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LTC3728

3728fg

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Dual, 550kHz, 2-Phase

Synchronous Step-Down

Switching Regulator

The LTC

3728 is a dual high performance step-down

switching regulator controller that drives all N-channel

synchronous power MOSFET stages. A constant-frequency

current mode architecture allows phase-lockable frequency

of up to 550kHz. Power loss and noise due to the ESR of

the input capacitors are minimized by operating the two

controller output stages out of phase.

OPTI-LOOP compensation allows the transient response to

be optimized over a wide range of output capacitance and

ESR values. The precision 0.8V reference and power good

output indicator are compatible with future microprocessor

generations, and a wide 3.5V to 30V (36V maximum) input

supply range encompasses all battery chemistries.

A RUN/SS pin for each controller provides both soft-

start and optional timed, short-circuit shutdown. Current

foldback limits MOSFET dissipation during short-circuit

conditions when overcurrent latchoff is disabled. Output

overvoltage protection circuitry latches on the bottom

MOSFET until V

OUT

returns to normal. The FCB mode

pin can select among Burst Mode, constant-frequency

mode and continuous inductor current mode or regulate

a secondary winding. The LTC3728 includes a power good

output pin that indicates when both outputs are within

7.5% of their designed set point.

Figure 1. High Efﬁ ciency Dual 5V/3.3V Step-Down Converter

Dual, 180° Phased Controllers Reduce Required

Input Capacitance and Power Supply Induced Noise

OPTI-LOOP

Compensation Minimizes C

OUT

±1% Output Voltage Accuracy

Power Good Output Voltage Indicator

Phase-Lockable Fixed Frequency 250kHz to 550kHz

Dual N-Channel MOSFET Synchronous Drive

Wide V

Range: 3.5V to 36V Operation

Very Low Dropout Operation: 99% Duty Cycle

Adjustable Soft-Start Current Ramping

Foldback Output Current Limiting

Latched Short-Circuit Shutdown with Defeat Option

Output Overvoltage Protection

Remote Output Voltage Sense

Low Shutdown I

: 20µA

5V and 3.3V Regulators

3 Selectable Operating Modes: Constant-Frequency,

Burst Mode

Operation and PWM

Available in 32-Pin 5mm × 5mm QFN and

28-Pin SSOP Packages

Notebook and Palmtop Computers

Telecom Systems

Portable Instruments

Battery-Operated Digital Devices

DC Power Distribution Systems

4.7μF

, 0.1μF

105k

1000pF

3.2μH

220pF

1μF

22μF

50V

OUT1

47μF

SENSE1

0.01Ω

20k

15k

OUT1

, 0.1μF

63.4k

3.2μH

220pF

1000pF

OUT

56μF

SENSE2

0.01Ω

20k

15k

OUT2

3.3V

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND

PGND

SENSE1

SENSE2

SENSE1

–

SENSE2

–

OSENSE1

OSENSE2

TH1

TH2

PGOOD INTV

RUN/SS1 RUN/SS2

5.2V TO 28V

M1, M2: FDS6982S

3728 F01

SS1

0.1μF

SS2

0.1μF

LTC3728

PLLIN

500kHz

L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and OPTI-LOOP are

registered trademarks of Linear Technology Corporation. All other trademarks are the property

of their respective owners. Protected by U.S. Patents including 5481178, 5929620, 6177787,

6144194, 6100678, 5408150, 6580258, 5705919.

