Datasheet

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LTC3778

3778f

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Wide Operating Range,

No R

SENSE

Step-Down Controller

■

Wide V

Range: 4V to 36V

■

Sense Resistor Optional

■

True Current Mode Control

■

2% to 90% Duty Cycle at 200kHz

■

ON(MIN)

≤ 100ns

■

Stable with Ceramic C

OUT

■

Dual N-Channel MOSFET Synchronous Drive

■

Power Good Output Voltage Monitor

■

±1% 0.6V Reference

■

Adjustable Current Limit

■

Adjustable Switching Frequency

■

Programmable Soft-Start

■

Output Overvoltage Protection

■

Optional Short-Circuit Shutdown Timer

■

Micropower Shutdown: I

≤ 30µA

■

Available in a 1mm 20-Lead TSSOP Package

■

Notebook and Palmtop Computers, PDAs

■

Battery Chargers

■

Distributed Power Systems

■

DDR Memory Power Supply

■

Automobile DC Power Supply

The LTC

3778 is a synchronous step-down switching

regulator controller for computer memory, automobile

and other DC/DC power supplies. The controller uses a

valley current control architecture to deliver very low duty

cycles without requiring a sense resistor. Operating fre-

quency is selected by an external resistor and is compen-

sated for variations in V

and V

OUT

Discontinuous mode operation provides high efficiency

operation at light loads. A forced continuous control pin

reduces noise and RF interference, and can assist second-

ary winding regulation when the main output is lightly

loaded. SENSE

and

SENSE

–

pins provide true Kelvin

sensing across the optional sense resistor or the

sychronous MOSFET.

Fault protection is provided by internal foldback current

limiting, an output overvoltage comparator, optional short-

circuit shutdown timer and input undervoltage lockout.

Soft-start capability for supply sequencing is accom-

plished using an external timing capacitor. The regulator

current limit level is user programmable. Wide supply

range allows operation from 4V to 36V at the input and

from 0.6V up to (0.9)V

at the output.

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

No R

SENSE

is a trademark of Linear Technology Corporation.

Figure 1. High Efficiency Step-Down Converter

Efficiency vs Load Current

LOAD CURRENT (A)

0.01

EFFICIENCY (%)

3778 F01b

0.1

100

= 5V

= 25V

CMDSH-3

B340A

1.8µH

VCC

4.7µF

10µF

35V

×3

5V TO 28V

OUT

2.5V

10A

OUT

180µF

×2

Si4874

3778 F01a

Si4884

1.4M

0.1µF

BOOST

RUN/SS

SGND

INTV

PGND

SENSE

–

SENSE

PGOOD

0.22µF

20k

LTC3778

500pF

40.2k

12.7k

DRV

Summary of content (24 pages)

