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LTC3708

3708fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Fast 2-Phase, No R

SENSE

Buck Controller with

Output Tracking

Digital Signal Processors

Network Servers

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

No R

SENSE

is a trademark of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

High Efﬁ ciency Dual Output Step-Down Converter

Very Low Duty Factor Operation (t

ON(MIN)

< 85ns)

No R

SENSE

™ Option for Maximum Efﬁ ciency

Very Fast Transient Response

Programmable Output Voltage Up/Down Tracking

2-Phase Operation Reduces Input Capacitance

0.6V ±1% Output Voltage Reference

External Frequency Synchronization

Monotonic Soft-Start

Onboard High Current MOSFET Drivers

Wide V

Range: Up to 36V

Adjustable Cycle-by-Cycle Current Limit

Instant Output Overvoltage Protection

Optional Short-Circuit Shutdown Timer

Power Good Output with 100μs Masking

Available in 5mm × 5mm QFN Package

The LTC

3708 is a dual, 2-phase synchronous step-down

switching regulator with output voltage up/down tracking

capability. The IC allows either coincident or ratiometric

tracking. Multiple LTC3708s can be daisy-chained in ap-

plications requiring more than two voltages to be tracked.

Power supply sequencing is accomplished using an

external soft-start timing capacitor.

The LTC3708 uses a constant on-time, valley current mode

control architecture to deliver very low duty factors without

requiring a sense resistor. Operating frequency is selected

by an external resistor and is compensated for variations in

input supply voltage. An internal phase-locked loop allows

the IC to be synchronized to an external clock.

Fault protection is provided by an output overvoltage

comparator and an optional short-circuit shutdown timer.

The regulator current limit level is user programmable. A

wide supply range allows voltages as high as 36V to be

stepped down to 0.6V output.

1μF

100k

4.7μF

10Ω

TG1

BOOST1

SW1

SENSE1

BG1

SENSE1

–

PGND1

FB1

TRACK2

ON1

TH1

INTLPF

RUN/SS

DRV

LTC3708

PGOOD

TG2

BOOST2

SW2

SENSE2

0.22μF

B340A

OUT1

2.5V

15A

19.1k12.1k

10k 1k

33k6.04k 6.04k6.04k

1.5M

25k

100k

POSCAP

330μF

1.4μH

1.2μH

M3 M4

12.1k

SENSE2

–

PGND2

FB2

FCB

ON2

TH2

EXTLPF

TRACK1

SGND V

RNG1

RNG2

BG2

0.1μF

0.01μF 0.1μF

180pF

0.01μF

L1: PANASONIC ETQP3HIR4BF

L2: PANASONIC ETQP2HIR2BF

M1, M2: RENESAS HAT2168

M3, M4: RENESAS HAT2165

33k

180pF

3708 TA01

OUT2

1.8V

15A

POSCAP

470μF

2.5V

10μF

50V

3.3V TO 28V

2ms/DIV

3708 TA01b

OUT2

(0.5V/DIV)

OUT1

(0.5V/DIV)

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

100

0.01 1 10

3708 TA01c

4.5

7.5

9.0

3.0

1.5

6.0

0.1

20V

TO 2.5V

OUT

TO 2.5V

OUT

20V

TO 1.8V

OUT

TO 1.8V

OUT

Summary of content (32 pages)