Summary of content (36 pages)

PAGE 1
LTC3728 Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator DESCRIPTION FEATURES n n n n n n n n n n n n n n n n n Dual, 180° Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise OPTI-LOOP® Compensation Minimizes COUT ±1% Output Voltage Accuracy Power Good Output Voltage Indicator Phase-Lockable Fixed Frequency 250kHz to 550kHz Dual N-Channel MOSFET Synchronous Drive Wide VIN Range: 3.
PAGE 2
LTC3728 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN) .........................36V to – 0.3V Topside Driver Voltages (BOOST1, BOOST2) ............................... 42V to –0.3V Switch Voltage (SW1, SW2) ......................... 36V to –5V INTVCC, EXTVCC, RUN/SS1, RUN/SS2, (BOOST1-SW1), (BOOST2-SW2), PGOOD ..... 7V to –0.3V SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages .........................(1.1)INTVCC to –0.3V PLLIN, PLLFLTR, FCB Voltages ............ INTVCC to –0.
PAGE 3
LTC3728 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 15V, VRUN/SS1, 2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.792 0.800 0.808 V –5 –50 nA 0.002 0.02 %/V 0.1 –0.1 0.5 –0.5 % % Main Control Loops VOSENSE1, 2 Regulated Feedback Voltage (Note 3); ITH1, 2 Voltage = 1.
PAGE 4
LTC3728 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 15V, VRUN/SS1, 2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS VEXTVCC EXTVCC Switchover Voltage ICC = 20mA, EXTVCC Ramping Positive VLDOHYS EXTVCC Hysteresis l MIN TYP MAX UNITS 4.5 4.7 V 0.2 V Oscillator and Phase-Locked Loop fNOM Nominal Frequency VPLLFLTR = 1.
PAGE 5
LTC3728 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency vs Output Current and Mode (Figure 13) Efﬁciency vs Output Current (Figure 13) 100 100 Burst Mode OPERATION 90 90 90 FORCED CONTINUOUS MODE (PWM) 60 50 40 CONSTANT FREQUENCY (BURST DISABLE) 30 20 0 0.001 VIN = 20V 70 50 0.001 50 0.1 0.01 1 OUTPUT CURRENT (A) 10 1000 400 200 200 150 100 50 SHUTDOWN 0 20 15 10 25 INPUT VOLTAGE (V) 30 0 35 0 10 30 20 CURRENT (mA) 40 4.90 4.85 4.
PAGE 6
LTC3728 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Current Sense Threshold vs VRUN/SS (Soft-Start) 80 90 80 VSENSE(CM) = 1.6V 80 70 76 40 60 VSENSE (mV) VSENSE (mV) 60 VSENSE (mV) Current Sense Threshold vs ITH Voltage Maximum Current Sense Threshold vs Sense Common Mode Voltage 72 68 50 40 30 20 10 20 0 64 –10 –20 0 0 1 2 3 5 4 60 6 –30 1 3 4 2 COMMON MODE VOLTAGE (V) 0 VRUN/SS (V) 5 3728 G10 1.5 VITH (V) 2 SENSE Pins Total Source Current 2.0 50 ISENSE (μA) –0.1 2.
PAGE 7
LTC3728 TYPICAL PERFORMANCE CHARACTERISTICS Soft-Start Up (Figure 13) Load Step (Figure 13) VOUT 5V/DIV Load Step (Figure 13) VOUT 200mV/DIV VOUT 200mV/DIV IL 2A/DIV IL 2A/DIV VRUN/SS 5V/DIV IL 2A/DIV VIN = 15V VOUT = 5V 5ms/DIV 20μs/DIV VIN = 15V VOUT = 5V VPLLFLTR = 0V LOAD STEP = 0A to 3A Burst Mode OPERATION 3728 G19 Input Source/Capacitor Instantaneous Current (Figure 13) IIN 2A/DIV 3728 G20 Constant-Frequency (Burst Inhibit) Operation (Figure 13) Burst Mode Operation (Figure 13) VOUT 2
PAGE 8
LTC3728 TYPICAL PERFORMANCE CHARACTERISTICS Undervoltage Lockout vs Temperature Shutdown Latch Thresholds vs Temperature 4.5 SHUTDOWN LATCH THRESHOLDS (V) UNDERVOLTAGE LOCKOUT (V) 3.50 3.45 3.40 3.35 3.30 3.25 3.20 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3728 G29 PIN FUNCTIONS LATCH ARMING 4.0 3.5 3.0 LATCHOFF THRESHOLD 2.5 2.0 1.5 1.0 0.
PAGE 9
