Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - voltage regulators - DC DC switch controllers PolyPhase® QFN 32

LTC3728L/LTC3728LX

3728lxff

Typical applicaTion

FeaTures

applicaTions

DescripTion

Dual, 550kHz, 2-Phase

Synchronous Regulators

The LTC

3728L/LTC3728LX are dual high performance

step-down switching regulator controllers that drive 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 microproces-

sor generations, and a wide 4.5V to 28V (30V 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 LTC3728L/LTC3728LX include

a power good output pin that indicates when both outputs

are within 7.5% of their designed set point.

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

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

property of their respective owners.

Figure 1. High Efficiency 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 (LTC3728LC)

Power Good Output Voltage Indicator

Phase-Lockable Fixed Frequency 250kHz to 550kHz

Dual N-Channel MOSFET Synchronous Drive

Wide V

Range: 4.5V to 28V 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

Low Shutdown I

: 20µA

5V and 3.3V Standby Regulators

3 Selectable Operating Modes: Constant-Frequency,

Burst Mode

Operation and PWM

5mm × 5mm QFN and 28-Lead Narrow 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

CERAMIC

22µF

50V

CERAMIC

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

LTC3728L/

LTC3728LX

PLLIN

500kHz

Summary of content (38 pages)

PAGE 1
LTC3728L/LTC3728LX Dual, 550kHz, 2-Phase Synchronous Regulators Description Features 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 (LTC3728LC) Power Good Output Voltage Indicator Phase-Lockable Fixed Frequency 250kHz to 550kHz Dual N-Channel MOSFET Synchronous Drive Wide VIN Range: 4.
PAGE 2
LTC3728L/LTC3728LX Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN)......................... 30V to – 0.3V Topside Driver Voltages (BOOST1, BOOST2)............................... 36V to –0.3V Switch Voltage (SW1, SW2)......................... 30V 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
LTC3728L/LTC3728LX Order Information LEAD FREE FINISH TAPE AND REEL LTC3728LCGN#PBF PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3728LCGN#TRPBF 28-Lead Narrow Plastic SSOP 0°C to 85°C LTC3728LEGN#PBF LTC3728LEGN#TRPBF 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3728LIGN#PBF LTC3728LIGN#TRPBF 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3728LCUH#PBF LTC3728LCUH#TRPBF 3728L 32-Lead (5mm × 5mm) Plastic QFN 0°C to 85°C LTC3728LEUH#PBF LTC3728LEUH#TRPBF 3728LE 32-Lead (5mm × 5m
PAGE 4
LTC3728L/LTC3728LX 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 0.788 0.800 0.800 0.808 0.812 V V Main Control Loops VOSENSE1, 2 Regulated Feedback Voltage (Note 3); ITH1, 2 Voltage = 1.2V (LTC3728LC) (Note 3); ITH1, 2 Voltage = 1.
PAGE 5
LTC3728L/LTC3728LX 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 4.8 5.0 5.2 UNITS INTVCC Linear Regulator VINTVCC Internal VCC Voltage 6V < VIN < 30V, VEXTVCC = 4V VLDO INT INTVCC Load Regulation ICC = 0 to 20mA, VEXTVCC = 4V 0.2 2.
PAGE 6
LTC3728L/LTC3728LX Typical Performance Characteristics Efficiency vs Output Current and Mode (Figure 13) 100 Efficiency vs Output Current (Figure 13) 100 Burst Mode OPERATION 90 80 90 50 40 CONSTANT FREQUENCY (BURST DISABLE) 30 20 0 0.001 VIN = 20V 70 VOUT = 5V f = 250kHz 0.1 0.01 1 OUTPUT CURRENT (A) EXTVCC VOLTAGE DROP (mV) SUPPLY CURRENT (µA) 400 200 SHUTDOWN 0 5 20 10 15 INPUT VOLTAGE (V) 25 100 50 0 30 10 0 20 CURRENT (mA) 30 4.95 4.90 4.85 4.80 EXTVCC SWITCHOVER THRESHOLD 4.
PAGE 7
