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LTC3727A-1

3727a1fa

TYPICAL APPLICATION

DESCRIPTION

High Efﬁ ciency, 2-Phase

Synchronous Step-Down

Switching Regulators

The LTC

3727A-1 is a high performance dual 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.

The LTC3727A-1 is an improved version of the LTC3727

family of parts. It has smaller output ripple while in the

drop-out condition and shorter minimum on-time.

OPTI-LOOP compensation allows the transient response to

be optimized over a wide range of output capacitance and

ESR values. There is a precision 0.8V reference and a power

good output indicator. A wide 4V to 30V (36V maximum)

input supply range encompasses all battery chemistries.

A RUN/SS pin for each controller provides soft-start. Current

foldback limits MOSFET heat dissipation during short-circuit

conditions. Output overvoltage protection circuitry protects

the controller until V

OUT

returns to normal.

Figure 1. High Efﬁ ciency Dual 12V/5V Step-Down Converter

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

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, 6304066, 5705919.

FEATURES

APPLICATIONS

Telecom Systems

Automotive Systems

Battery-Operated Digital Devices

Wide Output Voltage Range: 0.8V ≤ V

OUT

≤ 14V

Out-of-Phase Controllers Reduce Required Input

Capacitance and Power Supply Induced Noise

OPTI-LOOP

Compensation Minimizes C

OUT

1% Output Voltage Accuracy

Power Good Output Voltage Monitor

Phase-Lockable Fixed Frequency 250kHz to 550kHz

Dual N-Channel MOSFET Synchronous Drive

Wide V

Range: 4V to 36V Operation

Very Low Dropout Operation: 99% Duty Cycle

Adjustable Soft-Start Current Ramping

Foldback Output Current Limiting

Output Overvoltage Protection

Low Shutdown I

: 20μA

Selectable Constant Frequency or Burst Mode

Operation

Small 28-Lead SSOP Package

4.7μF

0.1μF

105k

1000pF

8μH

220pF

1μF

CERAMIC

22μF

50V

CERAMIC

47μF

0.015Ω

20k

15k

OUT1

0.1μF

280k

15μH

220pF

1000pF

56μF

15V

0.015Ω

20k

15k

OUT2

12V

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND

PGND

SENSE1

SENSE2

SENSE1

–

SENSE2

–

OSENSE1

OSENSE2

TH1

TH2

PGOOD INTV

RUN/SS1 RUN/SS2

18V TO 28V

M1, M2, M3, M4: FDS6680A

3727 F01

0.1μF

LTC3727A-1

PLLIN

Summary of content (32 pages)