PAGE 1
LTC3708 Fast 2-Phase, No RSENSE Buck Controller with Output Tracking DESCRIPTION FEATURES n n n n n n n n n n n n n n n The LTC®3708 is a dual, 2-phase synchronous step-down switching regulator with output voltage up/down tracking capability. The IC allows either coincident or ratiometric tracking. Multiple LTC3708s can be daisy-chained in applications requiring more than two voltages to be tracked. Power supply sequencing is accomplished using an external soft-start timing capacitor.
PAGE 2
LTC3708 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) Input Supply Voltage (VCC, DRVCC) ............. 7V to –0.3V Boosted Topside Driver Supply Voltage BOOST1, 2 ............................................ 42V to –0.3V Switch Voltage (SW1, 2) .............................. 36V to –5V SENSE1+, SENSE2+ Voltages ....................... 36V to –5V SENSE1–, SENSE2– Voltages .................... 10V to –0.3V ION1, ION2 Voltages .................................... 21V to –0.3V (BOOST – SW) Voltages ....
PAGE 3
LTC3708 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VCC = 5V, DRVCC = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS –0.05 –0.2 % 1.2 1.45 1.7 mS 94 186 116 233 138 280 ns ns 50 85 ns 270 350 ns 160 110 220 mV mV mV ΔVFB(LOADREG)1,2 Feedback Voltage Load Regulation ITH = 0.5V to 1.
PAGE 4
LTC3708 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VCC = 5V, DRVCC = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IINTLPF Internal Phase Detector Output Current Sourcing Capability Sinking Capability fSW1 < fSW2, VINTLPF = 0V fSW1 > fSW2, VINTLPF = 2.4 tON1 Modulation Range by External PLL Up Modulation Down Modulation ION1 = 60μA, VEXTLPF = 1.8V ION1 = 60μA, VEXTLPF = 0.
PAGE 5
LTC3708 TYPICAL PERFORMANCE CHARACTERISTICS Soft-Start Power Loss vs Input Voltage 6 VOUT1 2V/DIV VOUT2 2V/DIV POWER LOSS (W) 5 RUN/SS 5V/DIV IL1 5A/DIV VOUT = 2.5V 4 3 VOUT = 1.8V 2 1 3708 G05 50ms/DIV IOUT = 15A 0 5 10 15 20 INPUT VOLTAGE (V) 25 3708 G06 Power Loss vs Load Current VIN = 5V 3.0 VOUT = 2.5V Frequency vs Load Current 250 IOUT = 15A 240 2.5 FREQUENCY (kHz) POWER LOSS (W) Frequency vs Input Voltage 260 VOUT = 1.8V 2.0 1.5 1.0 220 200 FREQUENCY (kHz) 3.
PAGE 6
LTC3708 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Current Sense Threshold vs Temperature 300 250 200 150 100 50 0 0.5 1.5 1.25 1 VRNG VOLTAGE (V) 0.75 1.75 2 160 Load Regulation (Figure 13 Circuit) 0 VRNG = 1V –0.1 FORCED CONTINUOUS MODE 150 –0.2 $VOUT (%) 350 MAXIMUM CURRENT SENSE THRESHOLD (mV) MAXIMUM CURRENT SENSE THRESHOLD (mV) Maximum Current Sense Threshold vs VRNG Voltage 140 130 –0.3 –0.4 DISCONTINUOUS MODE –0.5 120 –0.6 110 –50 –25 –0.
PAGE 7
LTC3708 TYPICAL PERFORMANCE CHARACTERISTICS On-Time vs EXTLPF Voltage 500 450 450 400 400 ION1 = 30μA 350 300 250 200 ION1 = 60μA 150 VSW1 10V/DIV 250 200 100 50 0 1.2 1.1 1.3 EXTLPF VOLTAGE (V) 1.4 VSW2 10V/DIV ION2 = 60μA 150 50 1 IIN 2A/DIV VIN 200mV/DIV 300 100 0 2-Phase Operation ION2 = 30μA 350 tON2 (ns) tON1 (ns) On-Time vs INTLPF Voltage 500 0.6 0.8 1.2 1.4 1 1.6 INTLPF VOLTAGE (V) 1.8 2.0 VIN = 15V 1μs/DIV VOUT1 = 5V VOUT2 = 3.
PAGE 8
LTC3708 PIN FUNCTIONS RUN/SS (Pin 1): Run Control and Soft-Start Input. A capacitor to ground at this pin sets the ramp rate of the output voltage (approximately 0.5s/μF) and the time delay for overcurrent latchoff (see Applications Information). Forcing this pin below 0.8V shuts down the LTC3708. ITH1, ITH2 (Pins 2, 8): Error Ampliﬁer Compensation Point and Current Control Threshold. The current comparator threshold increases with this control voltage. The voltage ranges from 0V to 2.4V with 0.