LTC3728L/LTC3728LX Typical Performance Characteristics Maximum Current Sense Threshold vs Sense Common Mode Voltage 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 72 68 50 40 30 20 10 20 0 64 –10 –20 0 0 1 2 3 5 4 60 6 1 3 4 2 COMMON MODE VOLTAGE (V) 0 VRUN/SS (V) FCB = 0V VIN = 15V FIGURE 13 VOSENSE = 0.7V ISENSE (µA) 1.5 1.
PAGE 8
LTC3728L/LTC3728LX 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 3728L G19 VIN = 15V 20µs/DIV VOUT = 5V VPLLFLTR = 0V LOAD STEP = 0A TO 3A Burst Mode OPERATION Input Source/Capacitor Instantaneous Current (Figure 13) IIN 2A/DIV 3728L G20 3728L G21 Constant-Frequency (Burst Inhibit) Operation (Figure 13) Burst Mode Oper
PAGE 9
LTC3728L/LTC3728LX Typical Performance Characteristics Current Sense Pin Input Current vs Temperature 31 29 27 50 0 75 25 TEMPERATURE (°C) 100 125 600 8 6 4 2 0 –50 –25 50 0 75 25 TEMPERATURE (°C) 3728L G26 VPLLFLTR = 1.2V 400 300 VPLLFLTR = 0V 200 100 125 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3728L G28 Shutdown Latch Thresholds vs Temperature 3.50 4.5 3.45 3.40 3.35 3.30 3.25 50 25 75 0 TEMPERATURE (°C) 500 3728L G27 Undervoltage Lockout vs Temperature 3.
PAGE 10
LTC3728L/LTC3728LX Pin Functions VOSENSE1, VOSENSE2: Error Amplifier Feedback Input. Receives the remotely sensed feedback voltage for each controller from an external resistive divider across the output. TG2, TG1: High Current Gate Drives for Top N-Channel MOSFETs. These are the outputs of floating drivers with a voltage swing equal to INTVCC – 0.5V superimposed on the switch node voltage SW. PLLFLTR: Filter Connection for Phase-Locked Loop.
PAGE 11
+ 5V VIN R5 – + 4.7V 3V – + 0.8V 4.3V OSCILLATOR PHASE DET SGND (UH PACKAGE PAD) INTVCC EXTVCC VIN 3.3VOUT FCB 0.18µA PGOOD INTVCC R6 50k PLLIN PLLFLTR CLP RLP FIN – + + – 5V LDO REG VREF FCB BINH 0.74V INTERNAL SUPPLY + – VOSENSE2 0.86V – + 0.74V VOSENSE1 0.86V + – + – CLK2 CLK1 6V 1.2µA SLOPE COMP 0.86V 4(VFB) I1 Q R – + – SHDN RST 4(VFB) 3mV B FCB – RUN SOFT START OV – + – EA + 0.
PAGE 12
LTC3728L/LTC3728LX Operation (Refer to Functional Diagram) Main Control Loop Low Current Operation The IC 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 13
LTC3728L/LTC3728LX operation (Refer to Functional Diagram) Constant-Frequency Operation Power Good (PGOOD) Pin When the FCB pin is tied to INTVCC, Burst Mode operation is disabled and the forced minimum output current requirement is removed. This provides constant-frequency, discontinuous current (preventing reverse inductor current) operation over the widest possible output current range.
PAGE 14
LTC3728L/LTC3728LX Operation (Refer to Functional Diagram) Theory and Benefits of 2-Phase Operation The LTC1628 and the LTC3728L family of dual high efficiency DC/DC controllers brings the considerable benefits of 2-phase operation to portable applications for the first time.
PAGE 15
LTC3728L/LTC3728LX (Refer to Functional Diagram) duty cycles which, in turn, are dependent upon the input voltage VIN (Duty Cycle = VOUT/VIN). Figure 4 shows how the RMS input current varies for single-phase and 2-phase operation for 3.3V and 5V regulators over a wide input voltage range. It can readily be seen that the advantages of 2-phase operation are not just limited to a narrow operating range, but in fact extend over a wide region.
PAGE 16
LTC3728L/LTC3728LX Applications Information Figure 1 on the first page is a basic LTC3728L/LTC3728LX 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). PLLFLTR PIN VOLTAGE (V) 2.
PAGE 17
LTC3728L/LTC3728LX applications information 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. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation.
PAGE 18
