Datasheet

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LTC1929/LTC1929-PG

2-Phase, High Efficiency,

Synchronous Step-Down

Switching Regulators

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

OPTI-LOOP is a trademark of Linear Technology Corporation.

The LTC

1929/LTC1929-PG are 2-phase, single output,

synchronous step-down current mode switching regula-

tor controllers that drive N-channel external power MOSFET

stages in a phase-lockable fixed frequency architecture.

The 2-phase controllers drive their two output stages out

of phase at frequencies up to 300kHz to minimize the RMS

ripple currents in both input and output capacitors. The

2-phase technique effectively multiplies the fundamental

frequency by two, improving transient response while

operating each channel at an optimum frequency for

efficiency. Thermal design is also simplified.

An internal differential amplifier provides true remote

sensing of the regulated supply’s positive and negative

output terminals as required by high current applications.

The RUN/SS pin provides soft-start and a defeatable,

timed, latched short-circuit shutdown to shut down both

channels. Internal foldback current limit provides protec-

tion for the external synchronous MOSFETs in the event of

an output fault. OPTI-LOOP compensation allows the

transient response to be optimized over a wide range of

output capacitance and ESR values.

Figure 1. High Current 2-Phase Step-Down Converter

■

Desktop Computers

■

Internet/Network Servers

■

Large Memory Arrays

■

DC Power Distribution Systems

■

2-Phase Single Output Controller

■

Reduces Required Input Capacitance and Power

Supply Induced Noise

■

Current Mode Control Ensures Current Sharing

■

Phase-Lockable Fixed Frequency: 150kHz to 300kHz

■

True Remote Sensing Differential Amplifier

■

OPTI-LOOP

Compensation Improves Transient

Response

■

1% Output Voltage Accuracy

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Power Good Output Voltage Monitor (LTC1929-PG)

■

Wide V

Range: 4V to 36V Operation

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Very Low Dropout Operation: 99% Duty Cycle

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Adjustable Soft-Start Current Ramping

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Internal Current Foldback

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Short-Circuit Shutdown Timer with Defeat Option

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Overvoltage Soft-Latch Eliminates Nuisance Trips

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Available in 28-Lead SSOP Package

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

1929 F01

TG1

BOOST1

SW1

BG1

PGND

SENSE1

–

TG2

BOOST2

SW2

BG2

INTV

SENSE2

–

RUN/SS

EAIN

DIFFOUT

–

LTC1929

SGND

0.1µF

16k

1000pF

10Ω

10k

16k

10µF

35V

CERAMIC

×4

OUT

1000µF

×2

OUT

1.6V/40A

1µH

0.002Ω

5V TO 28V

1µH

0.47µF

100pF

10µF

0.002Ω

OUT

: T510E108K004AS L1, L2: CEPH149-1ROMC

Summary of content (28 pages)

