Datasheet

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LTC3805

3805fg

Features

applications

Description

Adjustable Frequency

Current Mode Flyback

DC/DC Controller

The LT C

3805 is a current mode controller for flyback

DC/DC converters designed to drive an N-channel MOSFET

in high input and output voltage converter applications.

Operating frequency and slope compensation can be

programmed by external resistors. Programmable overcur-

rent sensing protects the converter from short-circuits.

Soft-start can be programmed using an external capacitor

and the soft-start capacitor also programs an automatic

restart feature.

The LTC3805 provides ±1.5% output voltage accuracy

and consumes only 360µA of quiescent current during

normal operation and only 40µA during micropower start-

up. Using a 9.5V internal shunt regulator, the LTC3805

can be powered from a high V

through a resistor or it

can be powered directly from a low impedance DC voltage

of 9V or less.

The LTC3805 is available in the 10-lead MSOP package

and the 3mm × 3mm DFN package.

36V – 72V to 5V/2A Nonisolated Flyback Converter

and V

OUT

Limited Only by External Components

Adjustable Slope Compensation

Adjustable Overcurrent Protection with Automatic

Restart

Adjustable Operating Frequency (70kHz to 700kHz)

with One External Resistor

Synchronizable to an External Clock

±1.5% Reference Accuracy

Current Mode Operation for Excellent Line and Load

Transient Response

RUN Pin with Precision Threshold and Adjustable

Hysteresis

Programmable Soft-Start with One External Capacitor

Low Quiescent Current: 360µA

Small 10-Lead MSOP and 3mm × 3mm DFN

Telecom Power Supplies

42V and 12V Automotive Power Supplies

Isolated Electronic Equipment

LTC3805

GND

RUN

GATE

SENSE

SSFLT

FS FB

118k

68mΩ

2.2µF

×2

22µF

SYNC

1.33k

8.66k

221k 100k

36V TO

72V

OUT

AT 2A

13.7k

71.5k

100µF

× 2

3805 TA01a

0.1µF

20k

470pF

LOAD CURRENT (A)

0.01

EFFICIENCY (%)

POWER LOSS (W)

0.1

3805 TA01b

100 10

0.1

= 48V

LOSS

EFFICIENCY

Efficiency and Power Loss

vs Load Current; V

= 5V

typical application

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

No R

SENSE

and ThinSOT are trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Summary of content (20 pages)