LTC3728 PIN FUNCTIONS INTVCC (Pin 21/Pin 20): Output of the Internal 5V Linear Low Dropout Regulator and the EXTVCC Switch. The driver and control circuits are powered from this voltage source. Must be decoupled to power ground with a minimum of 4.7μF tantalum or other low ESR capacitor. EXTVCC (Pin 22/Pin 21): External Power Input to an Internal Switch Connected to INTVCC. This switch closes and supplies VCC power, bypassing the internal low dropout regulator, whenever EXTVCC is higher than 4.7V.
PAGE 10
LTC3728 FUNCTIONAL DIAGRAM PLLIN FIN INTVCC PHASE DET DUPLICATE FOR SECOND CONTROLLER CHANNEL 50k BOOST VIN DB PLLFLTR DROP OUT DET CLK1 RLP OSCILLATOR CLK2 CLP – 0.86V S Q R Q BOT VOSENSE1 – 0.86V TOP ON SWITCH LOGIC + 0.55V INTVCC BG COUT PGND B – + + SHDN VOSENSE2 CIN SW BOT 0.74V + FCB – + CB D1 + PGOOD TG TOP VOUT RSENSE – VSEC 4.5V 0.18μA R6 + 1.5V 0.74V I1 – + – BINH FCB – 3.3VOUT + 0.8V – ++ FCB SLOPE COMP – 45k 2.
PAGE 11
LTC3728 OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3728 uses a constant-frequency, current mode step-down architecture with the two controller channels operating 180 degrees out of phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current comparator, I1, resets the RS latch.
PAGE 12
LTC3728 OPERATION (Refer to Functional Diagram) Continuous Current (PWM) Operation Tying the FCB pin to ground will force continuous current operation. This is the least efﬁcient operating mode, but may be desirable in certain applications. The output can source or sink current in this mode. When sinking current while in forced continuous operation, current will be forced back into the main power supply.
PAGE 13
LTC3728 OPERATION (Refer to Functional Diagram) 5V SWITCH 20V/DIV 3.3V SWITCH 20V/DIV INPUT CURRENT 5A/DIV INPUT VOLTAGE 500mV/DIV IIN(MEAS) = 2.53ARMS 3728 F03a (a) IIN(MEAS) = 1.55ARMS 3728 F03b (b) Figure 3. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each.
PAGE 14
LTC3728 (Refer to Functional Diagram) A ﬁnal question: If 2-phase operation offers such an advantage over single-phase operation for dual switching regulators, why hasn’t it been done before? The answer is that, while simple in concept, it is hard to implement. Constant-frequency current mode switching regulators require an oscillator derived slope compensation signal to allow stable operation of each regulator at over 50% duty cycle.
PAGE 15
LTC3728 APPLICATIONS INFORMATION 2.5 PLLFLTR PIN VOLTAGE (V) Figure 1 on the ﬁrst page is a basic LTC3728 application circuit. External component selection is driven by the load requirement, and begins with the selection of RSENSE and the inductor value. Next, the power MOSFETs and D1 are selected. Finally, CIN and COUT are selected. The circuit shown in Figure 1 can be conﬁgured for operation up to an input voltage of 28V (limited by the external MOSFETs).
PAGE 16
LTC3728 APPLICATIONS INFORMATION load transient response and sufﬁcient ripple current signal in the current loop. The maximum ΔIL occurs at the maximum input voltage. The inductor value also has secondary effects. The transition to Burst Mode operation begins when the average inductor current required results in a peak current below 25% of the current limit determined by RSENSE.
PAGE 17
LTC3728 APPLICATIONS INFORMATION which are highest at high input voltages. For VIN < 20V the high current efﬁciency generally improves with larger MOSFETs, while for VIN > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actually provides higher efﬁciency. The synchronous MOSFET losses are greatest at high input voltage when the top switch duty factor is low or during a short-circuit when the synchronous switch is on close to 100% of the period.
PAGE 18
