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LTC3810

3810fc

100V Current Mode

Synchronous Switching

Regulator Controller

The LTC3810 is a synchronous step-down switching

regulator controller that can directly step-down voltages

from up to 100V, making it ideal for telecom and automo-

tive applications. The LTC3810 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 drivers

allow the LTC3810 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 LTC3810 to be turned off, reducing the supply

current to 240μA.

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 100V

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 Output Voltage Tracking/Soft-Start

Generates 10V Driver Supply from Input Supply

Synchronizable to External Clock

Selectable Pulse Skip Mode Operation

Power Good Output Voltage Monitor

Adjustable On-Time/Frequency: t

ON(MIN)

< 100ns

Adjustable Cycle-by-Cycle Current Limit

Programmable Undervoltage Lockout

Output Overvoltage Protection

28-Pin SSOP Package

High Efﬁ ciency High Voltage Step-Down Converter

Efﬁ ciency vs Load Current

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

PGOOD

MODE/SYNC

RNG

SGND

SS/TRACK

1000pF

47pF

5pF

15V TO 100V

22μF

OUT

12V/6A

ZXMN10A07F

200k

LTC3810

EXTV

SENSE

–

BGRTN

SENSE

DRV

INTV

NDRV

BOOST

3810 TA01

0.1μF

261k

100k

Si7456DP

1μF

MBR1100

FB1

14k

FB2

OUT

270μF

10μH

SHDN

LOAD (A)

EFFICIENCY (%)

100

1234

3810 TA01b

= 25V

= 75V

= 50V

L, LT, LTC, LTM, Linear Technology, Burst Mode 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.

Downloaded from Arrow.com.

Summary of content (38 pages)