PAGE 1
LTC1929/LTC1929-PG 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulators U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®1929/LTC1929-PG are 2-phase, single output, synchronous step-down current mode switching regulator controllers that drive N-channel external power MOSFET stages in a phase-lockable fixed frequency architecture.
PAGE 2
LTC1929/LTC1929-PG U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) ORDER PART NUMBER TOP VIEW Input Supply Voltage (VIN).........................36V to – 0.3V Topside Driver Voltages (BOOST1,2) .........42V to – 0.3V Switch Voltage (SW1, 2) .............................36V to – 5 V SENSE1+, SENSE2 +, SENSE1–, SENSE2 – Voltages ........................ (1.1)INTVCC to – 0.3V EAIN, VOS+, VOS–, EXTVCC, INTVCC, RUN/SS, AMPMD Voltages ..........................7V to – 0.
PAGE 3
LTC1929/LTC1929-PG ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VRUN/SS = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS ISCL RUN/SS Discharge Current Soft Short Condition VEAIN = 0.5V; VRUN/SS = 4.5V 0.5 2.0 4.0 µA ISDLHO Shutdown Latch Disable Current VEAIN = 0.5V 1.
PAGE 4
LTC1929/LTC1929-PG ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VRUN/SS = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN IB Input Bias Current Op Amp Mode (LTC1929 Only) AOL Open Loop DC Gain Op Amp Mode; 0.
PAGE 5
LTC1929/LTC1929-PG U W TYPICAL PERFOR A CE CHARACTERISTICS Supply Current vs Input Voltage and Mode EXTVCC VOLTAGE DROP (mV) 800 SUPPLY CURRENT (µA) 5.05 250 VOUT = 5V VEXTVCC = VOUT 600 ON 400 200 INTVCC AND EXTVCC SWITCH VOLTAGE (V) 1000 INTVCC and EXTVCC Switch Voltage vs Temperature EXTVCC Voltage Drop 200 LTC1929 150 LTC1929-PG 100 50 SHUTDOWN 0 0 5 20 15 10 25 INPUT VOLTAGE (V) 30 0 35 0 10 30 20 CURRENT (mA) 40 1929 G04 4.85 4.80 EXTVCC SWITCHOVER THRESHOLD 4.
PAGE 6
LTC1929/LTC1929-PG U W TYPICAL PERFOR A CE CHARACTERISTICS Load Regulation VITH vs VRUN/SS 2.5 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 –0.4 0 0 1 3 2 LOAD CURRENT (A) 4 5 0 1 2 3 5 4 –100 2 0 VRUN/SS (V) 4 1629 G14 Maximum Current Sense Threshold vs Temperature 1629 G15 RUN/SS Current vs Temperature 1.8 VSENSE– = 5V 1.
PAGE 7
LTC1929/LTC1929-PG U W TYPICAL PERFOR A CE CHARACTERISTICS Current SENSE Pin Input Current vs Temperature EXTVCC Switch Resistance vs Temperature 10 35 350 VOUT = 5V 33 31 29 27 50 25 0 75 TEMPERATURE (°C) 100 125 8 6 4 VFREQSET = OPEN 200 VFREQSET = 0V 150 100 50 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 – 50 – 25 50 25 75 0 TEMPERATURE (°C) 1929 G21 1929 G20 Undervoltage Lockout vs Temperature 100 125 1929 G22 VRUN/SS Shutdown Latch Thresholds vs Temperature 4.5 3.50 3.
PAGE 8
LTC1929/LTC1929-PG U U U PI FU CTIO S RUN/SS (Pin 1): Combination of Soft-Start, Run Control Input and Short-Circuit Detection Timer. A capacitor to ground at this pin sets the ramp time to full current output. Forcing this pin below 0.8V causes the IC to shut down all internal circuitry. All functions are disabled in shutdown. SENSE1+, SENSE2+ (Pins 2,14): The (+) Input to the Differential Current Comparators.
PAGE 9
LTC1929/LTC1929-PG W FU CTIO AL DIAGRA U U LTC1929-PG OPTIONAL PGOOD HOOKUP DIFFOUT PGOOD – 0.86V + VOS – EAIN A1 – – VOS + + + 0.
PAGE 10
LTC1929/LTC1929-PG U OPERATIO (Refer to Functional Diagram) Main Control Loop Low Current Operation The LTC1929 uses a constant frequency, current mode step-down architecture with inherent current sharing. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the main current comparator, I1, resets the RS latch.
PAGE 11
LTC1929/LTC1929-PG U OPERATIO (Refer to Functional Diagram) Differential Amplifier This amplifier provides true differential output voltage sensing. Sensing both VOUT + and VOUT – benefits regulation in high current applications and/or applications having electrical interconnection losses.
PAGE 12
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO When using the controller in very low dropout conditions, the maximum output current level will be reduced due to internal compensation required to meet stability criterion for buck regulators operating at greater than 50% duty factor. A curve is provided to estimate this reduction in peak output current level depending upon the operating duty factor.
PAGE 13
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO Accepting larger values of ∆IL allows the use of low inductances, but can result in higher output voltage ripple. A reasonable starting point for setting ripple current is ∆IL = 0.4(IOUT)/2, where IOUT is the total load current. Remember, the maximum ∆IL occurs at the maximum input voltage. The individual inductor ripple currents are determined by the inductor, input and output voltages. EXTVCC Pin Connection).
PAGE 14
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO 0.5 DC LOAD CURRENT The term (1 + δ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs. Temperature curve, but δ = 0.005/°C can be used as an approximation for low voltage MOSFETs. CRSS is usually specified in the MOSFET characteristics. The constant k = 1.7 can be used to estimate the contributions of the two terms in the main switch dissipation equation. 0.
PAGE 15
