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LTC3813

3813fb

100V Current Mode

Synchronous Step-Up Controller

The LTC3813 is a synchronous step-up switching regulator

controller that can generate output voltages up to 100V.

The LTC3813 uses a constant off-time peak current control

architecture with accurate cycle-by-cycle current limit,

without requiring a sense resistor.

A precise internal reference provides ±0.5% DC accuracy.

A high bandwidth (25MHz) error ampliﬁ er provides very

fast line and load transient response. Large 1Ω gate drivers

allow the LTC3813 to drive multiple MOSFETs for higher

current applications. The operating frequency is selected

by an external resistor and is compensated for variations

in V

and can also be synchronized to an external clock

for switching-noise sensitive applications. A shutdown pin

allows the LTC3813 to be turned off, reducing the supply

current to 240μA.

PARAMETER LTC3813 LTC3814-5

Maximum V

OUT

100V 60V

MOSFET Gate Drive 6.35V to 14V 4.5V to 14V

INTV

6.2V 4.2V

INTV

–

6V 4V

24V Fan Supplies

48V Telecom and Base Station Power Supplies

Networking Equipment, Servers

Automotive and Industrial Control Systems

High Output Voltages: Up to 100V

Large 1Ω Gate Drivers

No Current Sense Resistor Required

Dual N-Channel MOSFET Synchronous Drive

±0.5% 0.8V Voltage Reference

Fast Transient Response

Programmable Soft-Start

Generates 10V Driver Supply from Input Supply

Synchronizable to External Clock

Power Good Output Voltage Monitor

Adjustable Off-Time/Frequency: t

OFF(MIN)

< 100ns

Adjustable Cycle-by-Cycle Current Limit

Programmable Undervoltage Lockout

Output Overvoltage Protection

28-Pin SSOP Package

High Efﬁ ciency High Voltage Step-Up Converter

Efﬁ ciency vs Load Current

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U.S. Patents,

including 5481178, 5847554, 6304066, 6476589, 6580258, 6677210, 6774611.

PGOOD

SYNC

RNG

SGND

1000pF

0.01μF

100pF

10V TO 40V

22μF

OUT

50V/5A

OFF

OUT

100k

500k

50k

LTC3813

EXTV

SENSE

–

BGRTN

SENSE

DRV

INTV

NDRV

BOOST

3813 TA01

0.1μF

Si7850DP

10μH

Si7850DP

1μF

MBR1100

30.9k

499Ω

270μF

SHDN

LOAD (A)

EFFICIENCY (%)

100

3813 TA01b

= 12V

= 24V

= 36V

123 5

Summary of content (32 pages)