PAGE 1
LTC3810 100V Current Mode Synchronous Switching Regulator Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n n The LTC3810 is a synchronous step-down switching regulator controller that can directly step-down voltages from up to 100V, making it ideal for telecom and automotive applications. The LTC3810 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
LTC3810 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
LTC3810 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C (Note 2), INTVCC = DRVCC = VBOOST = VON = VRNG = SHDN = UVIN = VEXTVCC = VNDRV = 10V, VMODE/SYNC = VSENSE+ = VSENSE – = VBGRTN = VSW = 0V, unless otherwise speciﬁed. SYMBOL ∆VFB,LINE VSENSE(MAX) PARAMETER Feedback Voltage Line Regulation CONDITIONS 7V < INTVCC < 14V (Note 4) Maximum Current Sense Threshold VRNG = 2V, VFB = 0.
PAGE 4
LTC3810 ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C (Note 2), INTVCC = DRVCC = VBOOST = VON = VRNG = SHDN = UVIN = VEXTVCC = VNDRV = 10V, VMODE/SYNC = VSENSE+ = VSENSE– = VBGRTN = VSW = 0V, unless otherwise speciﬁed.
PAGE 5
LTC3810 TYPICAL PERFORMANCE CHARACTERISTICS Load Transient Response Short-Circuit/ Fault Timeout Operation Start-Up INTVCC 5V/DIV VOUT 100mV/ DIV VOUT 10V/DIV INTVCC VOUT 5V/DIV SS/TRACK 4V/DIV VIN 50V/DIV IOUT 5A/DIV IL 5A/DIV IL 5A/DIV 3810 G01 50μs/DIV VIN = 48V 0A TO 5A LOAD STEP FRONT PAGE CIRCUIT 3810 G02 VIN = 48V 500μs/DIV ILOAD = 1A MODE/SYNC = 0V FRONT PAGE CIRCUIT Short-Circuit/ Foldback Operation Tracking VOUT 5V/DIV VFB 0.
PAGE 6
LTC3810 TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Threshold vs ITH Voltage Frequency vs Load Current 350 400 300 300 On-Time vs ION Current 10000 VON = INTVCC FORCED CONTINUOUS 250 200 150 PULSE SKIP 100 50 1.4V 200 1V 0.7V 0.5V 100 0 –100 100 –200 –300 FRONT PAGE CIRCUIT 1 0 2 3 4 –400 5 0 1 1.5 2 ITH VOLTAGE (V) 0.5 LOAD CURRENT (A) 3810 G10 ION = 300μA 600 ON-TIME (ns) ON-TIME (ns) 500 400 300 ION = 300μA 640 620 600 200 580 100 560 –50 –25 0 2 1.
PAGE 7
LTC3810 TYPICAL PERFORMANCE CHARACTERISTICS Driver Peak Source Current vs Temperature 2.5 Driver Pull-Down RDS(ON) vs Temperature 1.50 VBOOST = VINTVCC = 10V Driver Peak Source Current vs Supply Voltage 3.0 VBOOST = VINTVCC = 10V PEAK SOURCE CURRENT (A) 1.00 2.0 RDS(ON) (Ω) PEAK SOURCE CURRENT (A) 1.25 0.75 0.50 1.5 0.25 1 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 2.0 1.5 1.0 0.5 0 –50 –25 125 2.
PAGE 8
LTC3810 TYPICAL PERFORMANCE CHARACTERISTICS INTVCC Shutdown Current vs INTVCC Voltage SS/TRACK Pull-Up Current vs Temperature 3 300 SS/TRACK CURRENT (μA) INTVCC CURRENT (μA) 250 200 150 100 2 1 50 0 –50 –25 0 0 2 8 6 10 4 INTVCC VOLTAGE (V) 12 14 50 25 75 0 TEMPERATURE (°C) 125 3810 G28 3810 G27 ITH Voltage vs Load Current Shutdown Threshold vs Temperature 2.2 3.0 2.0 SHUTDOWN THRESHOLD (V) 2.5 ITH VOLTAGE (V) 100 2.0 1.5 1.0 0.5 0 1 4 3 5 2 LOAD CURRENT (A) 1.6 1.4 1.2 1.0 0.
PAGE 9
LTC3810 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. VON (Pin 4): On-Time Voltage Input. Voltage trip point for the on-time comparator. Tying this pin to the output voltage or an external resistive divider from the output makes the on-time proportional to VOUT. The comparator defaults to 0.7V when the pin is grounded and defaults to 2.4V when the pin is connected to INTVCC.
PAGE 10
LTC3810 PIN FUNCTIONS BG (Pin 19): Bottom Gate Drive. The BG pin drives the gate of the bottom N-channel synchronous switch MOSFET. This pin swings from BGRTN to DRVCC. BGRTN (Pin 20): Bottom Gate Return. This pin connects to the source of the pull-down MOSFET in the BG driver and is normally connected to ground. Connecting a negative supply to this pin allows the synchronous MOSFET ’s gate to be pulled below ground to help prevent false turn-on during high dV/dt transitions on the SW node.
PAGE 11
LTC3810 FUNCTIONAL DIAGRAM EXTVCC NDRV INTVCC INTVCC 0.8V REF 5V REG VIN INTVCC MODE LOGIC 10V + NDRV M3 15 VIN RUV1 – 12V UVIN – – 14 INTVCC INTVCC UV VIN UV RUV2 200μA OFF 6.2V + 17 EXTVCC + 16 + 0.8V + F MODE/SYNC 270μA – 7 10V + ON 1.4μA PLL-SYNC 10 – + – PLL/LPF + VON 4 – 100nA RON VIN ION tON = 1 VVON (76pF) IION R S 20k + 6.
PAGE 12
LTC3810 OPERATION Main Control Loop The LTC3810 is a current mode controller for DC/DC stepdown 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 13
LTC3810 OPERATION Fault Monitoring/Protection Constant on-time current mode architecture provides accurate cycle-by-cycle current limit protection—a feature that is very important for protecting the high voltage power supply from output short circuits. The cycle-by-cycle current monitor guarantees that the inductor current will never exceed the value programmed on the VRNG pin. Foldback current limiting provides further protection if the output is shorted to ground.