PAGE 1
LTC3778 Wide Operating Range, No RSENSETM Step-Down Controller U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®3778 is a synchronous step-down switching regulator controller for computer memory, automobile and other DC/DC power supplies. The controller uses a valley current control architecture to deliver very low duty cycles without requiring a sense resistor. Operating frequency is selected by an external resistor and is compensated for variations in VIN and VOUT.
PAGE 2
LTC3778 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage (VIN, ION)..................36V to – 0.3V Boosted Topside Driver Supply Voltage (BOOST) ................................................... 42V to – 0.3V SENSE+, SW Voltage.................................... 36V to – 5V DRVCC, EXTVCC, (BOOST – SW), FCB, RUN/SS, PGOOD Voltages .................... 7V to – 0.3V VON, VRNG Voltages ................... INTVCC + 0.3V to – 0.3V ITH, VFB Voltages............
PAGE 3
LTC3778 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX VRUN/SS(ON) RUN Pin Start Threshold 0.8 1.5 2 V VRUN/SS(LE) RUN Pin Latchoff Enable Threshold VRUN/SS(LT) RUN Pin Latchoff Threshold RUN/SS Pin Rising 4 4.5 V RUN/SS Pin Falling 3.5 4.2 V IRUN/SS(C) Soft-Start Charge Current – 1.
PAGE 4
LTC3778 U W TYPICAL PERFOR A CE CHARACTERISTICS Transient Response (Discontinuous Mode) Transient Response Start-Up RUN/SS 2V/DIV VOUT 50mV/DIV VOUT 50mV/DIV VOUT 1V/DIV IL 5A/DIV IL 5A/DIV 20µs/DIV LOAD STEP 0A TO 10A VIN = 15V VOUT = 2.5V FCB = 0V FIGURE 1 CIRCUIT 20µs/DIV LOAD STEP 1A TO 10A VIN = 15V VOUT = 2.5V FCB = INTVCC FIGURE 1 CIRCUIT 3778 G01 Efficiency vs Load Current 100 DISCONTINUOUS MODE CONTINUOUS MODE 70 0.1 0.01 1 LOAD CURRENT (A) ILOAD = 1A 90 ILOAD = 10A 80 0 10 2.
PAGE 5
LTC3778 U W TYPICAL PERFOR A CE CHARACTERISTICS On-Time vs ION Current On-Time vs VON Voltage 10k VVON = 0V 1000 On-Time vs Temperature 300 IION = 30µA 250 ON-TIME (ns) ON-TIME (ns) ON-TIME (ns) 800 1k 600 400 100 0 10 10 ION CURRENT (µA) 1 100 2 1 VON VOLTAGE (V) 0 100 75 50 25 0 0.15 0.30 VFB (V) 0.45 300 250 200 150 100 50 0 0.60 0.5 0.75 1.0 1.25 1.5 VRNG VOLTAGE (V) 1.75 120 110 50 25 0 75 TEMPERATURE (°C) 75 50 25 0 1.5 100 125 3778 G11 2 2.
PAGE 6
LTC3778 U W TYPICAL PERFOR A CE CHARACTERISTICS Input and Shutdown Currents vs Input Voltage 60 EXTVCC OPEN 600 30 400 20 200 –0.1 ∆INTVCC (%) 40 SHUTDOWN SHUTDOWN CURRENT (µA) INPUT CURRENT (µA) 50 800 10 0 EXTVCC SWITCH RESISTANCE (Ω) 1200 1000 EXTVCC Switch Resistance vs Temperature INTVCC Load Regulation –0.2 –0.3 10 –0.4 0 –0.5 8 6 4 2 EXTVCC = 5V 0 0 5 20 15 25 10 INPUT VOLTAGE (V) 30 35 10 30 40 20 INTVCC LOAD CURRENT (mA) 0 3778 G24 –0.
PAGE 7
LTC3778 U U U PI FU CTIO S RUN/SS (Pin 1): Run Control and Soft-Start Input. A capacitor to ground at this pin sets the ramp time to full output current (approximately 3s/µF) and the time delay for overcurrent latchoff (see Applications Information). Forcing this pin below 0.8V shuts down the device. VON (Pin 2): On-Time Voltage Input. Voltage trip point for the on-time comparator. Tying this pin to the output voltage makes the on-time proportional to VOUT. The comparator input defaults to 0.
PAGE 8
