC boost converter applications in a tiny ThinSOT package. The LTC3803-3 is designed so that none of its pins need to come in contact with the input or output voltages of the power supply circuit of which it is a part, allowing the conversion of voltages well beyond the LTC3803-3’s absolute maximum ratings.
PAGE 8
LTC3803-3 OPERATION The undervoltage lockout (UVLO) mechanism prevents the LTC3803-3 from trying to drive a MOSFET with insufﬁcient VGS. The voltage at the VCC pin must exceed VTURNON (nominally 8.7V) at least momentarily to enable LTC3803-3 operation. The VCC voltage is then allowed to fall to VTURNOFF (nominally 5.7V) before undervoltage lockout disables the LTC3803-3.
PAGE 9
LTC3803-3 APPLICATIONS INFORMATION Many LTC3803-3 application circuits can be derived from the topology shown in Figure 2. The LTC3803-3 itself imposes no limits on allowed power output, input voltage VIN or desired regulated output voltage VOUT; these are all determined by the ratings on the external power components.
PAGE 10
LTC3803-3 APPLICATIONS INFORMATION CURRENT SENSE RESISTOR CONSIDERATIONS The external current sense resistor (RSENSE in Figure 2) allows the user to optimize the current limit behavior for the particular application. As the current sense resistor is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to several amperes. Care must be taken to ensure proper circuit operation, especially with small current sense resistor values.
PAGE 11
LTC3803-3 APPLICATIONS INFORMATION One potential design pitfall is undersizing the value of capacitor CVCC. In this case, the normal supply current drawn by the LTC3803-3 will discharge CVCC too rapidly; before the third winding drive becomes effective, the VCC turn-off threshold will be reached. The LTC3803-3 turns off, and the VCC node begins to charge via RSTART back up to the VCC turn-on threshold.
PAGE 12
LTC3803-3 TYPICAL APPLICATIONS 2W Isolated Housekeeping Telecom Converter BAS516 PRIMARY SIDE 10V, 100mA OUTPUT T1 • 2.2μF 1μF VIN 36V TO 75V • 22k 806Ω 2.2μF BAS516 9.2k 1nF BAS516 1k 1 LTC3803-3 6 ITH/RUN NGATE 2 5 3 GND VFB VCC SENSE 4 220k • SECONDARY SIDE 10V, 100mA OUTPUT SECONDARY SIDE GROUND FDC2512 T1: PULSE ENGINEERING PA0648 OR TYCO TTI8698 5.6k 1μF PRIMARY GROUND 0.
PAGE 13
LTC3803-3 TYPICAL APPLICATIONS 4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter T1 PA1277NL VIN+ 18V TO 72V 2.2μF VIN– • 220k MMBTA42 100k • GND BAS516 68Ω PDZ6.8B 100μF 6.3V s3 PDS1040 VOUT+ 3.3V 3A 150pF VCC 680Ω 10Ω 22Ω BAS516 • 1 ITH/RUN GATE 6 FDC2512 0.1μF LTC3803-3 2 5 VCC GND VFB SENSE 4 BAT760 0.1μF 0.040Ω 270Ω VCC 1 6.8k BAS516 PS2801-1 0.1μF 1 2 0.33μF BAS516 2 3 VIN OPTO LT4430 GND OC COMP FB VOUT+ 6 2.2nF 5 56k 47pF 100k 4 22.
PAGE 14
LTC3803-3 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 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3.
PAGE 15
LTC3803-3 REVISION HISTORY (Revision history begins at Rev D) REV DATE DESCRIPTION D 6/10 MP-grade part added. Reﬂected throughout the data sheet. PAGE NUMBER 1 to 16 38033fd 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 16
LTC3803-3 TYPICAL APPLICATION Efﬁciency vs Load 100 VOUT = 3.3V 90% Efﬁcient Synchronous Flyback Converter VIN 36V TO 72V VOUT* 3.3V 1.5A T1 • Q2 CIN 220k EFFICIENCY (%) 95 85 80 CO • 1nF 90 VIN = 36V VIN = 48V VIN = 60V VIN = 72V 75 D1 33k 1 ITH/RUN GATE 6 Q1 LTC3803-3 2 5 VCC GND 8.06k 3 VFB = 0.8V SENSE 70 500 • 4.7k 4 10μF 10V 1000 1250 1500 LOAD CURRENT (mA) 1750 2000 38033 TA04b Efﬁciency vs Load 38033 TA04a 25.5k* RFB 750 0.