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LTC3814-5

38145fc

60V Current Mode

Synchronous Step-Up Controller

The LTC3814-5 is a synchronous step-up switching regu-

lator controller that can generate output voltages up to

60V. The LTC3814-5 uses a constant off-time peak current

control architecture to deliver very high duty cycles 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 driv-

ers allow the LTC3814-5 to drive large power MOSFETs

for higher current applications. The operating frequency

is selected by an external resistor and is compensated for

variations in V

. A shutdown pin allows the LTC3814-5

to be turned off reducing the supply current to <230µA.

24V Fan Supplies

48V Telecom and Base Station Power Supplies

Networking Equipment, Servers

Automotive and Industrial Control Systems

High Output Voltages: Up to 60V

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 5.5V Driver Supply

Power Good Output Voltage Monitor

Adjustable Off-Time/Frequency: t

OFF(MIN)

< 100ns

Adjustable Cycle-by-Cycle Current Limit

Undervoltage Lockout On Driver Supply

Output Overvoltage Protection

Thermally Enhanced 16-Pin TSSOP Package

High Efﬁ ciency High Voltage Step-Up Converter

Efﬁ ciency vs Load Current

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

PGOOD

OFF

PGOOD

RNG

SGND

RUN/SS

OFF

1000pF

0.01µF

100pF

4.5V TO 14V

22µF

OUT

24V

OUT

100k

263k

LTC3814-5

EXTV

PGND

INTV

NDRV

BOOST

38145 TA01

0.1µF

Si7848DP

1µF

29.4k

270µF

× 2

4.7µH

MBR1100

LOAD (A)

EFFICIENCY (%)

100

38145 TA01b

= 12V

= 5V

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and No R

SENSE

is a trademark 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.

Summary of content (30 pages)