PAGE 9
LTC3708 FUNCTIONAL DIAGRAM RIPLL CIPLL INTLPF FCB ION CLOCK DETECTOR RON FROM CHANNEL 2 TG ENABLE PHASE DETECTOR (PD2) EXTLPF 0.6V REF PHASE DETECTOR (PD1) VIN + VCC CIN CVCC REPLL CEPLL BOOST TO CHANNEL 2 OST OST tON = 0.7 (10pF) IION TG FCNT R M1 ON Q SW S 20k + SENSE+ SWITCH LOGIC + ICMP L1 VOUT + DB COUT DRVCC IREV – CB – SHDN BG OV CDRVCC M2 1.4V PGND VRNG SENSE– s 0.7V – OV 3.3μA – EA VFB R2 + 1 240k 0.66V SGND Q4 R1 + – ITH UV CC 0.
PAGE 10
LTC3708 OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3708 uses a constant on-time, current mode stepdown architecture with two control channels operating at 180 degrees out of phase. In normal operation, each top MOSFET is turned on for a ﬁxed interval determined by its own 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 repeating the cycle.
PAGE 11
LTC3708 OPERATION (Refer to Functional Diagram) DRVCC Power for the top and bottom MOSFET drivers is derived from the DRVCC pin. The top MOSFET driver is powered from a ﬂoating bootstrap capacitor, CB. This capacitor is normally recharged from DRVCC through an external Schottky diode, DB, when the top MOSFET is turned off.
PAGE 12
LTC3708 (Refer to Functional Diagram) While this is an impressive reduction in itself, remember that the power losses are proportional to I2RMS, meaning that the actual power wasted is reduced by a factor of 2.66. The reduced input ripple current also means that less power is lost in the input power path, which could include batteries, switches, trace/connector resistances and protection circuitry.
PAGE 13
LTC3708 APPLICATIONS INFORMATION The basic LTC3708 application circuit is shown on the ﬁrst page of this data sheet. External component selection is primarily determined by the maximum load current and begins with the selection of the power MOSFET switches and/or sense resistor. For the LTC3708, the inductor current is determined by the RDS(ON) of the synchronous MOSFET or by a sense resistor when the user opts for more accurate current sensing.
PAGE 14
LTC3708 APPLICATIONS INFORMATION Operating Frequency The choice of operating frequency is a trade-off between efﬁciency and component size. Low frequency operation improves efﬁciency by reducing MOSFET switching and driving losses but requires larger inductance and/or capacitance to maintain low output ripple voltage. 1.5 1.0 0.
PAGE 15
LTC3708 APPLICATIONS INFORMATION PLL and Frequency Synchronization In the LTC3708, there are two onboard phase-locked loops (PLL). One PLL is used to achieve frequency locking and 180° phase shift between the two channels while the second PLL locks onto the rising edge of an external clock. Since the LTC3708 uses a constant on-time architecture, the error signal generated by the phase detector of the PLL is used to vary the on time to achieve frequency locking and phase separation.
PAGE 16
LTC3708 APPLICATIONS INFORMATION highest (VOUT)(IOUT) product needs to be used in the formula below to determine the maximum RMS current requirement. Increasing the output current, drawn from the other out-of-phase controller, will actually decrease the input RMS ripple current from this maximum value (see Figure 2). The type of input capacitor, value and ESR rating have efﬁciency effects that need to be considered in the selection process.
