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LTC3707

3707fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

High Efﬁ ciency, 2-Phase

Synchronous Step-Down

Switching Regulator

The LTC

3707 is a high performance dual step-down

switching regulator controller that drives N-channel

synchronous power MOSFET stages. A constant frequency

current mode architecture allows adjustment of the

frequency up to 300kHz. 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 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 operation, constant

frequency mode and continuous inductor current mode

or regulate a secondary winding.

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

■

180° Phased Dual Controllers Reduce Required

Input Capacitance and Power Supply Induced Noise

■

OPTI-LOOP

Compensation Minimizes C

OUT

■

±1.5% Output Voltage Accuracy over Temperature

■

Dual N-Channel MOSFET Synchronous Drive

■

Power Good Output Voltage Monitor

■

DC Programmed Fixed Frequency 150kHz to 300kHz

■

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

■

Remote Output Voltage Sense

■

Low Shutdown I

: 20µA

■

5V and 3.3V Standby Regulators

■

Selectable Constant Frequency, Burst Mode

Operation or PWM Operation

■

Small 28-Lead Narrow SSOP Package

■

Notebook and Palmtop Computers, PDAs

■

Battery Chargers

■

Portable Instruments

■

Battery-Operated Digital Devices

■

DC Power Distribution Systems

L, LT, LTC, LTM, 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, 5705919, 5929620, 6144194, 6177787, 6304066, 6580258.

4.7µF

, 0.1µF

105k

1000pF

6.3µH

220pF

1µF

CERAMIC

22µF

50V

CERAMIC

OUT1

47µF

SENSE1

0.01

20k

15k

OUT1

, 0.1µF

63.4k

6.3µH

220pF

1000pF

OUT

56µF

SENSE2

0.01

20k

15k

OUT2

3.3V

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND PGND

SENSE1

SENSE2

SENSE1

–

SENSE2

–

OSENSE1

OSENSE2

TH1

TH2

INTV

RUN/SS1 RUN/SS2

5.2V TO 28V

M1, M2, M3, M4: FDS6680A

3707 F01

SS1

0.1µF

SS2

0.1µF

LTC3707

Summary of content (32 pages)

