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LTC3812-5

38125fc

60V Current Mode

Synchronous Switching

Regulator Controller

The LTC3812-5 is a synchronous step-down switching

regulator controller that can directly step down voltages

from up to 60V input, making it ideal for telecom and

automotive applications. The LTC3812-5 uses a constant

on-time valley current control architecture to deliver very

low 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 LTC3812-5 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 LTC3812-5 to

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

Integrated bias control generates gate drive power from

the input supply during start-up and when an output short-

circuit occurs, with the addition of a small external SOT23

MOSFET. When in regulation, power is derived from the

output for higher efﬁ ciency.

48V Telecom and Base Station Power Supplies

Networking Equipment, Servers

Automotive and Industrial Control Systems

High Voltage Operation: Up to 60V

Large 1Ω Gate Drivers

No Current Sense Resistor Required

Dual N-Channel MOSFET Synchronous Drive

Extremely Fast Transient Response

±0.5% 0.8V Voltage Reference

Programmable Soft-Start

Generates 5.5V Driver Supply

Selectable Pulse-Skipping Mode Operation

Power Good Output Voltage Monitor

Adjustable On-Time/Frequency: t

ON(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-Down Converter

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

PGOOD

FCB

PGOOD

RNG

SGND

RUN/SS

1000pF

47pF

5pF

6V TO 60V

22μF

OUT

ZXMN-

10A07F

200k

100k

LTC3812-5

EXTV

PGND

INTV

NDRV

BOOST

38125 TA01

0.1μF

110k

100k

Si7850DP

1μF

MBR1100

10k

1.89k

OUT

270μF

4.7

μH

Efﬁ ciency vs Load Current

LOAD CURRENT (A)

EFFICIENCY (%)

100

1234

38125 TA01b

= 12V

= 42V

= 24V

L, LT, LTC, LTM, 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, 5847554, 6304066, 6476589, 6580258,

6677210, 6774611.

Summary of content (34 pages)