LTC3778 W FU CTIO AL DIAGRA U U RON VOUT VIN 2 VON 2.4V 0.7V 8 ION 6 FCB 11 VIN 10 EXTVCC + 4.7V CIN + 1µA – 0.6V REF 0.6V 5V REG 12 F tON = BOOST 20 VVON (10pF) IION R S Q TG 19 FCNT SENSE+ 17 SWITCH LOGIC IREV SHDN 1.4V 4 VOUT + COUT 13 BG 14 OV VRNG L1 DB SENSE – 16 DRVCC – – M1 18 + ICMP CB SW ON 20k + INTVCC + – × (0.5~2) M2 CVCC RSENSE (OPTIONAL)* PGND 0.7V 15 3 3.3µA PGOOD 1 240k + 1V Q2 Q4 0.
PAGE 9
LTC3778 U OPERATIO Main Control Loop The LTC3778 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. Inductor current is determined by sensing the voltage between the SENSE– and SENSE+ pins.
PAGE 10
LTC3778 U W U U APPLICATIO S I FOR ATIO The basic LTC3778 application circuit is shown in Figure 1. External component selection is primarily determined by the maximum load current and begins with the selection of the sense resistance and power MOSFET switches. The LTC3778 can use either a sense resistor or the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency largely determines the inductor value.
PAGE 11
LTC3778 U W U U APPLICATIO S I FOR ATIO The ρT term is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance with temperature, typically about 0.4%/°C as shown in Figure 2. For a maximum junction temperature of 100°C, using a value ρT = 1.3 is reasonable. Operating Frequency The choice of operating frequency is a tradeoff between efficiency and component size.
PAGE 12
LTC3778 U W U U APPLICATIO S I FOR ATIO load current increases. By lengthening the on-time slightly as current increases, constant frequency operation can be maintained. This is accomplished with a resistive divider from the ITH pin to the VON pin and VOUT. The values required will depend on the parasitic resistances in the specific 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 3a.
PAGE 13
LTC3778 U W U U APPLICATIO S I FOR ATIO commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to derate the capacitor. The selection of COUT is primarily determined by the ESR required to minimize voltage ripple and load step transients.
PAGE 14
LTC3778 U W U U APPLICATIO S I FOR ATIO VIN + CIN VIN 1N4148 OPTIONAL EXTVCC CONNECTION 5V < VOUT2 < 7V TG • + LTC3778 EXTVCC SW R4 T1 1:N FCB R3 • + VOUT2 CSEC 1µF VOUT1 COUT BG SGND PGND 3778 F04 Figure 4. Secondary Output Loop and EXTVCC Connection Fault Conditions: Current Limit and Foldback The maximum inductor current is inherently limited in a current mode controller by the maximum sense voltage.
PAGE 15
LTC3778 U W U U APPLICATIO S I FOR ATIO EXTVCC Connection Soft-Start and Latchoff with the RUN/SS Pin The EXTVCC pin can be used to provide MOSFET gate drive and control power from the output or another external source during normal operation. Whenever the EXTVCC pin is above 4.7V the internal 5V regulator is shut off and an internal 50mA P-channel switch connects the EXTVCC pin to INTVCC. INTVCC power is supplied from EXTVCC until this pin drops below 4.5V.
PAGE 16
LTC3778 U W U U APPLICATIO S I FOR ATIO Overcurrent latchoff operation is not always needed or desired. Load current is already limited during a shortcircuit by the current foldback circuitry and latchoff operation can prove annoying during troubleshooting. The feature can be overridden by adding a pull-up current greater than 5µA to the RUN/SS pin. The additional current prevents the discharge of CSS during a fault and also shortens the soft-start period.
PAGE 17