LTC3728L/LTC3728LX Applications Information where d is the temperature dependency of RDS(ON) and RDR (approximately 4Ω) is the effective driver resistance at the MOSFET’s Miller threshold voltage. VTH(MIN) is the typical MOSFET minimum threshold voltage. Both MOSFETs have I2R losses while the topside N‑channel equation includes an additional term for transition losses, which are highest at high input voltages.
PAGE 19
LTC3728L/LTC3728LX applications information current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the manufacturer if there is any question.
PAGE 20
LTC3728L/LTC3728LX Applications Information Panasonic SP, NEC Neocap, Cornell Dubilier ESRE and Sprague 595D series. Consult manufacturers for other specific recommendations. INTVCC Regulator An internal P‑channel low dropout regulator produces 5V at the INTVCC pin from the VIN supply pin. INTVCC powers the drivers and internal circuitry within the IC. The INTVCC pin regulator can supply a peak current of 50mA and must be bypassed to ground with a minimum of 4.
PAGE 21
LTC3728L/LTC3728LX applications information 4. EXTVCC Connected to an Output-Derived Boost Network. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage that has been boosted to greater than 4.7V. This can be done with either the inductive boost winding as shown in Figure 6a or the capacitive charge pump shown in Figure 6b. The charge pump has the advantage of simple magnetics.
PAGE 22
LTC3728L/LTC3728LX Applications Information Since VOSENSE is servoed to the 0.8V reference voltage, we can choose R1 in Figure 2 to have a maximum value to absorb this current.  0.8V R1(MAX) = 24k   2.4V – V OUT Each RUN/SS pin has an internal 6V Zener clamp (see the Functional Diagram). VIN   3.3V OR 5V D1 CSS for VOUT < 2.4V Regulating an output voltage of 1.8V, the maximum value of R1 should be 32k. Note that for an output voltage above 2.
PAGE 23
LTC3728L/LTC3728LX applications information with noise pickup or poor layout causing the protection circuit to latch off. Defeating this feature will easily allow troubleshooting of the circuit and PC layout. The internal short-circuit and foldback current limiting still remains active, thereby protecting the power supply system from failure. After the design is complete, a decision can be made whether to enable the latchoff feature.
PAGE 24
LTC3728L/LTC3728LX Applications Information The output of the phase detector is a complementary pair of current sources charging or discharging the external filter network on the PLLFLTR pin. If the external frequency (fPLLIN) is greater than the oscillator frequency, f0SC, current is sourced continuously, pulling up the PLLFLTR pin. When the external frequency is less than f0SC, current is sunk continuously, pulling down the PLLFLTR pin.
PAGE 25
LTC3728L/LTC3728LX applications information If VSEC drops below this level, the FCB voltage forces temporary continuous switching operation until VSEC is again above its minimum. In order to prevent erratic operation if no external connections are made to the FCB pin, the FCB pin has a 0.18µA internal current source pulling the pin high. Include this current when choosing resistor values R5 and R6.
PAGE 26
LTC3728L/LTC3728LX Applications Information Supplying INTVCC power through the EXTVCC switch input from an output-derived source will scale the VIN current required for the driver and control circuits by a factor of (Duty Cycle)/(Efficiency). For example, in a 20V to 5V application, 10mA of INTVCC current results in approximately 2.5mA of VIN current. This reduces the mid-current loss from 10% or more (if the driver was powered directly from VIN) to only a few percent. 3.
PAGE 27