PAGE 1
LTC3727A-1 High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators DESCRIPTION FEATURES n n n n n n n n n n n n n n n Wide Output Voltage Range: 0.
PAGE 2
LTC3727A-1 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Input Supply Voltage (VIN) ......................... 36V to –0.3V Top Side Driver Voltages BOOST1, BOOST2.................................. 42V to –0.3V Switch Voltage (SW1, SW2) ......................... 36V to –5V INTVCC, EXTVCC, (BOOST1-SW1), (BOOST2-SW2)......................................... 8.5V to –0.3V RUN/SS1, RUN/SS2, PGOOD ...................... 7V to –0.3V SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages ....................
PAGE 3
LTC3727A-1 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 gm1, 2 Transconductance Ampliﬁer gm ITH1, 2 = 1.2V; Sink/Source 5μA (Note 4) gmGBW1, 2 Transconductance Ampliﬁer GBW ITH1, 2 = 1.2V (Note 4) IQ Input DC Supply Current Normal Mode Shutdown (Note 5) VIN = 15V, EXTVCC Tied to VOUT1, VOUT1 = 8.
PAGE 4
LTC3727A-1 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 3.25 UNITS 3.3V Linear Regulator V3.3OUT 3.3V Regulator Output Voltage No Load 3.35 3.45 V V3.3IL 3.3V Regulator Load Regulation I3.3 = 0mA to 10mA 0.5 2.5 % V3.3VL 3.3V Regulator Line Regulation 6V < VIN < 30V 0.05 0.
PAGE 5
LTC3727A-1 TYPICAL PERFORMANCE CHARACTERISTICS Supply Current vs Input Voltage and Mode (Figure 13) EXTVCC Voltage Drop 1000 160 Internal 7.5V LDO Line Regulation 7.7 VEXTVCC = 8.5V BOTH CONTROLLERS ON 600 400 200 7.5 120 INTVCC VOLTAGE (V) EXTVCC VOLTAGE DROP (mV) SUPPLY CURRENT (μA) 800 100 80 60 40 20 10 INPUT VOLTAGE (V) 0 30 7.4 7.3 7.2 7.1 7.0 20 6.9 0 6.8 SHUTDOWN 0 ILOAD = 1mA 7.
PAGE 6
LTC3727A-1 TYPICAL PERFORMANCE CHARACTERISTICS Dropout Voltage vs Output Current (Figure 13) SENSE Pins Total Source Current 1.4 100 50 VOUT = 5V 1.6 DROPOUT VOLTAGE (V) –50 –100 –150 –200 –250 RUN/SS CURRENT (μA) 1.2 0 ISENSE (μA) RUN/SS Current vs Temperature 1.8 1.0 RSENSE = 0.015Ω 0.8 0.6 0.4 1.4 1.2 1.0 0.8 0.6 0.4 –300 0.2 –350 0.2 RSENSE = 0.
PAGE 7
LTC3727A-1 TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Pin Input Current vs Temperature 10 VOUT = 5V EXTVCC SWITCH RESISTANCE (Ω) CURRENT SENSE INPUT CURRENT (μA) 35 EXTVCC Switch Resistance vs Temperature 33 31 29 27 25 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 8 6 4 2 0 –50 125 –25 50 25 0 75 TEMPERATURE (°C) 3727 G22 Undervoltage Lockout vs Temperature 700 VPLLFLTR = 5V UNDERVOLTAGE LOCKOUT (V) FREQUENCY (kHz) 3.50 500 400 300 VPLLFLTR = 1.
PAGE 8
LTC3727A-1 PIN FUNCTIONS RUN/SS1, RUN/SS2 (Pins 1, 15): Combination of Soft-Start, Run Control Inputs. 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. SENSE1+, SENSE2+ (Pins 2, 14): The (+) Input to the Differential Current Comparators.
PAGE 9
LTC3727A-1 FUNCTIONAL DIAGRAM PLLIN FIN VIN INTVCC PHASE DET DUPLICATE FOR SECOND CONTROLLER CHANNEL 100k BOOST DB PLLFLTR DROP OUT DET CLK1 RLP OSCILLATOR CLK2 CLP – 0.86V S Q R Q BOT VOSENSE1 TOP ON SWITCH LOGIC INTVCC 0.86V + 0.55V COUT PGND B – + BG + 0.74V SHDN VOSENSE2 CIN SW BOT + + FCB – – CB D1 + PGOOD TG TOP VOUT RSENSE – + 1.5V 7V 0.18μA R6 0.74V I1 – + – BINH FCB – 3.3VOUT + 0.8V – ++ FCB SLOPE COMP – + 50k SENSE + 25k 2.
PAGE 10
LTC3727A-1 OPERATION (Refer to Functional Diagram) Main Control Loop Low Current Operation The LTC3727A-1 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
LTC3727A-1 OPERATION (Refer to Functional Diagram) Continuous Current (PWM) Operation Theory and Beneﬁts of 2-Phase 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 12