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO Where f = operating frequency of each stage, COUT = output capacitance and ∆IRIPPLE = combined inductor ripple currents. The output ripple varies with input voltage since ∆IL is a function of input voltage. The output ripple will be less than 50mV at max VIN with ∆IL = 0.
PAGE 16
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO EXTVCC Connection The LTC1929 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 and MOSFET gate driving power. The switch remains closed as long as the voltage applied to EXTVCC remains above 4.5V.
PAGE 17
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO SW, rises to VIN and the BOOST pin rises to VIN + VINTVCC. The value of the boost capacitor CB needs to be 30 to 100 times that of the total input capacitance of the topside MOSFET(s). The reverse breakdown of DB must be greater than VIN(MAX). The final arbiter when defining the best gate drive amplitude level will be the input supply current. If a change is made that decreases input current, the efficiency has improved.
PAGE 18
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO after CSS reaches 4.1V, CSS begins discharging on the assumption that the output is in an overcurrent condition. If the condition lasts for a long enough period as determined by the size of CSS, the controller will be shut down until the RUN/SS pin voltage is recycled.
PAGE 19
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO 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. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference.
PAGE 20
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO 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 the ratio (Duty Factor)/(Efficiency). For example, in a 20V to 5V application, 10mA of INTVCC current results in approximately 3mA 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.
PAGE 21
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO that will give a sense of the overall loop stability without breaking the feedback loop. The initial output voltage step resulting from the step change in output current may not be within the bandwidth of the feedback loop, so this signal cannot be used to determine phase margin. This is why it is better to look at the Ith pin signal which is in the feedback loop and is the filtered and compensated control loop response.
PAGE 22
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO Design Example As a design example, assume VIN = 5V (nominal), VIN␣ =␣ 5.5V (max), VOUT = 1.8V, IMAX = 20A, TA = 70°C 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: L≥ ≥ VOUT  VOUT   1−  VIN  f ∆I  ( ) (  1.
PAGE 23
LTC1929/LTC1929-PG U U W U APPLICATIO S I FOR ATIO continuous mode will be highest at the maximum input voltage since the duty factor is < 50%. The maximum output current ripple is: ( ) VOUT 0.3 at 33% D F fL 1.8 V 0.3 ∆ICOUTMAX = 300kHz 1.5µH ∆ICOUT = ( = 1.2ARMS )( ( ) ) VOUTRIPPLE = 20mΩ 1.2ARMS = 24mVRMS PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1929.
PAGE 24
LTC1929/LTC1929-PG U W U U APPLICATIO S I FOR ATIO Simplified Visual Explanation of How a 2-Phase Controller Reduces Both Input and Output RMS Ripple Current A multiphase power supply significantly reduces the amount of ripple current in both the input and output capacitors. The RMS input ripple current is divided by, and the effective ripple frequency is multiplied up by the number of phases used (assuming that the input voltage is greater than the number of phases used times the output voltage).
PAGE 25
LTC1929/LTC1929-PG U U W U APPLICATIO S I FOR ATIO SW1 L1 RSENSE1 D1 VIN VOUT RIN CIN + + SW2 L2 COUT RL RSENSE2 D2 BOLD LINES INDICATE HIGH, SWITCHING CURRENT LINES. KEEP LINES TO A MINIMUM LENGTH. 1929 F09 Figure 9. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator SINGLE PHASE SW V ICIN DUAL PHASE SW1 V SW2 V IL1 ICOUT IL2 ICIN ICOUT RIPPLE 1929 F10 Figure 10.
PAGE 26
R2 2.7k C11 1nF R3 10k R6 8.06k C15 470pF R5 10k C10 100pF C9 0.01µF C7 0.1µF R7 8.06k C17 1000pF C1 1000pF 14 13 12 11 10 9 8 7 6 5 4 3 2 1 SENSE1 – AMPMD 15 16 17 18 19 20 21 22 23 24 25 26 27 28 C13 2.2µF C8 0.47µF C3, C4: OS-CON 6SP680M C18–C21: T510E108M004 L1, L2: SUMIDA CEP149-1R0MC Q1–Q8: FDS6670A OR FDS7760A C12 1µF R1 10Ω C14 10µF 1 3 2 + Figure 11. 5V Input, 1.
PAGE 27
LTC1929/LTC1929-PG U TYPICAL APPLICATIO S 100 EFFICIENCY (%) 90 80 70 60 VIN = 5V VOUT = 1.6V 50 0 5 10 15 20 25 30 LOAD CURRENT (A) 35 40 1929 F12 Figure 12. Efficiency Plot for Circuit of Figure 11 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. G Package 28-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 10.07 – 10.33* (0.397 – 0.407) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 7.65 – 7.90 (0.301 – 0.311) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 5.20 – 5.
PAGE 28
LTC1929/LTC1929-PG U TYPICAL APPLICATIO LTC1929-PG 0.1µF 1000pF 2.7k RUN/SS NC 2 SENSE1 + TG1 3 SENSE1 – SW1 4 10k 5 INTVCC 6 51k 15k 7 47k 8 3.3nF 100pF 9 10 470pF 11 10k 15k PLLFLTR PLLIN BOOST1 VIN BG1 NC EXTVCC ITH INTVCC SGND VDIFFOUT PGND BG2 VOS – BOOST2 VOS + SW2 13 SENSE2 – TG2 14 SENSE2 + PGOOD 12 1000pF EAIN 28 L1 27 0.004Ω 26 25 0.22µF M1 M2 24 D1 MBRS140T3 23 22 10Ω 5V (OPT) CIN 47µF 35V + 21 20 0.