PAGE 1
LTC3814-5 60V Current Mode Synchronous Step-Up Controller FEATURES n n n n n n n n n n n n n n DESCRIPTION The LTC3814-5 is a synchronous step-up switching regulator controller that can generate output voltages up to 60V. The LTC3814-5 uses a constant off-time peak current control architecture to deliver very high duty cycles with accurate cycle-by-cycle current limit without requiring a sense resistor.
PAGE 2
LTC3814-5 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) Supply Voltages INTVCC ................................................... –0.3V to 14V (INTVCC – PGND), (BOOST – SW) ......... –0.3V to 14V BOOST (Continuous) ............................. –0.3V to 85V BOOST (≤400ms) .................................. –0.3V to 95V EXTVCC .................................................. –0.3V to 15V (EXTVCC – INTVCC).................................. –12V to 12V (NDRV – INTVCC) Voltage...............
PAGE 3
LTC3814-5 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C (Note 2), INTVCC = VBOOST = VRNG = VEXTVCC = VNDRV = VOFF = 5V, unless otherwise speciﬁed. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop l INTVCC INTVCC Supply Voltage IQ INTVCC Supply Current INTVCC Shutdown Current RUN/SS > 1.5V (Notes 4, 5) RUN/SS = 0V IBOOST BOOST Supply Current RUN/SS > 1.
PAGE 4
LTC3814-5 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C (Note 2), INTVCC = VBOOST = VRNG = VEXTVCC = VNDRV = VOFF = 5V, unless otherwise speciﬁed.
PAGE 5
LTC3814-5 TYPICAL PERFORMANCE CHARACTERISTICS Frequency vs Input Voltage Efﬁciency vs Load Current 300 VOUT = 50V 280 VIN = 24V FREQUENCY (kHz) EFFICIENCY (%) FRONT PAGE CIRCUIT VIN = 36V 95 Frequency vs Load Current 300 VIN = 12V 90 FRONT PAGE CIRCUIT 280 ILOAD = 0A FREQUENCY (kHz) 100 260 ILOAD = 1A 240 VIN = 12V 260 240 VIN = 5V 85 220 2 3 4 LOAD (A) 200 5 2 ITH VOLTAGE (V) CURRENT SENSE THRESHOLD (mV) 3 4 LOAD CURRENT (A) MAXIMUM CURRENT SENSE THRESHOLD (mV) 620 600 580
PAGE 6
LTC3814-5 TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature 0.803 1.5 0.801 0.800 0.799 1.75 VBOOST = VINTVCC = 5V 1.25 1.0 0.50 0.797 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 0.5 –50 125 –25 0 25 50 75 TEMPERATURE (°C) 100 0.25 –50 –25 125 50 25 75 0 TEMPERATURE (°C) Driver Peak Source Current vs Supply Voltage 125 EXTVCC Switch Resistance vs Temperature Driver Pulldown RDS(ON) vs Supply Voltage 7 1.1 3.0 100 38145 G16 38145 G15 38145 G14 6 2.
PAGE 7
LTC3814-5 TYPICAL PERFORMANCE CHARACTERISTICS INTVCC Shutdown Current vs INTVCC Voltage 3.5 350 3.0 300 INTVCC CURRENT (μA) INTVCC CURRENT (mA) INTVCC Current vs INTVCC Voltage 2.5 2.0 1.5 1.0 0.5 250 200 150 100 50 0 0 2 8 6 10 4 INTVCC VOLTAGE (V) 12 0 14 0 2 8 6 10 4 INTVCC VOLTAGE (V) 38145 G22 14 38145 G23 RUN/SS Pull-Up Current vs Temperature 3 12 Shutdown Threshold vs Temperature 2.2 RUN/SS = 0V SHUTDOWN THRESHOLD (V) SS/TRACK CURRENT (μA) 2.0 2 1 1.8 1.6 1.4 1.2 1.0 0.
PAGE 8
LTC3814-5 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. VOFF (Pin 2): 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. The comparator defaults to 0.7V when the pin is grounded and defaults to 2.4V when the pin is connected to INTVCC.
PAGE 9
LTC3814-5 FUNCTIONAL DIAGRAM VIN 5.5V + NDRV M3 9 – OFF INTVCC 11 INTVCC EXTVCC 10 5V REG + 0.8V REF – – VIN INTVCC VOFF UV 2 + 5.5V + ON + 4.2V – DB VIN 4.7V + BOOST VOUT CIN 16 ROFF IOFF 1 TG VVOFF tOFF = (76pF) IIOFF ON Q SW 14 20k + CB 15 R S L SHDN OVERTEMP SENSE ICMP SWITCH LOGIC VOUT M1 – INTVCC BG OV 12 CVCC M2 PGND × 13 PGOOD 1.4V 4 + COUT VRNG 3 FAULT ITH 0.7V + 5 RUN SHDN – 2.6V Σ+ RFB1 0.72V UV – – VFB 0.
PAGE 10
LTC3814-5 OPERATION Main Control Loop The LTC3814-5 is a current mode controller for DC/DC step-up 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. Inductor current is determined by sensing the voltage between the PGND and SW pins using the bottom MOSFET on-resistance.
PAGE 11