LTC3728 APPLICATIONS INFORMATION the maximum RMS current of one channel must be used. The maximum RMS capacitor current is given by: CIN Required IRMS IMAX VOUT ( VIN VOUT ) VIN 1/2 This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst case condition is commonly used for design because even signiﬁcant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours of life.
PAGE 19
LTC3728 APPLICATIONS INFORMATION have lower storage capacity per unit volume than other capacitor types. These capacitors offer a very cost-effective output capacitor solution and are an ideal choice when combined with a controller having high loop bandwidth. Tantalum capacitors offer the highest capacitance density and are often used as output capacitors for switching regulators having controlled soft-start.
PAGE 20
LTC3728 APPLICATIONS INFORMATION factor of (Duty Cycle)/(Efﬁciency). For 5V regulators this supply means connecting the EXTVCC pin directly to VOUT. However, for 3.3V and other lower voltage regulators, additional circuitry is required to derive INTVCC power from the output. The following list summarizes the four possible connections for EXTVCC: 1. EXTVCC Left Open (or Grounded).
PAGE 21
LTC3728 APPLICATIONS INFORMATION reference by the error ampliﬁer. The output voltage is given by the equation: R2 VOUT = 0.8V 1+ R1 where R1 and R2 are deﬁned in Figure 2. SENSE+/SENSE– PINS The common mode input range of the current comparator sense pins is from 0V to (1.1)INTVCC. Continuous linear operation is guaranteed throughout this range allowing output voltage setting from 0.8V to 7.7V, depending upon the voltage applied to EXTVCC.
PAGE 22
LTC3728 APPLICATIONS INFORMATION Fault Conditions: Overcurrent Latchoff The RUN/SS pins also provide the ability to latch off the controller(s) when an overcurrent condition is detected. The RUN/SS capacitor, CSS, is used initially to turn on and limit the inrush current. After the controller has been started and given adequate time to charge up the output capacitor and provide full load current, the RUN/SS capacitor is used for a short-circuit timer.
PAGE 23
LTC3728 APPLICATIONS INFORMATION Fault Conditions: Overvoltage Protection (Crowbar) The overvoltage crowbar is designed to blow a system input fuse when the output voltage of the regulator rises much higher than nominal levels. The crowbar causes huge currents to ﬂow that blow the fuse to protect against a shorted top MOSFET, if the short occurs while the controller is operating. A comparator monitors the output for overvoltage conditions. The comparator (OV) detects overvoltage faults greater than 7.
PAGE 24
LTC3728 APPLICATIONS INFORMATION Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3728 is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET.
PAGE 25
LTC3728 APPLICATIONS INFORMATION Voltage Positioning Voltage positioning can be used to minimize peak-to-peak output voltage excursions under worst-case transient loading conditions. The open-loop DC gain of the control loop is reduced depending upon the maximum load step speciﬁcations. Voltage positioning can easily be added to the LTC3728 by loading the ITH pin with a resistive divider having a Thevenin equivalent voltage source equal to the midpoint operating voltage range of the error ampliﬁer, or 1.
PAGE 26
LTC3728 APPLICATIONS INFORMATION RSENSE = 10mΩ and RESR = 40mΩ (sum of both input and output capacitance losses), then the total resistance is 130mΩ. This results in losses ranging from 3% to 13% as the output current increases from 1A to 5A for a 5V output, or a 4% to 20% loss for a 3.3V output. Efﬁciency varies as the inverse square of VOUT for the same external components and output power level.