LTC3727A-1 OPERATION (Refer to Functional Diagram) 5V SWITCH 20V/DIV 3.3V SWITCH 20V/DIV INPUT CURRENT 5A/DIV INPUT VOLTAGE 500mV/DIV 3727 G03a IIN(MEAS) = 2.53ARMS IIN(MEAS) = 1.55ARMS (a) 3727 G03b (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 13
LTC3727A-1 APPLICATIONS INFORMATION 2.5 PLLFLTR PIN VOLTAGE (V) Figure 1 on the ﬁrst page is a basic LTC3727A-1 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 14
LTC3727A-1 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 ΔIL) will cause this to occur at lower load currents, which can cause a dip in efﬁciency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to decrease.
PAGE 15
LTC3727A-1 APPLICATIONS INFORMATION 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. The Schottky diode D1 shown in Figure 2 conducts during the dead-time between the conduction of the two power MOSFETs.
PAGE 16
LTC3727A-1 APPLICATIONS INFORMATION The beneﬁt of the LTC3727A-1 multiphase can be calculated by using the equation above for the higher power controller and then calculating the loss that would have resulted if both controller channels switch on at the same time. The total RMS power lost is lower when both controllers are operating due to the interleaving of current pulses through the input capacitor’s ESR.
PAGE 17
LTC3727A-1 APPLICATIONS INFORMATION INTVCC Regulator An internal P-channel low dropout regulator produces 7.5V at the INTVCC pin from the VIN supply pin. INTVCC powers the drivers and internal circuitry within the LTC3727A-1. The INTVCC pin regulator can supply a peak current of 50mA and must be bypassed to ground with a minimum of 4.7μF tantalum, 10μF special polymer, or low ESR type electrolytic capacitor.
PAGE 18
LTC3727A-1 APPLICATIONS INFORMATION VIN OPTIONAL EXTVCC CONNECTION 7.5V < VSEC < 8.5V + CIN VSEC VIN + N-CH LTC3727A-1 1μF TG1 RSENSE of the topside MOSFET(s). The reverse breakdown of the external Schottky diode must be greater than VIN(MAX). When adjusting the gate drive level, the ﬁnal arbiter is the total input current for the regulator. If a change is made and the input current decreases, then the efﬁciency has improved.
PAGE 19
LTC3727A-1 APPLICATIONS INFORMATION Regulating an output voltage of 1.8V, the maximum value of R1 should be 32k. Note that for an output voltage above 2.4V, R1 has no maximum value necessary to absorb the sense currents; however, R1 is still bounded by the VOSENSE feedback current. Soft-Start/Run Function The RUN/SS1 and RUN/SS2 pins are multipurpose pins that provide a soft-start function and a means to shut down the LTC3727A-1.
PAGE 20
LTC3727A-1 APPLICATIONS INFORMATION A comparator monitors the output for overvoltage conditions. The comparator (OV) detects overvoltage faults greater than 7.5% above the nominal output voltage. When this condition is sensed, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared.
PAGE 21
LTC3727A-1 APPLICATIONS INFORMATION If the duty cycle falls below what can be accommodated by the minimum on-time, the LTC3727A-1 will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. The minimum on-time for the LTC3727A-1 is approximately 120ns. However, as the peak sense voltage decreases the minimum on-time gradually increases up to about 170ns.
PAGE 22
LTC3727A-1 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 23
LTC3727A-1 APPLICATIONS INFORMATION adequate charge storage and very low ESR at the switching frequency. A 25W supply will typically require a minimum of 22μF to 47μF of capacitance having a maximum of 20mΩ to 50mΩ of ESR. The LTC3727A-1 2-phase architecture typically halves this input capacitance requirement over competing solutions. Other losses, including Schottky diode conduction losses during dead-time and inductor core losses, generally account for less than 2% total additional loss.