PAGE 1
LTC3812-5 60V Current Mode Synchronous Switching Regulator Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n The LTC3812-5 is a synchronous step-down switching regulator controller that can directly step down voltages from up to 60V input, making it ideal for telecom and automotive applications. The LTC3812-5 uses a constant on-time valley current control architecture to deliver very low duty cycles with accurate cycle-by-cycle current limit without requiring a sense resistor.
PAGE 2
LTC3812-5 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (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
LTC3812-5 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 = 5V, VFCB = VSW = 0V, 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
LTC3812-5 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 = 5V, VFCB = VSW = 0V, unless otherwise speciﬁed.
PAGE 5
LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS Load Transient Response Short-Circuit/Fault Timeout Operation Start-Up VIN 20V/DIV VOUT 50mV/DIV VOUT 5V/DIV INTVCC INTVCC, VOUT 2V/DIV IOUT 5A/DIV SS/TRACK 2V/DIV VOUT IL 5A/DIV IL 2A/DIV 38125 G01 10μs/DIV FRONT PAGE CIRCUIT VIN = 25V 0A TO 5A LOAD STEP 38125 G02 2ms/DIV FRONT PAGE CIRCUIT VIN = 30V ILOAD = 0.
PAGE 6
LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Threshold vs ITH Voltage ITH Voltage vs Load Current 3.0 VRNG = 1V FRONT PAGE CIRCUIT 1.5 1.0 0.5 1 4 3 2 5 LOAD CURRENT (A) 1.4V 200 1V 0.7V 0.5V 100 –100 100 –200 –300 0 2.5 1 1.5 2 ITH VOLTAGE (V) 0.5 38125 G10 640 620 600 580 100 125 250 VRNG = INTVCC 200 150 100 50 0 0.2 0 0.4 0.6 300 200 100 0 0.5 1 230 Driver Peak Source Current vs Temperature 0.803 VRNG = INTVCC 1.
PAGE 7
LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS Driver Pull-Down RDS(ON) vs Temperature 1.1 3.0 VBOOST = VINTVCC = 5V PEAK SOURCE CURRENT (A) 1.50 1.25 RDS(ON) (Ω) Driver Pull-Down RDS(ON) vs Supply Voltage 1.00 0.75 0.50 2.5 1.0 2.0 RDS(ON) (Ω) 1.75 Driver Peak Source Current vs Supply Voltage 1.5 50 25 75 0 TEMPERATURE (°C) 100 0.8 1.0 0.7 0.5 0.25 –50 –25 0.9 0 125 0.
PAGE 8
LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Threshold vs Temperature RUN/SS Pull-Up Current vs Temperature 3 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 0.8 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 38125 G27 0.6 –50 –25 75 50 25 TEMPERATURE (°C) 0 100 125 38125 G28 PIN FUNCTIONS ION (Pin 1): On-Time Current Input. Tie a resistor from VIN to this pin to set the one-shot timer current and thereby set the switching frequency.
PAGE 9
LTC3812-5 PIN FUNCTIONS EXTVCC (Pin 10): External Driver Supply Voltage. When this voltage exceeds 4.2V, an internal switch connects this pin to INTVCC through an LDO and turns off the external MOSFET connected to NDRV, so that controller and gate drive are drawn from EXTVCC. INTVCC (Pin 11): Main Supply and Driver Supply Pin. All internal circuits and bottom gate output driver are powered from this pin.
PAGE 10
LTC3812-5 FUNCTIONAL DIAGRAM INTVCC 5V REG EXTVCC NDRV INTVCC VIN 0.8V REF INTVCC MODE LOGIC 5.5V + NDRV M3 9 – 9V 0.8V + + F FCB INTVCC INTVCC 11 UV EXTVCC – – 4 OFF 4.2V 10 + 270μA – + – 5.5V + ON 1.4μA + – TIMEOUT LOGIC 100nA RON VIN ION 1 tON = 2.4V (76pF) IION DRV OFF R S 20k + 4.7V BOOST + CIN 16 CB TG FCNT M1 15 ON Q VIN DB SW 14 + ICMP SWITCH LOGIC IREV L1 VOUT – – INTVCC SHDN CVCC BG OV M2 12 + PGND × OVERTEMP SENSE 1.
PAGE 11
LTC3812-5 OPERATION Main Control Loop Pulse-Skipping Mode The LTC3812-5 is a current mode controller for DC/DC step-down 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
LTC3812-5 OPERATION level to initiate another cycle. In this mode, frequency is proportional to load current at light loads. Pulse-skipping mode operation is disabled by comparator F when the FCB pin is brought below 0.8V, forcing continuous synchronous operation. Forced continuous mode is less efﬁcient due to resistive losses, but has the advantage of better transient response at low currents, approximately constant frequency operation, and the ability to maintain regulation when sinking current.
PAGE 13