PAGE 27
LTC3728 APPLICATIONS INFORMATION loop. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance. A second, more severe transient is caused by switching in loads with large (>1μF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT.
PAGE 28
LTC3728 APPLICATIONS INFORMATION Design Example As a design example for one channel, assume VIN = 12V (nominal), VIN = 22V(max), VOUT = 1.8V, IMAX = 5A, and f = 300kHz. The inductance value is chosen ﬁrst based on a 30% ripple current assumption. The highest value of ripple current occurs at the maximum input voltage. Tie the PLLFLTR pin to a resistive divider using the INTVCC pin generating 1V for 300kHz operation.
PAGE 29
LTC3728 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3728. These items are also illustrated graphically in the layout diagram of Figure 10. Figure 11 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1.
PAGE 30
LTC3728 APPLICATIONS INFORMATION SW1 L1 D1 RSENSE1 COUT1 VOUT1 + RL1 VIN RIN CIN + SW2 BOLD LINES INDICATE HIGH, SWITCHING CURRENT LINES. KEEP LINES TO A MINIMUM LENGTH. D2 L2 RSENSE2 COUT2 VOUT2 + RL2 3728 F11 Figure 11. Branch Current Waveforms signal ground. The R2 and R4 connections should not be along the high current input feeds from the input capacitor(s). 4.
PAGE 31
LTC3728 APPLICATIONS INFORMATION PC Board Layout Debugging Start with one controller on at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage. Check for proper performance over the operating voltage and current range expected in the application.
PAGE 32
LTC3728 TYPICAL APPLICATIONS 59k 1M 100k 0.1MF 2 180pF 20k 1% 1000pF 3 105k, 1% 4 5 6 33pF 15k 1000pF 9 33pF 3.3V 10 11 15k 1000pF 12 20k 1% 13 63.4k 1% 180pF 1000pF 14 SENSE1+ TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN BG1 PLLIN EXTVCC FCB SGND 3.3VOUT ITH2 INTVCC PGND BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ + 0.0157 27 VOUT1 5V 3A; 4A PEAK 26 8 0.1MF 25 M1 M2 5 D1 MBRM 140T3 LT1121 3 ON/OFF 2 1 220k 24 23 107 22 CMDSH-3TR 22MF 50V 150MF, 6.
PAGE 33
LTC3728 TYPICAL APPLICATIONS 0.1MF 27pF 20k 1% 10k 2 105k 1% 1000pF 3 4 0.01MF 5 1000pF fSYNC 6 7 8 220pF 9 33pF 3.3V 10 11 15k 220pF 12 20k 1% 27pF 13 63.4k 1% 1000pF 14 SENSE1+ TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN BG1 FCB EXTVCC LTC3728 ITH1 SGND 3.3VOUT ITH2 VOSENSE2 INTVCC PGND BG2 BOOST2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 28 L1 8MH PGOOD 27 0.0157 26 0.1MF 25 24 M1 47MF 6.3V 23 107 22 CMDSH-3TR 22MF 50V 0.1MF GND 21 20 19 1MF 10V 4.
PAGE 34
LTC3728 PACKAGE DESCRIPTION G Package 28-Lead Plastic SSOP (0.209) (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. DIMENSIONS ARE IN (INCHES) 0.65 (.
PAGE 35
LTC3728 PACKAGE DESCRIPTION UH32 Package 32-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1693 Rev D) 0.70 ±0.05 5.50 ±0.05 4.10 ±0.05 3.50 REF (4 SIDES) 3.45 ± 0.05 3.45 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 ± 0.05 R = 0.05 TYP 0.00 – 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER R = 0.115 TYP 31 32 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 3.
PAGE 36
LTC3728 TYPICAL APPLICATION IIN 12VIN CIN I1 PHASMD OPEN TG1 U1 TG2 LTC3729 90° 0° IIN* BUCK: 2.5V/15A 180° BUCK: 2.5V/15A 2.5VO/30A I2 CLKOUT I2 I3 I3 TG1 U2 TG2 LTC3728 90° 90° BUCK: 1.5V/15A 270° BUCK: 1.8V/15A 1.5VO/15A 1.8VO/15A I4 PLLIN I1 3728 F14 I4 *INPUT RIPPLE CURRENT CANCELLATION INCREASES THE RIPPLE FREQUENCY AND REDUCES THE RMS INPUT RIPPLE CURRENT THUS, SAVING INPUT CAPACITORS Figure 14.