LTC3814-5 OPERATION IC/Driver Supply Power The LTC3814-5’s internal control circuitry and top and bottom MOSFET drivers operate from a supply voltage (INTVCC pin) in the range of 4.5V to 14V. If the input supply voltage or another available supply is within this voltage range it can be used to supply IC/driver power. If a supply in this range is not available, two internal regulators are available to generate a 5.5V supply from the input or output.
PAGE 12
LTC3814-5 APPLICATIONS INFORMATION Power MOSFET Selection The LTC3814-5 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). Since the bottom MOSFET is used as the current sense element, particular attention must be paid to its on-resistance.
PAGE 13
LTC3814-5 APPLICATIONS INFORMATION When the controller is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by: Main Switch Duty Cycle = VOUT − VIN VOUT Synchronous Switch Duty Cycle = VIN VOUT The power dissipation for the main and synchronous MOSFETs at maximum output current are given by: Multiple MOSFETs can be used in parallel to lower RDS(ON) and meet the current and thermal requirements if desired.
PAGE 14
LTC3814-5 APPLICATIONS INFORMATION voltage changes, tie the VOFF pin to a resistive divider from VIN, as shown in Figure 5. Choose the resistor values so that the VRNG voltage equals about 1.55V at the mid-point of VIN as follows: VIN(MAX) + VIN(MIN) R1 VIN,MID = = 1.55V • 1+ R2 2 With these resistor values, the frequency will remain relatively constant at: 1+ R1/ R2 f= (Hz) ROFF (76pF) Changes in the load current magnitude will also cause a frequency shift.
PAGE 15
LTC3814-5 APPLICATIONS INFORMATION Minimum On-Time and Dropout Operation The minimum on-time tON(MIN) is the smallest amount of time that the LTC3814-5 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 350ns. The minimum on-time limit imposes a minimum duty cycle of tON(MIN)/(tON(MIN) + tOFF). If the minimum duty cycle is reached, due to a rising input voltage for example, then the output will rise out of regulation.
PAGE 16
LTC3814-5 APPLICATIONS INFORMATION For many designs it is possible to choose a single capacitor type that satisﬁes both the ESR and bulk C requirements for the design. In certain demanding applications, however, the ripple voltage can be improved signiﬁcantly by connecting two or more types of capacitors in parallel. For example, using a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic capacitor can be used to supply the required bulk C.
PAGE 17
LTC3814-5 APPLICATIONS INFORMATION Input Capacitor Selection The input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input and the input current waveform is continuous (see Figure 8b). The input voltage source impedance determines the size of the input capacitor, which is typically in the range of 10μF to 100μF. A low ESR capacitor is recommended though not as critical as for the output capacitor.
PAGE 18
LTC3814-5 APPLICATIONS INFORMATION Using the INTVCC Regulators One, both or neither of these regulators can be used to generate the 5.5V IC/driver supply depending on the circuit requirements, available supplies, and the voltage range of VIN or VOUT. Deriving the 5.5V 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 19
LTC3814-5 APPLICATIONS INFORMATION 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.
PAGE 20
LTC3814-5 APPLICATIONS INFORMATION The two types of compensation networks, Type 2 and Type 3 are shown in Figures 11 and 12. When component values are chosen properly, these networks provide a “phase bump” at the crossover frequency. Type 2 uses a single pole-zero pair to provide up to about 60° of phase boost while Type 3 uses two poles and two zeros to provide up to 150° of phase boost. signiﬁcantly.
PAGE 21
LTC3814-5 APPLICATIONS INFORMATION If breadboard measurement is not practical, mathematical software such as MATHCAD or MATLAB can be used to generate plots from the transfer function given in Equation 1. A SPICE simulation can also be used to generate approximate gain/phase curves. Plug the expected capacitor, inductor and MOSFET values into the following SPICE deck and generate an AC plot of VOUT/ VITH with gain in dB and phase in degrees.