PAGE 17
LTC3708 APPLICATIONS INFORMATION Manufacturers such as Nichicon, United Chemi-Con and Sanyo can be considered for high performance throughhole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest (ESR)(size) product of any aluminum electrolytic at a somewhat higher price. An additional ceramic capacitor in parallel with OS-CON capacitors is recommended to reduce the inductance effects.
PAGE 18
LTC3708 APPLICATIONS INFORMATION Caution should be used when setting the current limit based upon the RDS(ON) of the MOSFETs. The maximum current limit is determined by the minimum MOSFET on-resistance. Data sheets typically specify nominal and maximum values for RDS(ON), but not a minimum. A reasonable assumption is that the minimum RDS(ON) lies the same amount below the typical value as the maximum lies above it. Consult the MOSFET manufacturer for further guidelines.
PAGE 19
LTC3708 APPLICATIONS INFORMATION VIN 3.3V OR 5V RUN/SS D1 $V = 0.6V RUN/SS RSS* ON ON THRESHOLD CSS TIME 3708 F07 VOUT1 *OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated TIME Output Voltage Tracking 3708 F06 Figure 6. Monotonic Soft-Start Waveforms Controlled soft-start requires that the timing capacitor, CSS, be made large enough to guarantee that the output can track the voltage rise on the RUN/SS pin.
PAGE 20
LTC3708 APPLICATIONS INFORMATION VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 VOUT2 3708 F08 TIME TIME (8a) Coincident Tracking (8b) Ratiometric Tracking Figure 8. Two Different Modes of Output Voltage Tracking VOUT1 VOUT2 R3 R1 TO VFB1 PIN TO TRACK2 PIN R4 VOUT1 R3 VOUT2 R1 TO TRACK2 PIN TO VFB2 PIN R2 R4 R3 TO VFB1 PIN R2 TO VFB2 PIN R4 3708 F09 (9a) Coincident Tracking Setup (9b) Ratiometric Tracking Setup Figure 9.
PAGE 21
LTC3708 APPLICATIONS INFORMATION connected to a soft-start capacitor. All other LTC3708s should have their RUN/SS pins pulled up to VCC with a resistor between 50k and 300k. Figure 12 shows the circuit with four outputs. Three of them are programmed in the coincident mode while the fourth one tracks ratiometrically. If output tracking is not needed, connect the TRACK pins to VCC. Do Not Float These Pins.
PAGE 22
LTC3708 APPLICATIONS INFORMATION 2. Transition Loss. This loss arises from the brief amount of time the top MOSFET spends in the saturated region during switch node transitions. It depends upon the input voltage, load current, driver strength and MOSFET capacitance, among other factors. The loss is signiﬁcant at input voltages above 20V and can be estimated from: Transition Loss ≈ (0.5) • VIN2 • IOUT • CRSS • f • ⎛ 1 1 ⎞ RDS(ON)_ DRV ⎜ + ⎟ ⎝ DRVCC − VGS(TH) VGS(TH) ⎠ 3. DRVCC and VCC Current.
PAGE 23
LTC3708 APPLICATIONS INFORMATION Third, design the inductors for about 40% ripple current at the maximum VIN: L1 = 2.5V ⎛ 2.5V ⎞ ⎜ 1– ⎟ = 1.1μH (500kHz)(0.4)(10A) ⎝ 28V ⎠ A standard 1μH inductor will result in 45% of ripple current (4.5A) at worst case. L2 = 1.8 V ⎛ 1.8 V ⎞ ⎜ 1– ⎟ = 0.8 μH (500kHz)(0.4)(10A) ⎝ 28V ⎠ L2 can also use 1μH to save some BOM (Bill of Material) cost; the resulting ripple current is 3.4A.
PAGE 24