PAGE 14
LTC3810 OPERATION is < 6.7V or an external supply in the appropriate range can be used. The LTC3810 will automatically detect which mode is being used and operate properly. The four possible operating modes for generating this supply are summarized as follows (see Figure 4): 1.
PAGE 15
LTC3810 APPLICATIONS INFORMATION The basic LTC3810 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 LTC3810 uses either a sense resistor or the on-resistance of the synchronous power MOSFET for determining the inductor current.
PAGE 16
LTC3810 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. Since most MOSFETs in the 60V to 100V range have higher thresholds (typically VGS(MIN) ≥ 6V), the LTC3810 is designed to be used with a 6.
PAGE 17
LTC3810 APPLICATIONS INFORMATION the period. Since there is no transition loss term in the synchronous MOSFET, optimal efﬁciency is obtained by minimizing RDS(ON)— by using larger MOSFETs or paralleling multiple MOSFETs. Multiple MOSFETs can be used in parallel to lower RDS(ON) and meet the current and thermal requirements if desired. The LTC3810 contains large low impedance drivers capable of driving large gate capacitances without signiﬁcantly slowing transition times.
PAGE 18
LTC3810 APPLICATIONS INFORMATION application. A good starting point is to feed about 25% of the voltage change at the ITH pin to the VON pin as shown in Figure 8. Place capacitance on the VON pin to ﬁlter out the ITH variations at the switching frequency. VON CVON 0.01μF RVON2 30k 100k LTC3810 ITH 3810 F08 Figure 8.
PAGE 19
LTC3810 APPLICATIONS INFORMATION 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. Input Capacitor Selection In continuous mode, the drain current of the top MOSFET is approximately a square wave of duty cycle VOUT/VIN which must be supplied by the input capacitor.
PAGE 20
LTC3810 APPLICATIONS INFORMATION that have been surge tested for use in switching power supplies. Several excellent surge-tested choices are the AVX, TPS and TPSV or the KEMET T510 series. Aluminum electrolytic capacitors have signiﬁcantly higher ESR, but can be used in cost-driven applications providing that consideration is given to ripple current ratings and longterm reliability. Other capacitor types include Panasonic SP and Sanyo POSCAPs.
PAGE 21
LTC3810 APPLICATIONS INFORMATION is typically much higher than 14V a separate supply for the IC power (INTVCC) and driver power (DRVCC) must be used. The LTC3810 has integrated bias supply control circuitry that allows the IC/driver supply to be easily generated from VIN and/or VOUT with minimal external components. There are four ways to do this as shown in the simpliﬁed schematics of Figure 4 and explained in the following sections. from overheating.
PAGE 22
LTC3810 APPLICATIONS INFORMATION The external NMOS for the linear regulator should be a standard 3V threshold type (i.e., not a logic level threshold). The rate of charge of INTVCC from 0V to 10V is controlled by the LTC3810 to be approximately 75μs regardless of the size of the capacitor connected to the INTVCC pin.
PAGE 23
LTC3810 APPLICATIONS INFORMATION Using an External Supply Connected to the INTVCC/ DRVCC Pins If an external supply is available between 6.2V and 14V, the supply can be connected directly to the INTVCC/DRVCC pins. In this mode, INTVCC, EXTVCC and NDRV must be shorted together. INTVCC/DRVCC Supply and the EXTVCC Connection The LTC3810 contains an internal low dropout regulator to produce the 10V INTVCC/DRVCC supply from the EXTVCC pin voltage. This regulator turns on when the EXTVCC pin is above 6.
PAGE 24
LTC3810 APPLICATIONS INFORMATION C2 IN C1 R2 R1 FB –6dB/OCT GAIN –6dB/OCT – OUT RB 0 FREQ + VREF –90 –180 PHASE –270 –360 3810 F11 Figure 11. Type 2 Schematic and Transfer Function IN C2 C3 R1 R3 FB R2 C1 –6dB/OCT – GAIN OUT RB VREF PHASE (DEG) So far, the AC response of the loop is pretty well out of the user’s control.
PAGE 25
LTC3810 APPLICATIONS INFORMATION Feedback Component Selection Selecting the R and C values for a typical Type 2 or Type 3 loop is a nontrivial task. The applications shown in this data sheet show typical values, optimized for the power components shown. They should give acceptable performance with similar power components, but can be way off if even one major power component is changed signiﬁcantly.
PAGE 26