LTC3728L/LTC3728LX applications information produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability without breaking the feedback loop. Placing a power MOSFET directly across the output capacitor and driving the gate with an appropriate signal generator is a practical way to produce a realistic load step condition.
PAGE 28
LTC3728L/LTC3728LX 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 from the INTVCC pin, generating 0.7V for 300kHz operation.
PAGE 29
LTC3728L/LTC3728LX 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 IC. 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
LTC3728L/LTC3728LX Applications Information RPU PGOOD SENSE1+ TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN INTVCC 3.3VOUT ITH2 PGND CVIN CINTVCC VIN 1µF CERAMIC CIN M3 BOOST2 D1 COUT1 GND COUT2 1µF CERAMIC BG2 M4 D2 CB2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RIN M2 VOUT1 + FCB EXTVCC LTC3728L/LTC3728LX ITH1 INTVCC SGND 3.3V BG1 M1 RSENSE + PLLIN + fIN R4 L1 CB1 R1 R3 PGOOD + R2 RUN/SS1 VPULL-UP (<7V) RSENSE VOUT2 L2 RUN/SS2 3728 F10 Figure 10.
PAGE 31
LTC3728L/LTC3728LX Applications Information SW1 L1 D1 RSENSE1 COUT1 VOUT1 + RL1 CERAMIC VIN RIN CIN + SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 L2 RSENSE2 COUT2 VOUT2 + RL2 CERAMIC 3728 F11 Figure 11.
PAGE 32
LTC3728L/LTC3728LX 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 as well. Check for proper performance over the operating voltage and current range expected in the application.
PAGE 33
LTC3728L/LTC3728LX Typical Applications 59k 1M 100k 0.1µF 1000pF 20k 1% 105k, 1% TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN 3.3VOUT ITH2 1000pF 20k 1% 63.4k 1% 1000pF PGND VOUT1 5V 3A; 4A PEAK 8 Q1 Q2 D1 CMDSH-3TR 10Ω 22µF 50V 5 LT1121 3 150µF, 6.3V PANASONIC SP 2 1 220k 100k + 0.1µF 1µF 10V ON/OFF VOUT3 12V 120mA 1µF 25V GND + SGND 3.3V 180pF 0.015Ω + 1000pF + M1 BG1 EXTVCC FCB LTC3728L/LTC3728LX INTVCC ITH1 33pF 15k SENSE1+ 0.
PAGE 34
LTC3728L/LTC3728LX typical applications VPULL-UP (<7V) 0.1µF 20k 1% 10k 105k 1% 0.01µF SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN SGND 3.3VOUT ITH2 1000pF 20k 1% 63.4k 1% 1000pF PGND 0.008Ω VOUT1 5V/4A Q2 M1 1µF 50V 10Ω CMDSH-3TR 22µF 50V 150µF, 6.3V 0.1µF 1µF 1µF 50V 4.7µF, 10V GND + 1500pF Q1 PIN 4 BG1 FCB EXTVCC LTC3728L/LTC3728LX ITH1 INTVCC 3.3V 180pF TG1 + fSYNC 100pF 4.75k SENSE1+ L1 4.3µH PGOOD 0.1µF 1000pF 100pF 8.
PAGE 35
LTC3728L/LTC3728LX Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .386 – .393* (9.804 – 9.982) .045 ± .005 28 27 26 25 24 23 22 21 20 19 18 17 1615 .254 MIN .033 (0.838) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 BSC 1 RECOMMENDED SOLDER PAD LAYOUT .015 ± .004 × 45° (0.38 ± 0.10) .0075 – .
PAGE 36
LTC3728L/LTC3728LX Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UH 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 R = 0.
PAGE 37
LTC3728L/LTC3728LX Revision History (Revision history begins at Rev F) REV DATE DESCRIPTION F 12/11 Added labels D1 and D2 to the Typical Application. PAGE NUMBER 1 Corrected pin name for the GN Package Pin 3 to SENSE1–. 2 Changed Note 3 to Note 8 on gmGBW1,2 on the Electrical Characteristics Table. 4 Added new Note 8: Guaranteed by design. 5 Updated threshold on BINH to 4.3V on the Functional Diagram. 11 Updated threshold on EXTVCC to 4.7V on the Functional Diagram.
PAGE 38
LTC3728L/LTC3728LX Typical Application IIN CIN I1 PHASMD OPEN TG1 U1 TG2 LTC3729 90° 0° 180° BUCK: 2.5V/15A 2.5VO/30A I2 90° I1 I2 I3 I3 U2 TG2 LTC3728L/ LTC3728LX 90° PLLIN IIN* BUCK: 2.5V/15A CLKOUT TG1 12VIN BUCK: 1.5V/15A 270° BUCK: 1.8V/15A 1.5VO/15A 1.8VO/15A I4 3728 F14 I4 *INPUT RIPPLE CURRENT CANCELLATION INCREASES THE RIPPLE FREQUENCY AND REDUCES THE RMS INPUT RIPPLE CURRENT THUS, SAVING INPUT CAPACITORS Figure 14.