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LT3742

3742fa

Typical applicaTion

FeaTures

applicaTions

DescripTion

Dual, 2-Phase Step-Down

Switching Controller

The LT

3742 is a dual step-down DC/DC switching regulator

controller that drives high side N-channel power MOSFETs.

A 500kHz fixed frequency current mode architecture pro-

vides fast transient response with simple loop compensa-

tion components and cycle-by-cycle current limiting. The

output stages of the two controllers operate 180° out of

phase to reduce the input ripple current, minimizing the

noise induced on the input supply, and allowing less input

capacitance.

An internal boost regulator generates a bias rail of V

+ 7V

to provide gate drive for the N-channel MOSFETs allowing

low dropout and 100% duty cycle operation. The LT3742

can be used for applications where both controllers need

to operate independently, or where both controllers are

used to provide a single higher current output.

The device is available in a thermally enhanced 4mm ×

4mm QFN package.

L, LT, LTC, LTM, Linear Technology, the Linear logo, PolyPhase and Burst Mode are registered

trademarks and No R

SENSE

is a trademark of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

8V and 5V Dual Step-Down Converter

Wide Input Voltage Range: 4V to 30V

Wide Output Voltage Range: 0.8V to V

Low Shutdown I

: 20µA

Out-of-Phase Controllers Reduce Required Input

Capacitance and Power Supply Induced Noise

0.8V ±1.5% Voltage Reference

500kHz Current Mode Fixed Frequency Operation

Internal Boost Converter Provides Bias Rail for

N-channel MOSFET Gate Drive

Power Good Voltage Monitor for Each Output

Programmable Soft-Start

24-Lead 4mm × 4mm × 0.75mm Package

Satellite and Cable TV Set-Top Boxes

Distributed Power Regulation

Automotive Systems

Super Capacitor Charger

Efficiency vs Load Current

OUT1

14V

PG1

1.8k

45.3k

20.0k

1µF

10µF

6.5µH

0.010Ω

47µF

RUN1

1000pF

1nF

200Ω

30k

1.05k

47µF

1000pF

1nF

3742 TA01a

200Ω

20k

UVLO

10µH

4.7µF

SWB

GND

LT3742

BIAS

SW1

SENSE1

–

FB1

PG1

RUN/SS1

SW2

SENSE2

–

FB2

PG2

RUN/SS2

OUT2

PG2

RUN2

LOAD CURRENT (A)

EFFICIENCY (%)

1 2

3742 TA01b

100

0.5 1.5

2.5

3.5

OUT

Summary of content (26 pages)

PAGE 1
LT3742 Dual, 2-Phase Step-Down Switching Controller Features n n n n n n n n n n Description Wide Input Voltage Range: 4V to 30V Wide Output Voltage Range: 0.8V to VIN Low Shutdown IQ: 20µA Out-of-Phase Controllers Reduce Required Input Capacitance and Power Supply Induced Noise 0.8V ±1.
PAGE 2
LT3742 pIN CONFIGURATION VIN Voltage.................................................................30V UVLO Voltage.............................................................30V PG1, PG2 Voltage.......................................................30V SWB, BIAS Voltage....................................................40V SENSE1+, SENSE2+ Voltage.......................................30V SENSE1–, SENSE2– Voltage.......................................30V RUN/SS1, RUN/SS2 Voltage.......................
PAGE 3
LT3742 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V unless otherwise specified. PARAMETER CONDITIONS FB Pin Bias Current VFB = 0.8V, VC = 0.4V MIN FB Pin Voltage Matching –4 TYP MAX UNITS 50 200 nA 0 4 mV Error Amplifier Transconductance 250 µmho Error Amplifier Voltage Gain 500 V/V VC Pin Source Current VFB = 0.
PAGE 4
LT3742 Typical Performance Characteristics IQ-SHDN vs Temperature Controller Current Sense Voltage vs Temperature IQ-Running vs Temperature 10 50 70 8 CURRENT (mA) CURRENT (µA) 40 SENSE VOLTAGE (mV) 9 30 7 6 5 20 65 60 55 4 10 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 3 –50 –25 125 50 25 75 0 TEMPERATURE (°C) 100 50 –50 125 –25 75 0 25 50 TEMPERATURE (°C) Internal UVLO vs Temperature VFB vs Temperature UVLO Threshold vs Temperature 5 125 3742 G03 3742 G02 3742 G01 100 3.
PAGE 5
LT3742 Typical Performance Characteristics Controller Frequency vs Temperature PG Threshold vs Temperature 450 75 0 25 50 TEMPERATURE (°C) 100 125 POWER GOOD 0.8 8 6 0.7 0.6 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 4 –50 75 0 25 50 TEMPERATURE (°C) –25 3742 G10 3742 G09 100 125 3742 G11 RUN/SS Threshold vs Temperature SWB Current Limit vs Temperature 1.0 380 360 RUN/SS VOLTS (V) –25 0.
PAGE 6
LT3742 Pin Functions G1, G2 (Pins 1, 6): Gate Drives. These pins provide high current gate drive for the external N-channel MOSFETs. These pins are the outputs of floating drivers whose voltage swings between the BIAS and SW pins. VIN (Pin 2): