Datasheet

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LT3800

3800fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

High-Voltage Synchronous

Current Mode Step-Down

Controller

The LT

3800 is a 200kHz ﬁ xed frequency high voltage

synchronous current mode step-down switching regulator

controller. The IC drives standard gate N-channel power

MOSFETs and can operate with input voltages from 4V to

60V. An onboard regulator provides IC power directly from

and provides for output-derived power to minimize

quiescent current. MOSFET drivers employ an internal

dynamic bootstrap feature, maximizing gate-source “ON”

voltages during normal operation for improved operating

efﬁ ciencies. The LT3800 incorporates Burst Mode

opera-

tion, which reduces no load quiescent current to under

100µA. Light load efﬁ ciencies are also improved through

a reverse inductor current inhibit, allowing the controller

to support discontinuous operation. Both Burst Mode

operation and the reverse-current inhibit features can be

disabled if desired. The LT3800 incorporates a program-

mable soft-start that directly controls the voltage slew rate

of the converter output for reduced startup surge currents

and overshoot errors. The LT3800 is available in a 16-lead

thermally enhanced TSSOP package.

12V 75W DC/DC Converter with Reverse Current Inhibit and Input UVLO

Wide 4V to 60V Input Voltage Range

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

Output Slew Rate Controlled Soft-Start with

Auto-Reset

100µA No Load Quiescent Current

Low 10µA Current Shutdown

1% Regulation Accuracy

200kHz Operating Frequency

Standard Gate N-Channel Power MOSFETs

Current Limit Unaffected by Duty Cycle

Reverse Overcurrent Protection

16-Lead Thermally Enhanced TSSOP Package

12V and 42V Automotive and Heavy Equipment

48V Telecom Power Supplies

Avionics and Industrial Control Systems

Distributed Power Converters

Efﬁ ciency and Power Loss

SHDN

BURST_EN

SENSE

–

BOOST

PGND

SENSE

LT3800

SGND

1.5nF

200k

100pF

680pF

174k

20k

82.5k

0.015Ω

BAS19

1N4148

1µF

×3

56µF

×2

10µF

270µF

Si7850DP

Si7370DP

B160

15µH

20V TO 55V

OUT

12V AT 75W

3800 TA01a

LOAD

(A)

EFFICIENCY (%)

POWER LOSS (W)

100

3800 TA01b

0.1 1

= 24V

LOSS (48V)

= 48V

= 60V

= 36V

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

and No R

SENSE

and ThinSOT are 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.

Summary of content (24 pages)

PAGE 1
LT3800 High-Voltage Synchronous Current Mode Step-Down Controller DESCRIPTION FEATURES n n n n n n n n n n n n n Wide 4V to 60V Input Voltage Range Output Voltages up to 36V Adaptive Nonoverlap Circuitry Prevents Switch Shoot-Through Reverse Inductor Current Inhibit for Discontinuous Operation Improves Efﬁciency with Light Loads Output Slew Rate Controlled Soft-Start with Auto-Reset 100μA No Load Quiescent Current Low 10μA Current Shutdown 1% Regulation Accuracy 200kHz Operating Frequency Standard Gate N-
PAGE 2
LT3800 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION Supply Voltages Input Supply Pin (VIN)............................... –0.3V to 65V Boosted Supply Pin (BOOST).................... –0.3V to 80V Boosted Supply Voltage (BOOST – SW) ... –0.3V to 24V Boosted Supply Reference Pin (SW)............ –2V to 65V Local Supply Pin (VCC).............................. –0.3V to 24V Input Voltages SENSE+, SENSE– ...................................... –0.3V to 40V SENSE+ – SENSE– ....................................
PAGE 3
LT3800 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, SENSE – = SENSE+ = 10V, SGND = PGND = SW = 0V, CTG = CBG = 3300pF, unless otherwise noted.
PAGE 4
LT3800 ELECTRICAL CHARACTERISTICS LT3800I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 4: VIN voltages below the start-up threshold (7.5V) are only supported when VCC is externally driven above 6.5V. Note 5: Operating range dictated by MOSFET absolute maximum gatesource voltage ratings. Note 6: Supply current speciﬁcation does not include switch drive currents. Actual supply currents will be higher.
PAGE 5
LT3800 TYPICAL PERFORMANCE CHARACTERISTICS VCC UVLO Threshold (Rising) vs Temperature 25 ERROR AMP TRANSCONDUCTANCE (μmho) TA = 25°C 20 6.25 ICC (μA) VCC UVLO THRESHOLD, RISING (V) 6.30 15 10 6.20 5 6.15 –50 0 –25 0 25 50 75 TEMPERATURE (°C) 100 125 0 2 4 6 8 0 –200 –400 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE(CM) (V) 0 25 50 75 TEMPERATURE (°C) 125 1.
PAGE 6
LT3800 PIN FUNCTIONS and reverse-current inhibit functions are enabled. When the pin voltage is above 0.5V, Burst Mode operation is disabled, but reverse-current inhibit operation is maintained. DC/DC converters operating with reverse-current inhibit operation (BURST_EN = VFB) have a 1mA minimum load requirement. Reverse-current inhibit is disabled when the pin voltage is above 2.5V.
PAGE 7
LT3800 FUNCTIONAL DIAGRAM VIN UVLO (<4V) 8V REG 3.8V REG INTERNAL SUPPLY RAIL FEEDBACK REFERENCE 1.231V + – BST UVLO VCC UVLO (<6V) VIN 1 16 BOOST BOOSTED SWITCH DRIVER DRIVE CONTROL 15 TG 14 SW NOL SWITCH DRIVE LOGIC CONTROL 12 VCC + SHDN 3 SYNCHRONOUS SWITCH DRIVER DRIVE CONTROL 11 BG – 10 PGND + BURST_EN 5 – OSCILLATOR + 6 ERROR AMP – gm R + 0.
PAGE 8
LT3800 APPLICATIONS INFORMATION Overview The LT3800 is a high input voltage range step-down synchronous DC/DC converter controller IC that uses a 200kHz constant frequency, current mode architecture with external N-channel MOSFET switches. The LT3800 has provisions for high efﬁciency, low load operation for battery-powered applications. Burst Mode operation reduces total average input quiescent currents to 100μA during no load conditions.
PAGE 9
LT3800 APPLICATIONS INFORMATION The onboard VCC regulator will provide gate drive power for start-up under all conditions with total MOSFET gate charge loads up to 180nC. The regulator can operate the LT3800 continuously, provided the VIN voltage and/or MOSFET gate charge currents do not create excessive power dissipation in the IC. Safe operating conditions for continuous regulator use are shown in Figure 1.
PAGE 10
LT3800 APPLICATIONS INFORMATION Burst Mode The LT3800 employs low current Burst Mode functionality to maximize efﬁciency during no load and low load conditions. Burst Mode operation is enabled by shorting the BURST_EN pin to SGND. Burst Mode operation can be disabled by shorting BURST_EN to either VFB or VCC. When the required switch current, sensed via the VC pin voltage, is below 15% of maximum, the Burst Mode operation is employed and that level of sense current is latched onto the IC control path.
PAGE 11
LT3800 APPLICATIONS INFORMATION With the reverse-current inhibit feature enabled, an LT3800 converter will operate much like a nonsynchronous converter during light loads. Reverse-current inhibit reduces resistive losses associated with inductor ripple currents, which improves operating efﬁciencies during light-load conditions. An LT3800 DC/DC converter that is operating in reverseinhibit mode has a minimum load requirement of 1mA (BURST_EN = VFB).
PAGE 12
LT3800 APPLICATIONS INFORMATION Soft-Start Characteristic Showing Excessive Ripple Component 250μs/DIV VOUT VOUT VOUT(SS) V(VC) VOUT(SS) V(VC) 3800 AI07 The soft-start cycle should be evaluated to verify that the reduced RSS value allows operation without excessive modulation of the VC pin before ﬁnalizing the design. If the VC pin has an excessive ripple component during the soft-start cycle, converter output ripple should be reduced or RSS increased.
PAGE 13
LT3800 APPLICATIONS INFORMATION Current Limit Foldback Circuit for Applications That Use Soft-Start Alternative Current Limit Foldback Circuit for Applications That Have Soft-Start Disabled VC VC 1N4148 1N4148 1N4148 47k 39k 27k VOUT 3800 AI10 3800 AI09 VOUT Adaptive Nonoverlap (NOL) Output Stage Shutdown The FET driver output stages implement adaptive nonoverlap control.
PAGE 14
LT3800 APPLICATIONS INFORMATION Programming LT3800 VIN UVLO VIN LT3800 RB 3 SHDN RA SGND 17 3800 AI02 The UVLO voltage, VIN(UVLO), is set using the following relation: R A = RB • VIN(UVLO) – 1.35V 1.35V If additional hysteresis is desired for the enable function, an external positive feedback resistor can be used from the LT3800 regulator output. The shutdown function can be disabled by connecting the SHDN pin to VIN through a large value pull-up resistor.
PAGE 15
LT3800 APPLICATIONS INFORMATION Some magnetics vendors specify a volt-second product in their data sheet. If they do not, consult the vendor to make sure the speciﬁcation is not being exceeded by your design. The required volt-second product is calculated as follows: Volt-Second VOUT fO V • 1– OUT VIN Magnetics vendors specify either the saturation current, the RMS current, or both.
PAGE 16
LT3800 APPLICATIONS INFORMATION Power MOSFET Selection External N-channel MOSFET switches are used with the LT3800. The positive gate-source drive voltage of the LT3800 for both switches is roughly equivalent to the VCC supply voltage, for use of standard threshold MOSFETs. Selection criteria for the power MOSFETs include the “ON” resistance (RDS(ON)), total gate charge (QG), reverse transfer capacitance (CRSS), maximum drain-source voltage (VDSS) and maximum current.
PAGE 17
LT3800 APPLICATIONS INFORMATION To prevent large input voltage transients and avoid bypass capacitor heating, a low ESR input capacitor sized for the maximum RMS current must be used. This maximum capacitor RMS current follows the relation: IRMS = ( IMAX VOUT ( VIN – VOUT ) ) 1 2 VIN which peaks at a 50% duty cycle, when IRMS = IMAX/2.
PAGE 18
LT3800 APPLICATIONS INFORMATION current. Commutating this diode requires a signiﬁcant charge contribution from the main switch. At the instant the body diode commutates, a current discontinuity is created and parasitic inductance causes the switch node to ﬂy up in response to this discontinuity. High currents and excessive parasitic inductance can generate extremely fast dV/dt rise times.
PAGE 19
LT3800 APPLICATIONS INFORMATION The LT3800 packaging 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. This footprint should be made as large as possible to reduce the thermal resistance of the IC case to ambient air.
PAGE 20
LT3800 TYPICAL APPLICATIONS 9V-38V to 3.3V 10A DC/DC Converter with Input UVLO and Burst Mode Operation No Load I(VIN) = 100μA VIN 9V TO 38V + C8 100μF 50V ×2 RA RB 1M 187k R3 82k C2 330pF C10 100pF C3 100pF BOOST NC TG C5 1μF 16V X7R M1 Si7884DP LT3800 C1 1nF R1 100k 1% VIN C9 4.7μF 50V X7R ×3 R2 169k 1% SHDN SW CSS NC R4 39k BURST_EN VCC VFB BG VC D1 MBR520 L1 3.3μH C4 1μF 16V X7R M2 Si7884DP DS1 SS14 ×2 PGND SENSE– SENSE+ SGND RS 0.01Ω C7 10μF 6.
PAGE 21
LT3800 TYPICAL APPLICATIONS 9V-38V to 5V 6A DC/DC Converter with All Ceramic Capacitors, Input UVLO, Burst Mode Operation and Current Limit Foldback VIN 9V TO 38V C8 22μF ×3 VIN RA 1M C9 1nF NC R1 49.9k 1% R2 154k 1% TG SHDN SW CSS NC R4 51k C6 47pF BURST_EN L1 10μH C4 1μF 10V X7R BG VC C3 100pF M2 Si7884DP DS1 SS14 PGND SENSE– SENSE+ SGND RS 0.
PAGE 22
LT3800 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev I) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 0.48 (.019) REF 3.58 (.141) 16 1514 13 12 11 6.60 t0.10 2.94 (.116) 4.50 t0.10 10 9 DETAIL B 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 t0.05 1.05 t0.10 0.51 (.
PAGE 23
LT3800 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 9/11 Minor correction to TA04a schematic 24 3800fc 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 24
LT3800 TYPICAL APPLICATION 24V-48V to –12V 75W Inverting DC/DC Converter with VIN UVLO VIN 24V TO 48V + R7 1M C10 1μF 100V X7R w4 C9 56μF 63V w2 D1 BAS19 R8 1M VIN 2N3906 BOOST NC TG R6 130k R2 174k 1% C1 1nF SHDN M1 FDD3570 C2 1μF 16V X7R LT3800 R3 1M D2 1N4148 SW L1 15μH RS 0.01Ω R1 200k CSS NC BURST_EN VFB R5 20k 1% M2 FDD3570 BG VC R4 39k C6 1μF 16V X7R VCC C7 150pF 100V DS1 B180 PGND SENSE– SENSE+ SGND C3 470pF C8 4.