LTC3812-5 OPERATION The four possible operating modes for generating this supply are summarized as follows (see Figure 4): 1. LTC3812-5 generates a 5.5V start-up supply from a small external SOT-23 NMOS acting as linear regulator with drain connected to VIN and gate controlled by the LTC3812-5’s internal linear regulator controller through the NDRV pin. As soon as the output voltage reaches 4.7V, the 5.
PAGE 14
LTC3812-5 APPLICATIONS INFORMATION MAXIMUM SENSE VOLTAGE AND VRNG PIN Inductor current is determined by measuring the voltage across a sense resistance (the on-resistance of the bottom MOSFET) that appears between the PGND and SW pins. The maximum sense voltage is set by the voltage applied to the VRNG pin and is equal to approximately: VSENSE(MAX) = 0.173VRNG – 0.026 The current mode control loop will not allow the inductor current valleys to exceed VSENSE(MAX)/RSENSE.
PAGE 15
LTC3812-5 APPLICATIONS INFORMATION The most important parameter in high voltage applications is breakdown voltage BVDSS. Both the top and bottom MOSFETs will see full input voltage plus any additional ringing on the switch node across its drain-to-source during its off-time and must be chosen with the appropriate breakdown speciﬁcation. The LTC3812-5 is designed to be used with a 4.5V to 14V gate drive supply (INTVCC pin) for driving logic-level MOSFETs (VGS(MIN) ≥ 4.5V).
PAGE 16
LTC3812-5 APPLICATIONS INFORMATION drain current. CMILLER is the calculated capacitance using the gate charge curve from the MOSFET data sheet and the technique described above. Both MOSFETs have I2R losses while the topside N-channel equation incudes an additional term for transition losses, which peak at the highest input voltage.
PAGE 17
LTC3812-5 APPLICATIONS INFORMATION MINIMUM OFF-TIME AND DROPOUT OPERATION The minimum off-time tOFF(MIN) is the smallest amount of time that the LTC3812-5 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 250ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation.
PAGE 18
LTC3812-5 APPLICATIONS INFORMATION it is on for only a fraction of the duty cycle. In order for the diode to be effective, the inductance between it and the bottom MOSFET must be as small as possible, mandating that these components be placed adjacently. The diode can be omitted if the efﬁciency loss is tolerable.
PAGE 19
LTC3812-5 APPLICATIONS INFORMATION handling and load step requirements. Dry tantalum, special polymer and aluminum electrolytic capacitors are available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies.
PAGE 20
LTC3812-5 APPLICATIONS INFORMATION the drivers are turned off to prevent the SOT-23 MOSFET from overheating. Soft-start cycles are then attempted at low duty cycle intervals to try to bring the output back up (see Figure 9). This fault timeout operation is enabled by choosing the choosing RNDRV such that the resistor current INDRV is greater than 270μA by using the following formulas: RNDRV ≤ The external NMOS for the linear regulator should be a standard 3V threshold type (i.e.
PAGE 21
LTC3812-5 APPLICATIONS INFORMATION to boost this voltage above 4.7V. In this mode, EXTVCC is grounded and the NMOS is chosen to handle the worstcase power dissipation: PMOSFET = (VIN(MAX))[(f)(QG(TOP) + QG(BOTTOM) + 3mA] To operate properly, the fault timeout operation must be disabled by choosing RNDRV > (VIN(MAX) – 5.
PAGE 22
LTC3812-5 APPLICATIONS INFORMATION IEXTVCC = f(QG(TOP) + QG(BOTTOM)) + 3mA < 50mA The junction temperature can be estimated from the equations given in Note 2 of the Electrical Characteristics as follows: TJ = TA + IEXTVCC • (VEXTVCC – VINTVCC)(38°C/W) < 125°C If absolute maximum ratings are exceeded, consider using an external supply connected directly to the INTVCC pin. FEEDBACK LOOP/COMPENSATION Feedback Loop Types So far, the AC response of the loop is pretty well out of the user’s control.
PAGE 23
LTC3812-5 APPLICATIONS INFORMATION 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 38125 F11 Figure 11. Type 3 Schematic and Transfer Function 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.
PAGE 24
