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LT3845

3845fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

High Voltage Synchronous

Current Mode Step-Down

Controller with Adjustable

Operating Frequency

The LT

3845 is a high voltage, synchronous, current

mode controller used for medium to high power, high

efﬁ ciency supplies. It offers a wide 4V to 60V input range

(7.5V minimum start-up voltage). An onboard regulator

simpliﬁ es the biasing requirements by providing IC power

directly from V

Burst Mode

operation maintains high efﬁ ciency at light

loads by reducing IC quiescent current to 120μA. Light

load efﬁ ciency is also improved with the reverse inductor

current inhibit function which supports discontinuous

operation.

Additional features include adjustable ﬁ xed operating

frequency that can be synchronized to an external clock

for noise sensitive applications, gate drivers capable of

driving large N-channel MOSFETs, a precision undervoltage

lockout, 10μA shutdown current, short-circuit protection

and a programmable soft-start.

The LT3845 is available in a 16-lead thermally enhanced

TSSOP package and 16-pin through hole N package.

■

High Voltage Operation: Up to 60V

■

Synchronizable Up to 600kHz

■

Adjustable Constant Frequency: 100kHz to 500kHz

■

Output Voltages Up to 36V

■

Adaptive Nonoverlap Circuitry Prevents Switch

Shoot-Through

■

Reverse Inductor Current Inhibit for Discontinuous

Operation Improves Efﬁ ciency with Light Loads

■

Programmable Soft-Start

■

120μA No Load Quiescent Current

■

10μA Shutdown Supply Current

■

1% Regulation Accuracy

■

Standard Gate N-Channel Power MOSFETs

■

Current Limit Unaffected by Duty Cycle

■

Reverse Overcurrent Protection

■

16-Lead Thermally Enhanced TSSOP Package,

16-Pin PDIP

■

12V and 42V Automotive and Heavy Equipment

■

48V Telecom Power Supplies

■

Avionics and Industrial Control Systems

■

Distributed Power Converters

High Voltage Step-Down Regulator 48V to 12V at 75W

Efﬁ ciency and Power Loss vs

Load Current

L, LT, LTC, LTM, Burst Mode, 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 5481178, 6611131, 6304066, 6498466, 6580258.

SHDN

BURST_EN

SYNC

SET

BOOST

PGND

SENSE

–

SGND

BAS521

1N4148

3845 TA01a

0.1μF

1500pF

Si7370DP

B160

15μH

0.01Ω

LT3845

1μF

33μF

×3

OUT

12V

75W

82.5k

16.2k

20V TO 55V

2.2μF

100V

49.9k

20k

100pF

2200pF

143k

47μF

63V

LOAD CURENT (A)

100

3845 TA01b

EFFICIENCY(%)

POWER LOSS (W)

0.1

= 48V

LOSS

Summary of content (26 pages)