PAGE 1
LTC3813 100V Current Mode Synchronous Step-Up Controller FEATURES DESCRIPTION n The LTC3813 is a synchronous step-up switching regulator controller that can generate output voltages up to 100V. The LTC3813 uses a constant off-time peak current control architecture with accurate cycle-by-cycle current limit, without requiring a sense resistor. n n n n n n n n n n n n n n High Output Voltages: Up to 100V Large 1Ω Gate Drivers No Current Sense Resistor Required Dual N-Channel MOSFET Synchronous Drive ±0.
PAGE 2
LTC3813 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) Supply Voltages INTVCC, DRVCC....................................... –0.3V to 14V (DRVCC - BGRTN), (BOOST - SW).......... –0.3V to 14V BOOST ................................................. –0.3V to 114V BGRTN ........................................................ –5V to 0V EXTVCC .................................................. –0.3V to 15V (NDRV - INTVCC) Voltage ........................... –0.3V to 10V SW, SENSE+ Voltage ................
PAGE 3
LTC3813 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, INTVCC = DRVCC = VBOOST = VOFF = VRNG = SHDN = UVIN = VEXTVCC = VNDRV = 10V, VSYNC = VSENSE+ = VSENSE – = VBGRTN = VSW = 0V, unless otherwise speciﬁed. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VFB Feedback Voltage (Note 4) 0°C to 85°C –40°C to 85°C –40°C to 125°C (I-Grade) l l l 0.796 0.794 0.792 0.792 0.800 0.800 0.800 0.800 0.
PAGE 4
LTC3813 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, INTVCC = DRVCC = VBOOST = VOFF = VRNG = SHDN = UVIN = VEXTVCC = VNDRV = 10V, VSYNC = VSENSE+ = VSENSE– = VBGRTN = VSW = 0V, unless otherwise speciﬁed.
PAGE 5
LTC3813 TYPICAL PERFORMANCE CHARACTERISTICS Load Transient Response VOUT 200mV/ DIV VOUT 20V/DIV VOUT 20V/DIV SS 4V/DIV IOUT 2A/DIV IL 5A/DIV IL 5A/DIV 3813 G01 100μs/DIV FIGURE 14 CIRCUIT VIN = 12V 0A TO 4A LOAD STEP VOUT = 24V 3813 G02 1ms/DIV FRONT PAGE CIRCUIT VIN = 24V ILOAD = 2A 300 FRONT PAGE CIRCUIT VIN = 24V RSHORT = 1Ω Frequency vs Load Current 280 FRONT PAGE CIRCUIT VIN = 12V FRONT PAGE CIRCUIT 270 280 VIN = 5V 260 ILOAD = 0A FREQUENCY (kHz) FREQUENCY (kHz) 95 90 3813 G03
PAGE 6
LTC3813 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Current Sense Threshold vs VRNG Voltage IOFF = 300μA OFF-TIME (ns) 660 640 620 600 580 560 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 400 300 200 100 0 0.5 1 1.5 2 VRNG = INTVCC 220 210 200 190 180 –50 –25 0.803 2.5 0.800 0.799 125 Driver Pulldown RDS(ON) vs Temperature 1.50 VBOOST = VINTVCC = 10V VBOOST = VINTVCC = 10V 1.25 2.0 1.00 RDS(ON) (Ω) PEAK SOURCE CURRENT (A) 0.
PAGE 7
LTC3813 TYPICAL PERFORMANCE CHARACTERISTICS EXTVCC LDO Resistance at Dropout vs Temperature INTVCC Shutdown Current vs Temperature INTVCC Current vs Temperature 14 4 400 3 300 10 8 6 4 INTVCC CURRENT (μA) INTVCC CURRENT (mA) RESISTANCE (Ω) 12 2 1 200 100 2 0 –50 –25 75 50 25 TEMPERATURE (°C) 0 100 0 –50 125 50 25 0 75 TEMPERATURE (°C) –25 0 –50 –25 125 75 50 25 TEMPERATURE (°C) 0 3813 G19 3813 G18 3.5 125 SS Pull-Up Current vs Temperature 300 4.
PAGE 8
LTC3813 PIN FUNCTIONS IOFF (Pin 1): Off-Time Current Input. Tie a resistor from VOUT to this pin to set the one-shot timer current and thereby set the switching frequency. SS (Pin 11): Soft-Start Input. A capacitor to ground at this pin sets the ramp rate of the maximum current sense threshold. VOFF (Pin 4): Off-Time Voltage Input. Voltage trip point for the on-time comparator. Tying this pin to an external resistive divider from the input makes the off-time proportional to VIN.
PAGE 9
LTC3813 PIN FUNCTIONS BG (Pin 19): Bottom Gate Drive. The BG pin drives the gate of the bottom N-channel main switch MOSFET. This pin swings from BGRTN to DRVCC. BGRTN (Pin 20): Bottom Gate Return. This pin connects to the source of the pulldown MOSFET in the BG driver and is normally connected to ground. Connecting a negative supply to this pin allows the main MOSFET’s gate to be pulled below ground to help prevent false turn-on during high dV/dt transitions on the SW node.
PAGE 10
LTC3813 FUNCTIONAL DIAGRAM VIN 10V + NDRV M3 15 – OFF INTVCC 17 VIN 5V REG RUV1 UVIN RUV2 SYNC 14 EXTVCC 0.8V REF 16 – – INTVCC VIN UV 7 UV + 0.8V + + – 6.2V PLL/LPF + 10V ON PLL-SYNC 10 + VOFF – 6.7V 4 ROFF VOUT 1 ON Q L CB SW 26 20k + CIN 28 27 R S + BOOST TG VVOFF tOFF = (76pF) IIOFF IOFF VIN DB OVERTEMP SENSE ICMP SENSE+ SWITCH LOGIC 25 M1 DRVCC – VOUT 18 SHDN BG OV s CVCC 19 M2 BGRTN + 20 COUT SENSE– 1.