PAGE 1
LTC3707 High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®3707 is a high performance dual step-down switching regulator controller that drives N-channel synchronous power MOSFET stages. A constant frequency current mode architecture allows adjustment of the frequency up to 300kHz. Power loss and noise due to the ESR of the input capacitors are minimized by operating the two controller output stages out of phase.
PAGE 2
LTC3707 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Input Supply Voltage (VIN) .........................30V to – 0.3V Top Side 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.
PAGE 3
LTC3707 which apply over the full operating ELECTRICAL CHARACTERISTICS The l denotes the speciﬁ=cations temperature range, otherwise speciﬁcations are at T = 25°C. V = 15V, V 5V unless otherwise noted. A IN RUN/SS1, 2 SYMBOL PARAMETER CONDITIONS gmGBW1, 2 Transconductance Ampliﬁer GBW ITH1, 2 = 1.2V; (Note 4) IQ Input DC Supply Current Normal Mode Standby Shutdown (Note 5) EXTVCC Tied to VOUT1 = 5V VRUN/SS1, 2 = 0V, VSTBYMD > 2V VRUN.
PAGE 4
LTC3707 which apply over the full operating ELECTRICAL CHARACTERISTICS The l denotes the speciﬁ=cations 5V unless otherwise noted. temperature range, otherwise speciﬁcations are at T = 25°C. V = 15V, V A IN RUN/SS1, 2 SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS fHIGH Highest Frequency VFREQSET = 2.4V 280 310 360 kHz IFREQSET FREQSET Input Current VFREQSET = 0V –2 –1 μA 3.35 3.45 V 3.3V Linear Regulator l V3.3OUT 3.3V Regulator Output Voltage No Load 3.20 V3.3IL 3.
PAGE 5
LTC3707 TYPICAL PERFORMANCE CHARACTERISTICS Supply Current vs Input Voltage and Mode (Figure 13) 1000 200 600 BOTH CONTROLLERS ON 400 200 5.05 INTVCC AND EXTVCC SWITCH VOLTAGE (V) EXTVCC VOLTAGE DROP (mV) 800 SUPPLY CURRENT (μA) INTVCC and EXTVCC Switch Voltage vs Temperature EXTVCC Voltage Drop 150 100 50 STANDBY SHUTDOWN 0 0 5 20 15 10 INPUT VOLTAGE (V) 25 0 30 10 0 20 CURRENT (mA) 30 4.95 4.90 4.85 4.80 4.70 –50 –25 40 100 125 3707 G06 75 80 ILOAD = 1mA 70 5.0 60 4.8 4.
PAGE 6
LTC3707 TYPICAL PERFORMANCE CHARACTERISTICS Load Regulation VITH VS VRUN/SS 2.5 FCB = 0V VIN = 15V FIGURE 1 SENSE Pins Total Source Current 100 VOSENSE = 0.7V 2.0 –0.2 50 ISENSE (μA) –0.1 VITH (V) NORMALIZED VOUT (%) 0.0 1.5 1.0 –0.3 0 –50 0.5 –0.
PAGE 7
LTC3707 TYPICAL PERFORMANCE CHARACTERISTICS Input Source/Capacitor Instantaneous Current (Figure 13) IIN 2A/DIV VIN 200mV/DIV Constant Frequency (Burst Inhibit) Operation (Figure 13) Burst Mode Operation (Figure 13) VOUT 20mV/DIV VOUT 20mV/DIV IOUT 0.5A/DIV IOUT 0.5A/DIV VSW1 10V/DIV VSW2 10V/DIV 3707 G22 1μs/DIV VIN = 15V VOUT = 5V IOUT5 = IOUT3.
PAGE 8
LTC3707 PIN FUNCTIONS RUN/SS1, RUN/SS2 (Pins 1, 15): Combination of soft-start, run control inputs and short-circuit detection timers. A capacitor to ground at each of these pins sets the ramp time to full output current. Forcing either of these pins back below 1.0V causes the IC to shut down the circuitry required for that particular controller. Latchoff overcurrent protection is also invoked via this pin as described in the Applications Information section. 3.
PAGE 9
LTC3707 FUNCTIONAL DIAGRAM VIN INTVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL 1.19V BOOST 1M DROP OUT DET CLK1 OSCILLATOR CLK2 PGOOD – 0.86V S Q R Q TG TOP BOT SWITCH LOGIC INTVCC BOT VFB 4.5V – SHDN + I1 + – FCB – ++ + VREF SLOPE COMP – – INTVCC I2 – + 30k SENSE + 45k 45k 2.4V – EA + EXTVCC 5V + – OV 5V LDO REG + SGND INTERNAL SUPPLY 6V VFB VOSENSE 0.80V R1 0.86V ITH SHDN RST 4(VFB) R2 + – 1.2μA INTVCC CSEC – 30k SENSE VIN 4.8V DSEC 3mV 0.
PAGE 10
LTC3707 OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3707 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 11
LTC3707 OPERATION (Refer to Functional Diagram) be forced back into the main power supply potentially boosting the input supply to dangerous voltage levels— BEWARE! Frequency Setting The FREQSET pin provides frequency adjustment of the internal oscillator from approximately 140kHz to 310kHz. This input is nominally biased through an internal resistor to the 1.19V reference, setting the oscillator frequency to approximately 220kHz.
PAGE 12
LTC3707 OPERATION (Refer to Functional Diagram) THEORY AND BENEFITS OF 2-PHASE OPERATION The LTC1628 and the LTC3707 are the ﬁrst dual high efﬁciency DC/DC controllers to bring the considerable beneﬁts of 2-phase operation to portable applications. Notebook computers, PDAs, handheld terminals and automotive electronics will all beneﬁt from the lower input ﬁltering requirement, reduced electromagnetic interference (EMI) and increased efﬁciency associated with 2-phase operation.
PAGE 13
LTC3707 (Refer to Functional Diagram) 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. A good rule of thumb for most applications is that 2-phase operation will reduce the input capacitor requirement to that for just one channel operating at maximum current and 50% duty cycle.
PAGE 14
LTC3707 APPLICATIONS INFORMATION Selection of Operating Frequency The LTC3707 uses a constant frequency architecture with the frequency determined by an internal oscillator capacitor. This internal capacitor is charged by a ﬁxed current plus an additional current that is proportional to the voltage applied to the FREQSET pin. A graph for the voltage applied to the FREQSET pin vs frequency is given in Figure 5.
PAGE 15