PAGE 1
LT3845 High Voltage Synchronous Current Mode Step-Down Controller with Adjustable Operating Frequency DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ High Voltage Operation: Up to 60V Synchronizable Up to 600kHz Adjustable Constant Frequency: 100kHz to 500kHz Output Voltages Up to 36V Adaptive Nonoverlap Circuitry Prevents Switch Shoot-Through Reverse Inductor Current Inhibit for Discontinuous Operation Improves Efﬁciency with Light Loads Programmable Soft-Start 120μA No Load Quiescent Current 10μA Shu
PAGE 2
LT3845 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN) ........................................65V Boosted Supply Voltage (BOOST) .............................80V Switch Voltage (SW) (Note 8) ....................... 65V to –2V Differential Boost Voltage (BOOST to SW).....................................................24V Bias Supply Voltage (VCC) .........................................24V SENSE+ and SENSE– Voltages ..................................
PAGE 3
LT3845 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9kΩ, SENSE – = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted.
PAGE 4
LT3845 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9kΩ, SENSE – = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS TG, BG Drive On Voltage (Note 7) TG, BG Drive Off Voltage CLOAD = 3300pF CLOAD = 3300pF 9.8 0.
PAGE 5
LT3845 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Threshold (Rising) vs Temperature Shutdown Threshold (Falling) vs Temperature 1.37 1.36 1.35 1.34 1.33 1.32 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 8.2 1.25 8.1 1.23 1.22 1.20 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 7.5 –50 –25 125 ICC Current Limit vs Temperature ICC = 20mA TA = 25°C 200 ICC CURRENT LIMIT (mA) VCC (V) VCC (V) 6 5 7.90 4 15 20 25 ICC(LOAD) (mA) 30 35 3 40 100 4 5 8 7 6 9 10 11 50 –50 12 –25 6.
PAGE 6
LT3845 TYPICAL PERFORMANCE CHARACTERISTICS Operating Frequency vs Temperature I(SENSE+ + SENSE–) vs VSENSE(CM) 800 Error Amp Reference vs Temperature 1.234 308 TA = 25°C 306 400 200 0 –200 1.233 ERROR AMP REFERENCE (V) OPERATING FREQUENCY (kHz) I(SENSE+ + SENSE–) (μA) 600 304 302 300 298 296 294 290 –50 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE(CM) (V) –25 0 25 50 75 TEMPERATURE (°C) 100 3845 G10 125 1.227 –50 –25 102 100 98 96 100 125 3845 G13 100 125 3845 G12 3.
PAGE 7
LT3845 PIN FUNCTIONS BG: The BG pin is the gate drive for the bottom N-channel MOSFET. Since very fast high currents are driven from this pin, connect it to the gate of the power MOSFET with a short and wide, typically 0.02" width, PCB trace to minimize inductance. BOOST: The BOOST pin is the supply for the bootstrapped gate drive and is externally connected to a low ESR ceramic boost capacitor referenced to SW pin.
PAGE 8
LT3845 PIN FUNCTIONS TG: The TG pin is the bootstrapped gate drive for the top N-Channel MOSFET. Since very fast high currents are driven from this pin, connect it to the gate of the power MOSFET with a short and wide, typically 0.02” width, PCB trace to minimize inductance. VC: The VC pin is the output of the error ampliﬁer whose voltage corresponds to the maximum (peak) switch current per oscillator cycle.
PAGE 9
R1 R2 CIN CSS CC2 CC1 RC RB RA CSS VC VFB BURST_EN SHDN VIN + – 100mV ERROR AMP – gm 1V 0.5V + FEEDBACK REFERENCE 1.231V 3.
PAGE 10
LT3845 APPLICATIONS INFORMATION Overview Power Requirements The LT3845 is a high input voltage range step-down synchronous DC/DC converter controller IC that uses a programmable constant frequency, current mode architecture with external N-channel MOSFET switches. The LT3845 is biased using an internal linear regulator to generate operational voltages from the VIN pin. Virtually all of the circuitry in the LT3845 is biased via this internal linear regulator output (VCC).
PAGE 11
LT3845 APPLICATIONS INFORMATION Inductor Auxiliary Winding where QG(TOTAL) is the total MOSFET gate charge of the TG and BG. In applications where these conditions are exceeded, VCC must be derived from an external source after start-up. Maximum continuous regulator power dissipation may be exceeded for short duration VIN transients.
PAGE 12
LT3845 APPLICATIONS INFORMATION VIN pin, giving a total VIN current of 120μA. Burst current can be reduced further when VCC is driven using an output derived source, as the VCC component of VIN current is then reduced by the converter buck ratio. Reverse-Current Inhibit The LT3845 contains a reverse-current inhibit feature to maximize efﬁciency during light load conditions. This mode of operation allows discontinuous operation and pulse-skipping mode at light loads. Refer to Figure 1.
PAGE 13
LT3845 APPLICATIONS INFORMATION at start-up the soft-start circuit will discharge the VC pin voltage below the DC control voltage equivalent to zero inductor current. This will reduce the input supply inrush current. The soft-start circuit is disabled once the CSS pin voltage has been charged to 200mV above the internal reference of 1.231V. During a VIN UVLO, VCC UVLO or SHDN UVLO event, the CSS pin voltage is discharged with a 50μA current source.
PAGE 14
LT3845 APPLICATIONS INFORMATION has a maximum voltage threshold of, typically, 100mV. Therefore, the peak inductor current is 100mV/RSENSE. The maximum output load current, IOUT(MAX), is the peak inductor current minus half the peak-to-peak ripple current, ΔIL.