PAGE 11
LTC3813 OPERATION Main Control Loop The LTC3813 is a current mode controller for DC/DC stepup converters. In normal operation, the top MOSFET is turned on for a ﬁxed interval determined by a one-shot timer (OST). When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator ICMP trips, restarting the one-shot timer and initiating the next cycle.
PAGE 12
LTC3813 OPERATION from DRVCC when the top MOSFET turns off. In an output overvoltage condition, where it is possible that the bottom MOSFET will be off for an extended period of time, an internal timeout guarantees that the bottom MOSFET is turned on at least once every 25μs for one top MOSFET on-time period to refresh the bootstrap capacitor. The bottom driver has an additional feature that helps minimize the possibility of external MOSFET shoot-thru.
PAGE 13
LTC3813 APPLICATIONS INFORMATION The basic LTC3813 application circuit is shown on the ﬁrst page of this data sheet. External component selection is primarily determined by the maximum input voltage and load current and begins with the selection of the sense resistance and power MOSFET switches. The LTC3813 uses either a sense resistor or the on-resistance of the synchronous power MOSFET for determining the inductor current.
PAGE 14
LTC3813 APPLICATIONS INFORMATION Power MOSFET Selection The LTC3813 requires two external N-channel power MOSFETs, one for the bottom (main) switch and one for the top (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage BVDSS, threshold voltage V(GS)TH, on-resistance RDS(ON), Miller capacitance and maximum current IDS(MAX). When the bottom MOSFET is used as the current sense element, particular attention must be paid to its on-resistance.
PAGE 15
LTC3813 APPLICATIONS INFORMATION increase in coulombs on the horizontal axis from a to b while the curve is ﬂat) is speciﬁed for a given VDS drain voltage, but can be adjusted for different VDS voltages by multiplying by the ratio of the application VDS to the curve speciﬁed VDS values. A way to estimate the CMILLER term is to take the change in gate charge from points a and b on a manufacturers data sheet and divide by the stated VDS voltage speciﬁed.
PAGE 16
LTC3813 APPLICATIONS INFORMATION Operating Frequency The choice of operating frequency is a tradeoff between efﬁciency and component size. Low frequency operation improves efﬁciency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC3813 applications is determined implicitly by the one-shot timer that controls the on-time tOFF of the synchronous MOSFET switch.
PAGE 17
LTC3813 APPLICATIONS INFORMATION f= 1+ R1/ R2 (Hz) ROFF (76pF) for the range of 0.45VIN to 1.55 • VIN , and will be proportional to VIN outside of this range. Changes in the load current magnitude will also cause a frequency shift. Parasitic resistance in the MOSFET switches and inductor reduce the effective voltage across the inductance, resulting in increased duty cycle as the load current increases.
PAGE 18
LTC3813 APPLICATIONS INFORMATION The required inductance can then be calculated to be: L= VIN(MIN) • DMAX f • IL The required saturation of the inductor should be chosen to be greater than the peak inductor current: IL(SAT) IO(MAX) 1 DMAX + IL 2 where the ﬁrst term is due to the bulk capacitance and second term due to the ESR. For many designs it is possible to choose a single capacitor type that satisﬁes both the ESR and bulk C requirements for the design.
PAGE 19
LTC3813 APPLICATIONS INFORMATION Once the output capacitor ESR and bulk capacitance have been determined, the overall ripple voltage waveform should be veriﬁed on a dedicated PC board (see PC Board Layout Checklist section for more information on component placement). Lab breadboards generally suffer from excessive series inductance (due to inter-component wiring), and these parasitics can make the switching waveforms look signiﬁcantly worse than they would be on a properly designed PC board.
PAGE 20
LTC3813 APPLICATIONS INFORMATION Input Voltage Undervoltage Lockout A resistor divider connected from the input supply to the UVIN pin (see Functional Diagram) is used to program the input supply undervoltage lockout thresholds. When the rising voltage at UVIN reaches 0.88V, the LTC3813 turns on, and when the falling voltage at UVIN drops below 0.8V, the LTC3813 is shut down—providing 10% hysteresis.
PAGE 21