LTC3708 APPLICATIONS INFORMATION VIN 7V TO 28V CIN 10μF 35V s4 BAT54A 5V + 10Ω 4.7μF 1μF BOOST2 VCC PGND1 17 4 31 21 VCC TRACK1 FCB DRVCC 27 TG1 TG2 28 BOOST2 BOOST1 0.22μF 26 SW2 SW1 25 SENSE2+ SENSE1+ 22 BG1 LTC3708EUH BG2 24 SENSE2– SENSE1– 23 PGND1 PGND2 32 VRNG2 VRNG1 VRNG2 29 ION1 ION2 3 VFB2 VFB1 6 TRACK2 PWRGD 9 EXTLPF INTLPF 0.01μF 2 ITH1 ITH2 1μF 1μF VOUT1 2.5V 10A 22μF 6.3V X7R L1 1μH + COUT1 470μF 4V B340A M1 M3 PGND1 20k 1% 56pF 10k 1% 31.
PAGE 25
LTC3708 APPLICATIONS INFORMATION RON1 FCB TG1 R1 PGOOD VFB1 R3 SW1 ITH1 BOOST ION1 CB1 COUT1 SENSE1+ EXTLPF D1 L1 SENSE1– TRACK1 PGND1 VRNG1 M3 CDR1 CIN1 VIN CIN2 CVCC LTC3708 DRVCC 5V SGND CDR2 BG2 M2 5V VCC BG1 M1 M4 PGND2 CSS RUN/SS VRNG2 SENSE2– TRACK2 L2 D2 SENSE2+ COUT2 INTLPF BOOST2 ION2 SW2 ITH2 TG2 VFB2 CB2 R4 R2 RON2 3708 F15 Figure 14. LTC3708 Layout Diagram • Connect the decoupling capacitors CDR1 and CDR2 close to the DRVCC and PGND pins.
PAGE 26
LTC3708 APPLICATIONS INFORMATION SW1 L1 D1 VOUT1 COUT1 + RL1 CERAMIC VIN RIN CIN + SW2 L2 D2 BOLD LINES INDICATE HIGH, SWITCHING CURRENT LINES. KEEP LINES TO A MINIMUM LENGTH. VOUT2 COUT2 + RL2 CERAMIC 3708 F15 Figure 15. Branch Current Waveforms D G D S D S D S RSENSE MOSFET SENSE+ SENSE– SENSE+ SENSE– (16a) Sensing the Bottom MOSFET 3708 F16 (16b) Sensing a Resistor Figure 16.
PAGE 27
LTC3708 APPLICATIONS INFORMATION 5V 10Ω 1μF 4.7μF DB1 B340LA M3 19.1k 1% VIN 6.04k 1% CB1 0.1μF 6.04k 1% 15k 470pF 1000pF 3.32k 470pF LTC3708 150pF CSS 0.1μF L2 1.22μH + B340LA M4 BG2 SENSE2– PGND2 VFB1 VFB2 TRACK2 FCB ION1 ION2 ITH1 ITH2 INTLPF EXTLPF RUN/SS TRACK1 VRNG2 SGND VRNG1 0.01μF M2 CB2 0.1μF SW2 SENSE2+ SW1 SENSE1+ SENSE1– PGND1 RON1 1.5M DB2 VCC DRVCC PGOOD TG1 TG2 BOOST2 BOOST1 BG1 56pF 100k + VOUT1 2.5V 15A COUT1 + 330μF 4V s2 12.1k 1% M1 L1 1.
PAGE 28
+ VIN RUN/SS SGND GND 1.8V 6A GND 9V TO 20V 10k 1% 30.1k 1% COUT1 470μF 2.5V 100pF 6.04k 1% 100pF B240A L1 2.2μH C1 10μF 25V 12.1k 1% C2 10μF 25V 10k 59k VIN VCC Si4860DY Si4860DY 1.2M 0Ω 0Ω BOOST1 1nF 15k 150pF 100pF 0.22μF 0Ω BOOST2 BAT54A 1nF * 1μF VCC 10Ω VCC 5V 1M VIN SW2 SENSE2+ * VFB2 PWRGD INTLPF ITH2 SGND ION2 SENSE2– BG2 PGND2 VRNG2 0.
PAGE 29
LTC3708 TYPICAL APPLICATIONS Dual-Phase, 30A Power Supply with 10mV Output Ripple VIN 5V + CIN1 100μF 6.3V CIN2-CIN7 4.7μF 6.3V s6 TRACK BAT54A 1μF PGND1 10Ω 1μF BOOST2 VCC 1μF VOUT 1V 30A COUT1 1μF 6.3V M1 L1 0.19μH + 0.22μF PGND2 VCC TRACK1 FCB DRVCC TG1 TG2 BOOST2 BOOST1 SW1 B340A M3 100k R1 VCC 10k 0.1% 274k VIN LTC3708 BG2 PGND1 PGND2 VRNG1 VRNG2 RUN/SS 0.
PAGE 30
LTC3708 TYPICAL APPLICATIONS 12V/12A and 5V/12A at 300kHz Application VIN 20V TO 28V + 100μF 50V C1 3.3μF 50V X5R C2 3.3μF 50V X5R 5V BAT54A BOOST1 BOOST2 10Ω 5V VCC C3 3.3μF 50V X5R C4 3.3μF 50V X5R C6 1μF V TRACK1 FCB DRV CC CC Q1 HAT2167H 0Ω TG2 TG1 BOOST1 VOUT1 12V 12A L1 3.5μH 0.1μF COUT1 150μF 16V s2 B340LA 22pF 75k 10k 191k 10k 22pF Q2 HAT2167H 0Ω 100pF 24.9k SENSE1+ SENSE2+ 100pF BG2 PGND2 VRNG2 ION1 1nF 1nF 88.7k + 75k SGND 5V 47pF 100k 5.11k 24.
PAGE 31
LTC3708 PACKAGE DESCRIPTION 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 3.45 ± 0.05 3.50 REF (4 SIDES) 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 s 45° CHAMFER R = 0.115 TYP 31 32 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 3.
PAGE 32
LTC3708 TYPICAL APPLICATION Area = 650mm2, Height = 3mm VIN 7V TO 24V 5V BAT54A 2.2μF 6.3V BOOST1 BOOST2 5V 10Ω 1μF VCC C9 10μF 25V 1μF Q1A Si4816BDY VCC TRACK1 FCB DRVCC TG1 C16 100μF 6.3V L1 1.8μH 0.1μF BOOST2 Q1B Si4816BDY 20k 31.6k 100k SENSE2+ SENSE1– VIN VCC SENSE2– PGND1 VRNG1 PGND2 VRNG2 ION1 10k 10k 150pF 220pF VIN VCC 1000pF 20k 220pF 560pF 100k 5.11k 20k 10k 150pF 470pF 560pF 0.