PAGE 1
LTC3805 Adjustable Frequency Current Mode Flyback DC/DC Controller Description Features n n n n n n n n n n n VIN and VOUT Limited Only by External Components Adjustable Slope Compensation Adjustable Overcurrent Protection with Automatic Restart Adjustable Operating Frequency (70kHz to 700kHz) with One External Resistor Synchronizable to an External Clock ±1.
PAGE 2
LTC3805 Absolute Maximum Ratings (Note 1) VCC to GND Low Impedance Source......................... –0.3V to 8.8V Current Fed.........................................25mA into VCC* SYNC............................................................ –0.3V to 6V SSFLT........................................................... –0.3V to 5V FB, ITH, FS.................................................. –0.3V to 3.5V RUN............................................................ –0.3V to 18V OC, ISENSE............
PAGE 3
LTC3805 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C, VCC = 8V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN TYP MAX l 7.75 8.4 9 l 3.75 3.95 VTURNON VCC Turn-On Voltage VTURNOFF VCC Turn-Off Voltage VHYST VCC Hysteresis 4.15 VCLAMP1mA VCC Shunt Regulator Voltage VCC Sinking 1mA, VRUN = 0 l 8.8 9.25 9.
PAGE 4
LTC3805 electrical Characteristics Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3805 is tested under pulsed conditions such that TJ ≈ TA. The LTC3805E is guaranteed to meet specifications from 0°C to 85°C.
PAGE 5
LTC3805 Typical Performance Characteristics Reference Voltage vs Temperature Reference Voltage vs Supply Voltage 812 0.800300 808 0.800200 804 0.800100 Oscillator Frequency vs RFS 800 700 800 fOSC (kHz) VFB (V) VFB VOLTAGE (mV) 600 0.800000 796 0.799900 792 0.799800 500 400 300 200 788 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0.799700 100 4 5 6 7 8 203 200 199 198 197 5 6 7 8 1.200 201 RFS = 124kΩ 200 199 VCC (V) 1.185 1.
PAGE 6
LTC3805 typical performance characteristics ICC Supply Current vs Temperature 100.8 9.55 340 PEAK CURRENT SENSE VOLTAGE (mV) 9.60 350 VCLAMP25mA 9.50 330 VCLAMP (V) SUPPLY CURRENT (µA) Peak Current Sense Voltage vs Temperature VCC Shunt Regulator Voltage vs Temperature 320 9.45 9.40 9.35 9.30 310 VCLAMP1mA 9.25 300 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 9.20 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 100.4 100.2 100.0 99.8 99.6 99.4 99.2 99.
PAGE 7
LTC3805 pin functions SSFLT (Pin 1): Soft-Start Pin. A capacitor placed from this pin to GND (Exposed Pad) controls the rate of rise of converter output voltage during start-up. This capacitor is also used for time out after a fault prior to restart. ITH (Pin 2): Error Amplifier Compensation Point. Normal operating voltage range is clamped between 0.7V and 1.9V. FB (Pin 3): Receives the feedback voltage from an external resistor divider across the output.
PAGE 8
LTC3805 Operation The LTC3805 is a programmable-frequency current mode controller for flyback, boost and SEPIC DC/DC converters. The LTC3805 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 LTC3805’s absolute maximum ratings. Main Control Loop Please refer to the Block Diagram of this data sheet and the Typical Application shown on the front page.
PAGE 9
LTC3805 Operation Start-Up/Shutdown Setting the Oscillator Frequency The LTC3805 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the VCC supply pin voltage, and a precision-threshold RUN pin. The voltage on both pins must exceed the appropriate threshold before operation is enabled. The LTC3805 transitions into and out of shutdown according to the state diagram shown in Figure 1. Operation in fault timeout is discussed in a subsequent section.
PAGE 10
LTC3805 Operation First, consider operation without overcurrent protection. For some maximum converter output current, the voltage on the ITH pin rises to and is clamped at approximately 1.9V. This corresponds to a 100mV limit on the voltage at the ISENSE pin. As the output current is further increased, the duty cycle is reduced as the output voltage sags. However, the peak current in the external MOSFET is limited by the 100mV threshold at the ISENSE pin.
PAGE 11
LTC3805 Applications Information Many LTC3805 application circuits can be derived from the topology shown on the first page of this data sheet and from the topology shown in Figure 2. built-in shunt regulator limits the voltage on the VCC pin to around 9.5V as long as the internal shunt regulator is not forced to sink more than 25mA.
PAGE 12
LTC3805 Applications Information CVCC is charged to VTURNON. At this point, assuming VRUN > VRUNON, the converter begins switching the external MOSFET and ramps up the converter output voltage at a rate set by the capacitor CSS on the SSFLT pin. Since RSTART cannot supply enough current to operate the external MOSFET, CVCC begins discharging and VCC drops.
PAGE 13
LTC3805 Applications Information Leakage Inductance where R1(5µA) is the additional hysteresis introduced by the 5µA current sourced by the RUN pin. When in shutdown, the RUN pin does not source the 5µA current and the rising threshold for VIN is simply Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the MOSFET (Q1) turn-off. This is increasingly prominent at higher load currents, where more stored energy must be dissipated.
PAGE 14
LTC3805 Applications Information Feedback in Isolated Applications Isolated applications do not use the FB pin and error amplifier but control the ITH pin directly using an optoisolator driven on the other side of the isolation barrier as shown in Figure 5. A detailed version is shown in Figure 9. For isolated converters, the FB pin is grounded which provides pull-up on the ITH pin. This pull-up is not enough to properly bias the opto-isolator which is typically biased using a resistor to VCC.
PAGE 15
LTC3805 Applications Information Overcurrent Threshold Adjustment Figure 6 shows the connection of the overcurrent pin, OC, along with the ISENSE pin and the current sense resistor RSENSE located in the source circuit of the power NMOS which is driven by the GATE pin. The internal overcurrent threshold on the OC pin is set at VOCT = 100mV which is the same as the peak current sense voltage VI(MAX) = 100mV on the ISENSE pin.
PAGE 16
LTC3805 Applications Information be slowed by using a larger value of SSFLT capacitor. It is also possible to trip overcurrent protection during a load step especially if the trip threshold is lowered by making ROC > ROC(CRIT). Another overcurrent protection strategy is keep the converter running as current limiting reduces the duty cycle and the output voltage sags.
PAGE 17
LTC3805 Applications Information internal soft-start time tSS(IN) = 1.8ms. However, in noisy environments a small CSS can be valuable to limit jitter in the oscillator. At this point, the LTC3805 attempts a restart. In the event of a persistent fault, such as a short-circuit on the converter output, the converter enters a “hiccup” mode where it continues to try and restart at repetition rate determined by CSS. If the fault is eventually removed the converter successfully restarts.
PAGE 18
LTC3805 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) R = 0.125 TYP 6 0.40 ±0.10 10 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) PACKAGE OUTLINE 0.25 ±0.05 1.65 ±0.10 (2 SIDES) 0.75 ±0.05 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) 0.00 – 0.
PAGE 19
LTC3805 Revision History (Revision history begins at Rev E) REV DATE DESCRIPTION PAGE NUMBER E 04/10 Changes to Typical Application 1, 17 Updates to Absolute Maximum Ratings 2 Changes to Electrical Characteristics Tables 3 Updates to Note 2 4 Update to Pin 11 in Pin Functions 7 Edits to Start-Up/Shutdown in Operation Section 9 Updated Related Parts Table 20 F 05/11 Added MP-grade part.
PAGE 20
LTC3805 Typical Application VIN+ 36V TO 72V CIN2 2.2µF 100V CIN1 2.2µF 100V T1 PA1861NL 4 7 8 10 1 9 5 R23 100k D4 BAS516 R30 51Ω D1 UPS840 VOUT+ 3.3V 3A +VOUT C01 100µF 6.3V C02 100µF 6.3V C03 100µF 6.3V 2 VOUT– – VIN C16 150pF 200V C25 100pF C26 470pF C18 330pF R6 220Ω R12 42.2k R24 20k C23, 0.1µF 3 Q1 FDC2512 R14 13.7k VCC 1 2 3 4 5 R5 221k R8 8.66k R20 118k C17 10µF 10 9 8 7 6 RCS 0.068Ω Efficiency and Power Loss vs Load Current and VIN; VO = 3.3V R16, 1.33k 100 R19 3.