LTC3812-5 APPLICATIONS INFORMATION Mathematical software such as MATHCAD or MATLAB can also be used to generate plots using the following transfer function of the modulator: VSENSE(MAX) 1+ s • R ESR • COUT • R H(s) = • L (2) 1.2 • RDS(ON) 1+ s • RL • COUT s = j2 f With the gain/phase plot in hand, a loop crossover frequency can be chosen. Usually the curves look something like Figure 12. Choose the crossover frequency about 25% of the switching frequency for maximum bandwidth.
PAGE 25
LTC3812-5 APPLICATIONS INFORMATION where H(s) was given in equation 2 and A(s) depends on compensation circuit used: Type 2: A (s) = 1+ s • R2 • C1 C1• C2 ⎞ ⎛ s • R1• (C1+ C2) • ⎜ 1+ s • R2 • ⎝ C1+ C2 ⎟⎠ Type 3: A (s)= 1 • s • R1• (C1+C2) (1+ s • (R1+R3) • C3) • (1+ s • R2 • C1) ⎛ C1• C2 ⎞ (1+ s • R3 • C3) • ⎜⎝1+ s • R2 • C1+C2 ⎟⎠ For SPICE, replace VSTIM line in the previous PSPICE code with following code and generate a gain/phase plot of V(out)/V(outin): rfb1 outin vfb 52.
PAGE 26
LTC3812-5 APPLICATIONS INFORMATION FAULT CONDITIONS: CURRENT LIMIT AND FOLDBACK RUN/SOFT-START FUNCTION The maximum inductor current is inherently limited in a current mode controller by the maximum sense voltage. In the LTC3812-5, the maximum sense voltage is controlled by the voltage on the VRNG pin. With valley current control, the maximum sense voltage and the sense resistance determine the maximum allowed inductor valley current.
PAGE 27
LTC3812-5 APPLICATIONS INFORMATION EFFICIENCY CONSIDERATIONS The percent efﬁciency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efﬁciency and which change would produce the most improvement. Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3812-5 circuits: 1. DC I2R losses.
PAGE 28
LTC3812-5 APPLICATIONS INFORMATION (max) plus any ringing, choose an 60V MOSFET. The Si7850DP has: BVDSS = 60V RDS(ON) = 25mΩ (max)/31mΩ (nom), δ= 0.007/°C, CMILLER = (8.3nC – 2.8nC)/30V = 183pF, VGS(MILLER) = 3.8V, θJA= 22°C/W. This yields a nominal sense voltage of: VSNS(NOM) = 6A • 1.3 • 0.025Ω = 195mV To guarantee proper current limit at worst-case conditions, increase nominal VSNS by at least 50% to 320mV (by tying VRNG to 2V).
PAGE 29
LTC3812-5 APPLICATIONS INFORMATION • Use compact plane for switch node (SW) to improve cooling of the MOSFETs and to keep EMI down. • Place M2 as close to the controller as possible, keeping the PGND, BG and SW traces short. • Use planes for VIN and VOUT to maintain good voltage ﬁltering and to keep power losses low. • Connect the input capacitor(s) CIN close to the power MOSFETs. This capacitor carries the MOSFET AC current. • Flood all unused areas on all layers with copper.
PAGE 30
LTC3812-5 TYPICAL APPLICATIONS 7V to 60V Input Voltage to 5V/5A with IC Power from 12V Supply and All Ceramic Output Capacitors CON 100pF DB BAS19 1 2 3 PGOOD BOOST ION LTC3812-5 VRNG TG PGOOD 4 FCB 5 ITH 6 VFB CSS 1000pF 7 8 SW PGND BG RUN/SS INTVCC SGND EXTVCC NDRV CC2 200pF RFB2 1.89k RC 100k CC1 5pF CIN1 68μF 100V 12V RON 110k SGND CIN2 1μF 80V VIN 7V TO 60V PGND 16 15 CB 0.1μF M1 Si7850DP 14 L1 4.7μH VOUT 5V 5A 13 12 CDRVCC 0.1μF M2 Si7850DP 11 10 COUT1 47μF 6.
PAGE 31
LTC3812-5 TYPICAL APPLICATIONS 15V to 60V Input Voltage to 3.3V/5A with Fault Timeout and Pulse-Skipping Disabled RNDRV 250k RON 71.5k M3 ZVN4210G CON 100pF 2 3 BOOST ION LTC3812-5 VRNG TG PGOOD 4 FCB 5 ITH 6 VFB CSS 1000pF 7 8 SW PGND BG RUN/SS INTVCC SGND EXTVCC NDRV CC2 47pF RFB2 3.2k RC 200k CC1 5pF SGND CIN2 1μF 100V PGND DB BAS19 1 PGOOD CIN1 68μF 100V VIN 15V TO 60V 16 15 CB 0.1μF 14 M1 Si7850DP L1 4.7μH VOUT 3.3V 5A 13 12 CDRVCC 0.1μF 11 10 COUT1 270μF 6.
PAGE 32
LTC3812-5 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) 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 33
LTC3812-5 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION C 01/11 Changed Operating Junction Temperature Range in Absolute Maximum Ratings and Order Information sections PAGE NUMBER 2 Remove Lead Based Part Numbers from Order Information 2 Updated Equations 25 Updated Related Parts 34 38125fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 34
LTC3812-5 TYPICAL APPLICATION 15V to 60V Input Voltage to 12V/5A with Trickle Charger Start-Up RNDRV 250k RON 261k CON 100pF 2 3 BOOST ION LTC3812-5 VRNG TG PGOOD 4 FCB 5 ITH 6 VFB CSS 1000pF 7 8 SW PGND BG INTVCC RUN/SS SGND EXTVCC NDRV CC2 47pF RFB2 1k RC 200k VIN 15V TO 60V 16 15 CB 0.1μF M1 Si7850DP 14 L1 10μH VOUT 12V 5A 13 12 CDRVCC 0.