LTC3813 APPLICATIONS INFORMATION Using the INTVCC Regulators One, both or neither of these regulators can be used to generate the 10V IC/driver supply depending on the circuit requirements, available supplies, and the voltage range of VIN or VOUT. Deriving the 10V supply from VIN is more efﬁcient, however deriving it from VOUT has the advantage of maintaining regulation of VOUT when VIN drops below the UV threshold.
PAGE 22
LTC3813 APPLICATIONS INFORMATION Power Dissipation Considerations Applications using large MOSFETs and high frequency of operation may result in a large DRVCC /INTVCC supply current. Therefore, when using the linear regulators, it is necessary to verify that the resulting power dissipation is within the maximum limits. The DRVCC /INTVCC supply current consists of the MOSFET gate current plus the LTC3813 quiescent current: H(s) = VOUT (s) RL • VIN • VSENSE(MAX) = VITH (s) 2.
PAGE 23
LTC3813 APPLICATIONS INFORMATION C1 R2 R1 FB GAIN (dB) IN –6dB/OCT GAIN –6dB/OCT – OUT RB VREF PHASE (DEG) C2 0 FREQ + –90 –180 PHASE –270 –360 3813 F11 Figure 11. Type 2 Schematic and Transfer Function IN R1 R3 FB R2 GAIN (dB) C3 C1 –6dB/OCT – GAIN OUT RB VREF PHASE (DEG) C2 +6dB/OCT –6dB/OCT 0 FREQ + –90 PHASE –180 –270 –360 3813 F12 Figure 12.
PAGE 24
LTC3813 APPLICATIONS INFORMATION the appropriate parasitic values are known, simulated or generated from the modulator transfer function. Measurement will give more accurate results, but simulation or transfer function can often get close enough to give a working system. To measure the modulator gain and phase directly, wire up a breadboard with an LTC3813 and the actual MOSFETs, inductor and input and output capacitors that the ﬁnal design will use.
PAGE 25
LTC3813 APPLICATIONS INFORMATION Finally, choose a convenient resistor value for R1 (10k is usually a good value).
PAGE 26
LTC3813 APPLICATIONS INFORMATION self-consistency between the assumed MOSFET junction temperature and the resulting value of ILIMIT which heats the MOSFET switches. Caution should be used when setting the current limit based upon the RDS(ON) of the MOSFETs. The maximum current limit is determined by the minimum MOSFET on-resistance. Data sheets typically specify nominal and maximum values for RDS(ON), but not a minimum.
PAGE 27
LTC3813 APPLICATIONS INFORMATION If the external frequency (fSYNC) is greater than the oscillator frequency fO, current is sourced continuously, pulling up the PLL/LPF pin. When the external frequency is less than fO, current is sunk continuously, pulling down the PLL/LPF pin. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference.
PAGE 28
LTC3813 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD (ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value.
PAGE 29
LTC3813 APPLICATIONS INFORMATION Verify that the Si7848DP is also a good choice for the bottom MOSFET by checking its power dissipation at current limit and minimum input voltage, assuming a junction temperature of 30°C above a 70°C ambient (ρ100°C = 1.4): Since VIN is always between 6.2V and 14V, it can be connected directly to the INTVCC and DRVCC pins. COUT is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.
PAGE 30
LTC3813 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out a PC board follow one of two suggested approaches. The simple PC board layout requires a dedicated ground plane layer. Also, for higher currents, it is recommended to use a multilayer board to help with heat sinking power components. • The ground plane layer should not have any traces and it should be as close as possible to the layer with power MOSFETs.
PAGE 31
LTC3813 PACKAGE DESCRIPTION G Package 28-Lead Plastic SSOP (5.3mm) (Reference LTC DWG # 05-08-1640) 9.90 – 10.50* (.390 – .413) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1.25 ±0.12 7.8 – 8.2 5.3 – 5.7 0.42 ±0.03 7.40 – 8.20 (.291 – .323) 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 RECOMMENDED SOLDER PAD LAYOUT 2.0 (.079) MAX 5.00 – 5.60** (.197 – .221) 0° – 8° 0.09 – 0.25 (.0035 – .010) 0.55 – 0.95 (.022 – .037) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
PAGE 32
LTC3813 TYPICAL APPLICATION 24V Input Voltage to 50V/5A Synchronized at 250kHz VOUT RNDRV 100k ROFF 806k 143k COFF 100pF 1 10k 4 10k CSS 1000pF RUV1 140k BOOST 19 BG 18 DRVCC 17 INTVCC 16 EXTVCC 15 NDRV SS 12 SGND 13 SHDN 14 UVIN SGND RUV2 10k CIN2 1μF 50V PGND L1 10μH CB, 0.1μF M1 Si7850DP VOUT 50V 5A 25 SENSE+ SENSE– 21 20 BGRTN 11 SHDN 28 27 TG 26 SW VOFF 5 V RNG 6 PGOOD 7 SYNC 8 ITH 9 VFB 10 PLL/LPF PGOOD 250kHz CLOCK 0.