LTC3778 U W U U APPLICATIO S I FOR ATIO During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH pin external components shown in Figure 7 will provide adequate compensation for most applications. For a detailed explanation of switching control loop theory see Linear Technology Application Note 76. As a design example, take a supply with the following specifications: VIN = 7V to 28V (15V nominal), VOUT = 2.
PAGE 18
LTC3778 U W U U APPLICATIO S I FOR ATIO 1 2 R3 11k R4 39k CC1 500pF RPG 100k 3 4 RC 20k 5 6 CC2 100pF CON, 0.01µF R1 12.7k 7 8 9 RON 400k R2 40.2k 10 LTC3778 20 RUN/SS BOOST VON TG PGOOD SW 19 CB 0.22µF 18 VRNG SENSE + 17 ITH SENSE – 16 FCB DB CMDSH-3 M1 Si4884 + PGND SGND BG ION DRVCC VFB INTVCC EXTVCC VIN VIN 5V TO 28V CIN 10µF 35V ×3 VOUT 2.5V 10A L1, 1.8µH 15 M2 Si4874 D1 B340A COUT1-2 180µF 4V ×2 COUT3 22µF 6.3V X7R 14 13 + CSS 0.1µF CVCC 4.
PAGE 19
LTC3778 U W U U APPLICATIO S I FOR ATIO  12V  ∆ITH =   RSENSE ∆IOUT  VRNG  ( )( )( ) = 24 0.002Ω 20A = 0.96V The corresponding change in the output voltage is determined by the gain of the error amplifier and feedback divider. The LTC3778 error amplifier has a transconductance gm that is constant over both temperature and a wide ± 40mV input range. Thus, by connecting a load resistance RVP to the ITH pin, the error amplifier gain can be precisely set for accurate voltage positioning.  0.
PAGE 20
LTC3778 U W U U APPLICATIO S I FOR ATIO • Use an immediate via to connect the components to ground plane including SGND and PGND of LTC3778. Use several bigger vias for power components. PC Board Layout Checklist When laying out a PC board follow one of the 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.
PAGE 21
LTC3778 U W U U APPLICATIO S I FOR ATIO • Segregate the signal and power grounds. All small signal components should return to the SGND pin at one point which is then tied to the PGND pin close to the source of M2. • Keep the high dV/dT SW, BOOST and TG nodes away from sensitive small-signal nodes. • Place M2 as close to the controller as possible, keeping the PGND, BG and SW traces short. • Connect the top driver boost capacitor CB closely to the BOOST and SW pins.
PAGE 22
LTC3778 U TYPICAL APPLICATIO S Intel Compatible Tualatin Mobile CPU Power Supply with AVP 1 3.3V VRON 3 3 2 1 R2 100k Q2 2N7002 2 R1 100k Q1 2N7002 2 R3 OPT 1 VIN INTVCC C1, 0.01µF SGND C2, 0.01µF R4 2k R5 100Ω 1 R6 1k R8 1k 2 VOUT PGND LTC3778E RUN/SS BOOST VON TG PGOOD SW 20 R16 0 + 2 1 QT IRF7811A ×2 D1 CMDSH-3 C10 0.22µF 19 L1 0.68µH SUMIDA CEP125-4712F011 PWRGOOD 3 1 Q3 MMMT3906-7 3 2 1 Q6 MMBT3904-7 R14 2 10k 3 R10 3.2k C7 1µF 6.3V 0603 VIN 7.
PAGE 23
LTC3778 U PACKAGE DESCRIPTIO F Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1650) 6.40 – 6.60* (.252 – .260) 20 19 18 17 16 15 14 13 12 11 6.25 – 6.50 (.246 – .256) 1 2 3 4 5 6 7 8 9 10 1.10 (.0433) MAX 4.30 – 4.48** (.169 – .176) 0° – 8° .09 – .18 (.0035 – .0071) .50 – .70 (.020 – .028) .65 (.0256) BSC .18 – .30 (.0071 – .0118) .05 – .15 (.002 – .006) F20 TSSOP 0501 NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3.
PAGE 24
LTC3778 U TYPICAL APPLICATIO 12V/5A at 300kHz 2 100k 3 4 5 RC CC1 20k 2.2nF 6 CC2 100pF 7 8 R1 10k R2 190k DB CMDSH-3 LTC3778 1 9 10 RON 1.6M RUN/SS BOOST VON TG PGOOD SW 20 19 VRNG ITH SENSE – PGND SGND BG ION DRVCC VFB INTVCC EXTVCC VIN VIN CIN 14V TO 28V 22µF 50V M1 18 17 SENSE + FCB CB 0.22µF L1 10µH VOUT 12V 5A 16 + M2 15 D1 COUT 220µF 16V 14 13 CVCC 4.7µF 12 + CSS 0.1µF 11 C2 2200pF CF 0.