LTC3810 APPLICATIONS INFORMATION Finally, choose a convenient resistor value for R1 (10k is usually a good value). Now calculate the remaining values: (K is a constant used in the calculations) f = chosen crossover frequency G = 10(GAIN/20) (this converts GAIN in dB to G in absolute gain) TYPE 2 Loop: BOOST + 45° K = tan 2 1 C2 = 2 • f • G • K • R1 ( ) C1= C2 K 2 1 K 2 • f • C1 V (R1) RB = REF VOUT VREF R2 = GAIN (dB) 0 0 PHASE –90 –180 FREQUENCY (Hz) 3810 F13 Figure 13.
PAGE 27
LTC3810 APPLICATIONS INFORMATION where H(s) was given in Equation 2 and A(s) depends on compensation circuit used: Pulse Skip Mode Operation and MODE/SYNC Pin The MODE/SYNC pin determines whether the bottom MOSFET remains on when current reverses in the inductor. Tying this pin above its 0.8V threshold enables pulse skip mode operation where the bottom MOSFET turns off when inductor current reverses.
PAGE 28
LTC3810 APPLICATIONS INFORMATION voltage VOUT2(MIN) below which continuous operation is forced until VOUT2 has risen above its minimum. R4 VOUT2(MIN) = 0.8V 1+ R3 Table 1. MODE/SYNC PIN CONDITION DC Voltage: 0V to 0.75V Forced Continuous Current Reversal Enabled DC Voltage: ≥ 0.85V Pulse Skip Mode Operation No Current Reversal Feedback Resistors Regulating a Secondary Winding Ext.
PAGE 29
LTC3810 APPLICATIONS INFORMATION Output Voltage Tracking To implement the coincident tracking in Figure 15a, connect an additional resistive divider to VOUT1 and connect its midpoint to the TRACK/SS pin of the slave IC. The ratio of this divider should be selected the same as that of the slave IC’s feedback divider shown in Figure 16. In this tracking mode, VOUT1 must be set higher than VOUT2.
PAGE 30
LTC3810 APPLICATIONS INFORMATION error on the tracking voltage depending on the absolute values of the tracking resistive divider. By selecting different resistors, the LTC3810 can achieve different modes of tracking including the two in Figure 15. So which mode should be programmed? While either mode in Figure 15 satisﬁes most practical applications, there do exist some tradeoffs. The ratiometric mode saves a pair of resistors, but the coincident mode offers better output regulation.
PAGE 31
LTC3810 APPLICATIONS INFORMATION 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. Thus the voltage on the PLL/LPF pin is adjusted until the phase and frequency of the external and internal oscillators are identical.
PAGE 32
LTC3810 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 ∆ILOAD (ESR), where ESR is the effective series resistance of COUT. ∆ILOAD 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 33
LTC3810 APPLICATIONS INFORMATION Since power dissipation at VIN(MIN) = 36V already exceeds 0.4W, calculate RNDRV(MAX) such that fault timeout is always enabled: RNDRV 36V 10V – 3.5V = 83.3k 270μA So, choose RNDRV = 80.6k. CIN is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.018Ω to minimize output voltage changes due to inductor ripple current and load steps. The ripple voltage will be only: ∆VOUT(RIPPLE) = ∆IL(MAX) • ESR = 4A • 0.
PAGE 34
LTC3810 APPLICATIONS INFORMATION RNDRV 80.6k RON 261k CON 100pF 1 20k 4 80.6k 0.01μF 10k CSS 1000pF RUV1 470k 11 BOOST VON 27 TG 26 SW 25 SENSE+ RUV2 12k CC2 47pF 19 BG 18 DRVCC 17 INTVCC 16 EXTVCC 15 NDRV RFB2 1k RC 200k SGND CC1 5pF CB 0.
PAGE 35
LTC3810 TYPICAL APPLICATIONS 7V to 80V Input Voltage to 5V/5A with IC Power from 12V Supply and All Ceramic Output Capacitors RON 110k 12V CON 100pF 1 BOOST TG 4 PGOOD RUV1 470k CSS 1000pF SENSE CC2 200pF RC 100k RFB2 1.89k VOUT 5V 5A 20 CDRVCC 0.1μF COUT 47μF 6.3V s3 M2 Si7852DP D1 B1100 CVCC 1μF SGND CC1 5pF M1 Si7852DP L1 4.7μH 19 BG 18 DRVCC 17 INTVCC 16 EXTVCC 15 NDRV SS/TRACK 12 SGND 13 SHDN 14 UVIN RUV2 61.9k CB 0.
PAGE 36
LTC3810 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 37
LTC3810 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 01/11 Changed Operating Junction Temperature Range in Absolute Maximum Ratings and Order Information sections 2 Remove Lead Based Part Numbers from Order Information 2 Updated Equations 27 Updated Related Parts 38 3810fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 38
LTC3810 TYPICAL APPLICATION 15V to 100V Input Voltage to 12V/5A with Trickle Charger Start-Up RNDRV 100k RON 261k CON 100pF BOOST ION 28 27 4 VON 5 VRNG 6 PGOOD 7 MODE/SYNC 8 I TH 9 V FB 10 PLL/LPF PGOOD RUV1 470k CSS 1000pF 11 SS/TRACK TG 26 SW + 25 SENSE CC2 47pF RFB2 1k RC 200k CC1 5pF M1 Si7456DP L1 10μH SENSE– 21 20 BGRTN 19 BG DRVCC 18 17 INTV CDRVCC 0.1μF EXTVCC 16 15 NDRV SGND RUV2 28k PGND CB 0.