PAGE 22
LTC3814-5 APPLICATIONS INFORMATION TYPE 2 Loop: Type 2: BOOST + 45° K = tan 2 1 C2 = 2 • f • G • K • R1 ( 2 ) A (s) = Type 3: C1= C2 K 1 K R2 = 2 • f • C1 V (R1) RB = REF VOUT VREF TYPE 3 Loop: BOOST + 45° K = tan2 4 1 C2 = 2 • f • G • R1 C1= C2 (K 1) K 2 • f • C1 R1 R3 = K 1 1 C3 = 2 f K • R3 V (R1) RB = REF VOUT VREF R2 = SPICE or mathematical software can be used to generate the gain/phase plots for the compensated power supply to do a sanity check on the compo
PAGE 23
LTC3814-5 APPLICATIONS INFORMATION A reasonable assumption is that the minimum RDS(ON) lies the same percentage below the typical value as the maximum lies above it. Consult the MOSFET manufacturer for further guidelines. Note that in a boost mode architecture, it is only possible to provide protection for “soft” shorts where VOUT > VIN . For hard shorts, the inductor current is limited only by the input supply capability.
PAGE 24
LTC3814-5 APPLICATIONS INFORMATION about 3mA and driver current can be calculated by: IGATE = f(QG(TOP) + QG(BOT)), where QG(TOP) and QG(BOT) are the gate charges of the top and bottom MOSFETs. This loss is proportional to the supply voltage that INTVCC is derived from, i.e., VIN, VOUT or an external supply connected to INTVCC. 4. COUT loss. The output capacitor has the difﬁcult job of ﬁltering the large RMS input current out of the synchronous MOSFET.
PAGE 25
LTC3814-5 APPLICATIONS INFORMATION To guarantee proper current limit at worst-case conditions, increase nominal VSNS by 50% to 190mV. To check if the current limit is acceptable at VSNS = 190mV, assume a junction temperature of about 30°C above a 70°C ambient (ρ100°C = 1.4): IIN(MAX) ≥ 190mV 1 − • 4A = 13A 1.4 • 0.009Ω 2 IOUT(MAX) = IIN(MAX) • (1-DMAX) = 6.5A and double-check the assumed TJ in the MOSFET: 1 2 PTOP = 6.5A ) (1.4)(0.009 ) = 1.06W ( 1 0.5 TJ = 70°C + 1.
PAGE 26
LTC3814-5 APPLICATIONS INFORMATION VOUT ROFF 403k 133k COFF 100pF DB BAS19 1 20k BOOST IOFF LTC3814-5 2 VOFF 3 V RNG 4 PGOOD 5 ITH 6 VFB PGOOD CSS 1000pF 7 8 TG SW PGND BG RUN/SS INTVCC SGND EXTVCC NDRV CC2 470pF RFB2 1k RC 250k CC1 47pF SGND 16 15 CB 0.1μF L1 5.9μH PGND M1 Si7848DP 14 13 12 VIN CIN2 12V 1μF 20V CIN1 68μF 20V CDRVCC 0.1μF 11 M2 Si7848DP COUT1 330μF 35V × 2 D1 B1100 VOUT 24V 5A COUT2 10μF 50V 10 9 CVCC 1μF PGND RFB1, 29.4k 38145 F14 Figure 14.
PAGE 27
LTC3814-5 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. • Place CIN, COUT, MOSFETs, D1 and inductor all in one compact area.
PAGE 28
LTC3814-5 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev H) Exposed Pad Variation BA 4.90 – 5.10* (.193 – .201) 2.74 (.108) 2.74 (.108) 16 1514 13 12 1110 6.60 ±0.10 9 2.74 (.108) 4.50 ±0.10 2.74 6.40 (.108) (.252) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±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) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
PAGE 29
LTC3814-5 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION C 01/11 Updated Description section PAGE NUMBER 1 Changed Operating Junction Temperature Range in Absolute Maximum Ratings and Order Information sections 2 Remove Lead Based Part Numbers from Order Information 2 Updated Note 2 4 Updated Fault Monitoring/Protection section 10 Updated Equations 22 Updated Related Parts 30 38145fc Information furnished by Linear Technology Corporation is believed to be accu
PAGE 30
LTC3814-5 TYPICAL APPLICATION 24V Input Voltage to 50V/5A VIN VOUT 143k 150k ROFF 806k DB BAS19 PGOOD CSS 1000pF RC 300k 16 2 100k CC2 330pF BOOST IOFF LTC3814-5 15 VOFF TG 3 14 VRNG SW 4 PGOOD 13 5 PGND ITH 6 VFB 12 BG 7 11 INTVCC RUN/SS 8 10 SGND EXTVCC 9 NDRV 10k RFB2 499Ω M3 ZXMN10A07F COFF 100pF 1 CIN1 68μF 50V RNDRV 100k SGND CC1 150pF CB 0.1μF CIN2 1μF 50V VIN 12V* TO 40V PGND L1 10μH M1 Si7850DP CDRVCC 0.