LTC3707 APPLICATIONS INFORMATION However, designs for surface mount are available that do not increase the height signiﬁcantly. Power MOSFET and D1 Selection Two external power MOSFETs must be selected for each controller with the LTC3707: One N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the INTVCC voltage. This voltage is typically 5V during start-up (see EXTVCC Pin Connection).
PAGE 16
LTC3707 APPLICATIONS INFORMATION 30% to 70% when compared to a single phase power supply solution. The type of input capacitor, value and ESR rating have efﬁciency effects that need to be considered in the selection process. The capacitance value chosen should be sufﬁcient to store adequate charge to keep high peak battery currents down. 20μF to 40μF is usually sufﬁcient for a 25W output supply operating at 200kHz.
PAGE 17
LTC3707 APPLICATIONS INFORMATION that the output capacitance does not signiﬁcantly discharge during the operating frequency period due to ripple current. The choice of using smaller output capacitance increases the ripple voltage due to the discharging term but can be compensated for by using capacitors of very low ESR to maintain the ripple voltage at or below 50mV.
PAGE 18
LTC3707 APPLICATIONS INFORMATION EXTVCC Connection The LTC3707 contains an internal P-channel MOSFET switch connected between the EXTVCC and INTVCC pins. When the voltage applied to EXTVCC rises above 4.7V, the internal regulator is turned off and the switch closes, connecting the EXTVCC pin to the INTVCC pin thereby supplying internal power. The switch remains closed as long as the voltage applied to EXTVCC remains above 4.5V.
PAGE 19
LTC3707 APPLICATIONS INFORMATION Topside MOSFET Driver Supply (CB, DB) External bootstrap capacitors CB connected to the BOOST pins supply the gate drive voltages for the topside MOSFETs. Capacitor CB in the functional diagram is charged though external diode DB from INTVCC when the SW pin is low. When one of the topside MOSFETs is to be turned on, the driver places the CB voltage across the gate-source of the desired MOSFET. This enhances the MOSFET and turns on the topside switch.
PAGE 20
LTC3707 APPLICATIONS INFORMATION By pulling both RUN/SS pins below 1V and/or pulling the STBYMD pin below 0.2V, the LTC3707 is put into low current shutdown (IQ = 20μA). The RUN/SS pins can be driven directly from logic as shown in Figure 7. Diode D1 in Figure 7 reduces the start delay but allows CSS to ramp up slowly providing the soft-start function. Each RUN/SS pin has an internal 6V zener clamp (See Functional Diagram). VIN 3.
PAGE 21
LTC3707 APPLICATIONS INFORMATION but less than in normal operation.
PAGE 22
LTC3707 APPLICATIONS INFORMATION FCB Pin Operation The FCB pin can be used to regulate a secondary winding or as a logic level input. Continuous operation is forced when the FCB pin drops below 0.8V. During continuous mode, current ﬂows continuously in the transformer primary. The secondary winding(s) draw current only when the bottom, synchronous switch is on.
PAGE 23
LTC3707 APPLICATIONS INFORMATION Efﬁciency Considerations The percent efﬁciency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efﬁciency and which change would produce the most improvement. Percent efﬁciency can be expressed as: %Efﬁciency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power.
PAGE 24
LTC3707 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ΔILOAD (ESR), where ESR is the effective series resistance of COUT.
PAGE 25
LTC3707 APPLICATIONS INFORMATION Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile, there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main power line in an automobile is the source of a number of nasty potential transients, including load-dump, reverse-battery, and double-battery.
PAGE 26
LTC3707 APPLICATIONS INFORMATION Design Example As a design example for one channel, assume VIN = 12V(nominal), VIN = 22V(max), VOUT = 2V, 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 FREQSET pin to the INTVCC pin for 300kHz operation. The minimum inductance for 30% ripple current is: ⎛ V ⎞ V ΔIL = OUT ⎜ 1– OUT ⎟ (f)(L) ⎝ VIN ⎠ A 4.
PAGE 27
LTC3707 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 LTC3707. These items are also illustrated graphically in the layout diagram of Figure 10. The 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 28
LTC3707 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 3707 F11 Figure 11. Branch Current Waveforms 4. Are the SENSE – and SENSE + leads routed together with minimum PC trace spacing? The ﬁlter capacitor between SENSE+ and SENSE– should be as close as possible to the IC.
PAGE 29
LTC3707 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 30
LTC3707 TYPICAL APPLICATION 59k 1M 100k 0.1μF 2 180pF 1000pF 3 105k, 1% 4 20k 1% 5 INTVCC 6 33pF 7 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 FREQSET VIN STBYMD BG1 FCB EXTVCC LTC3707 ITH1 SGND 3.3VOUT ITH2 INTVCC PGND BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 + T1, 1:1.8 10μH 33μF 25V 0.015Ω 27 VOUT1 5V 3A; 4A PEAK 26 8 0.
PAGE 31
LTC3707 PACKAGE DESCRIPTION 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 – .0098 (0.19 – 0.25) 2 3 4 5 6 7 8 9 10 11 12 13 14 .0532 – .0688 (1.35 – 1.75) .004 – .0098 (0.102 – 0.
PAGE 32
LTC3707 TYPICAL APPLICATION 0.1μF 27pF 2 1000pF 105k 1% 3 4 20k 1% INTVCC 5 6 0.01μF 7 8 220pF 9 3.3V 10 11 15k 220pF 12 20k 1% 27pF 13 63.4k 1% 1000pF 14 SENSE1+ TG1 SENSE1– SW1 VOSENSE1 BOOST1 FREQSET VIN STBYMD BG1 FCB LTC3707 ITH1 SGND EXTVCC INTVCC PGND 3.3VOUT ITH2 BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 L1 8μH PGOOD 27 0.015Ω VOUT1 5V 3A; 4A PEAK 26 0.1μF 25 M1A M1B 24 23 47μF 6.3V 10Ω 22 22μF 50V CMDSH-3TR 0.