PAGE 24
LTC3727A-1 APPLICATIONS INFORMATION automobile is the source of a number of nasty potential transients, including load-dump, reverse-battery, and double-battery. Load-dump is the result of a loose battery cable. When the cable breaks connection, the ﬁeld collapse in the alternator can cause a positive spike as high as 60V which takes several hundred milliseconds to decay.
PAGE 25
LTC3727A-1 APPLICATIONS INFORMATION CIN is chosen for an RMS current rating of at least 3A at temperature assuming only this channel is on. COUT is chosen with an ESR of 0.02Ω for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage. The output voltage ripple due to ESR is approximately: VORIPPLE = RESR (ΔIL) = 0.
PAGE 26
LTC3727A-1 APPLICATIONS INFORMATION drops below the low current operation threshold—typically 10% to 20% of the maximum designed current level in Burst Mode operation. The duty cycle percentage should be maintained from cycle to cycle in a well-designed, low noise PCB implementation. Variation in the duty cycle at a subharmonic rate can suggest noise pickup at the current or voltage sensing inputs or inadequate loop compensation.
PAGE 27
LTC3727A-1 APPLICATIONS INFORMATION RPU 3 R2 R1 4 5 fIN 6 12 R4 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN BG1 13 14 L1 27 VOUT1 RSENSE 26 25 CB1 M1 D1 24 23 COUT1 RIN CIN FCB SGND 3.3VOUT ITH2 PGND BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 M2 20 CVIN CINTVCC GND + 10 11 R3 TG1 22 EXTVCC LTC3727A-1 21 8 ITH1 INTVCC 7 9 3.
PAGE 28
LTC3727A-1 APPLICATIONS INFORMATION SW1 L1 D1 RSENSE1 COUT1 VOUT1 + RL1 VIN RIN CIN + SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH D2 L2 RSENSE2 COUT2 VOUT2 + RL2 3727 F11 Figure 11.
PAGE 29
LTC3727A-1 TYPICAL APPLICATIONS 0.1μF 2 27pF 105k 1% 20k 1% 10k 1000pF 3 4 0.01μF 5 1000pF 7 220pF 9 3.3V 10 11 220pF 12 20k 1% 27pF TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN BG1 13 280k 1% 1000pF 14 SGND PGND 3.3VOUT ITH2 BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 0.015Ω VOUT1 5V 5A; 6A PEAK 26 M1A 0.1μF 25 M1B D1 MBRM 140T3 24 COUT1 47μF 6.
PAGE 30
LTC3727A-1 TYPICAL APPLICATIONS 0.1μF 2 27pF 105k 1% 20k 1% 1000pF 3 4 5 6 7 220pF 9 3.3V 10 11 220pF 12 20k 1% 27pF TG1 SENSE1– SW1 VOSENSE1 BOOST1 PLLFLTR VIN PLLIN BG1 13 192.5k 1% 1000pF 14 SGND 3.3VOUT ITH2 PGND BG2 BOOST2 VOSENSE2 SW2 SENSE2– TG2 SENSE2+ RUN/SS2 0.1μF 25 M1A VOUT1 5V 5A; 6A PEAK M1B D1 MBRM 140T3 24 COUT1 47μF 6.3V 23 10Ω 20 19 CMDSH-3 22μF 50V 0.1μF 1μF 10V GND 4.7μF COUT2 100μF 16V CMDSH-3 18 17 0.
PAGE 31
LTC3727A-1 PACKAGE DESCRIPTION G Package 28-Lead Plastic SSOP (5.3mm) (Reference 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 p 0.12 7.8 – 8.2 5.3 – 5.7 0.42 p0.03 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) 0o – 8o 0.09 – 0.25 (.0035 – .010) 0.55 – 0.95 (.022 – .037) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
PAGE 32
LTC3727A-1 RELATED PARTS PART NUMBER DESCRIPTION LTC1438/LTC1439 Dual High Efﬁciency Low Noise Synchronous Step-Down Switching Regulators POR, Auxiliary Regulator LTC1438-ADJ Dual Synchronous Controller with Auxiliary Regulator POR, External Feedback Divider LTC1538-AUX Dual High Efﬁciency Low Noise Synchronous Step-Down Switching Regulator Auxiliary Regulator, 5V Standby LTC1539 Dual High Efﬁciency Low Noise Synchronous Step-Down Switching Regulator 5V Standby, POR, Low-Battery, Aux Regulator