PAGE 15
LT3845 APPLICATIONS INFORMATION will increase the peak currents, requiring more ﬁltering on the input and output of the supply. If ΔIL is too high, the slope compensation circuit is ineffective and current mode instability may occur at duty cycles greater than 50%. To satisfy slope compensation requirements the minimum inductance is calculated as follows: L MIN > VOUT • 2DCMAX – 1 RSENSE • 8.33 • DCMAX fSW The magnetics vendors specify either the saturation current, the RMS current or both.
PAGE 16
LT3845 APPLICATIONS INFORMATION Choose the MOSFET VDSS speciﬁcation to exceed the maximum voltage across the drain to the source of the MOSFET, which is VIN(MAX) plus any additional ringing on the switch node. Ringing on the switch node can be greatly reduced with good PCB layout and, if necessary, an RC snubber.
PAGE 17
LT3845 APPLICATIONS INFORMATION The maximum ESR required to meet a ΔVOUT design requirement can be calculated by: ( VOUT )(L)(fSW) ESR(MAX) = VOUT • 1– VOUT The VFB pin input bias current is typically 25nA, so use of extremely high value feedback resistors could cause a converter output that is slightly higher than expected. Bias current error at the output can be estimated as: ΔVOUT(BIAS) = 25nA • R2 VIN(MAX) Supply UVLO and Shutdown Worst-case ΔVOUT occurs at highest input voltage.
PAGE 18
LT3845 APPLICATIONS INFORMATION If low supply current in standby mode is required, select a higher value of RB. The supply turn off voltage is 9% below turn on. In the example the VSUPPLY(OFF) would be 13.2V. If additional hysteresis is desired for the enable function, an external positive feedback resistor can be used from the LT3845 regulator output. The shutdown function can be disabled by connecting the SHDN pin to the VIN through a large value pull-up resistor.
PAGE 19
LT3845 APPLICATIONS INFORMATION FET carry PGND currents. SGND originates at the negative terminal of the VOUT bypass capacitor, and is the small signal reference for the LT3845. Don’t be tempted to run small traces to separate ground paths. A good ground plane is important as always, but PGND referred bypass elements must be oriented such that transient currents in these return paths do not corrupt the SGND reference.
PAGE 20
LT3845 APPLICATIONS INFORMATION Locate the feedback resistors in close proximity to the LT3845 to minimize the length of the high impedance VFB node. The SENSE– and SENSE+ traces should be routed together and kept as short as possible. The LT3845 TSSOP package has been designed to efﬁciently remove heat from the IC via the Exposed Pad on the backside of the package. The Exposed Pad is soldered to a copper footprint on the PCB.
PAGE 21
LT3845 TYPICAL APPLICATIONS 9V-16V to 3.3V at 10A DC/DC Converter Capable of Withstanding 60V Transients, All Ceramic Capacitors and Soft-Start Enabled VIN 9V TO 16V 60V TRANSIENTS CIN 2.2μF 100V ×4 CIN2 0.1μF 100V C5 1μF 16V R3 1.1M VIN BOOST SHDN CSS C3 8200pF BURST_EN R2 16.9k R4 25k R5 100k D1 VCC D2A BAV99 BG VC SYNC SENSE+ fSET SENSE– SGND VOUT 3.3V 10A COUT 100μF 6.3V ×5 C4 2.2μF 16V C2 R6 6800pF 49.9k RSENSE 0.
PAGE 22
LT3845 TYPICAL APPLICATIONS 9V-16V to 5V at 10A DC/DC Converter, 500kHz Frequency Operation, Capable of Withstanding 36V Transients, All Ceramic Capacitors, Soft-Start and Burst Mode Enabled VIN 9V TO 16V 36V TRANSIENTS CIN 6.8μF 50V ×4 CIN2 0.1μF 50V C5 1μF 16V R3 1.1M VIN C3 8200pF BOOST SHDN CSS BURST_EN VC R2 154k C1 100pF R4 10k L1 2.7μH SW D1 VCC D2 BAS19 BG VFB R1 49.9k M1 Si7884DP TG LT3845 COUT 100μF 6.3V ×4 + SENSE fSET SENSE– C4 2.
PAGE 23
LT3845 TYPICAL APPLICATIONS 9V-24V to 3.3V, 2-Phase at 10A per Phase, DC/DC Converter with Spread Spectrum Operation VIN 24V CIN 6.8μF 50V ×2 C5 1μF 16V R3 1.21M VIN BOOST SHDN C3 8200pF R4 1.21M CSS BURST_EN SYNC SENSE+ fSET SENSE– R6 130k D2 BAS19 L1 4.7μH D1 B160 M2 Si7850DP SGND C4 2.2μF 16V VOUT 3.3V 20A R12 25k Q1 D5 5.7V CIN3 0.1μF 100V R11 500k C10 1μF 16V VIN 1 C11 0.1μF 6 SYNC1 V+ OUT1 LTC6908-1 2 5 SYNC2 GND OUT2 3 4 SET3 MOD CSS R2 16.
PAGE 24
LT3845 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 p 0.10 9 2.94 (.116) 4.50 p 0.10 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 p 0.05 1.05 p 0.10 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 1.10 (.0433) MAX 0o – 8o 0.65 (.0256) BSC 0.50 – 0.75 (.020 – .030) NOTE: 1.
PAGE 25
LT3845 REVISION HISTORY (Revision history begins at Rev D) REV DATE DESCRIPTION PAGE NUMBER D 1/10 Updated Features and Description 1 Revised Absolute Maximum Ratings, Pin Conﬁguration and Order Information to Add New Package and Grade Options 2 Revised Electrical Characteristics 3, 4 Revised Pin Functions 7, 8 Updated Block Diagram 9 Revised Oscillator SYNC Section Revised Typical Applications Updated Related Parts List 18 21, 22, 23, 26 26 3845fd Information furnished by Linear Techno
PAGE 26
LT3845 TYPICAL APPLICATION 9V-16V to 3.3V at 5A DC/DC Converter, Frequency Synchronization Range 150kHz to 300kHz, Capable of Withstanding 60V Transients, All Ceramic Capacitors, Soft-Start and Burst Mode Enabled VIN 9V TO 16V 60V TRANSIENTS CIN 2.2μF 100V ×4 CIN2 0.1μF 100V C5 1μF 16V R3 1.1M VIN C3 8200pF BOOST SHDN CSS C1 R2 16.8k 100pF SYNC R4 10k C2 5600pF R5 100k RSENSE 0.01Ω VOUT 3.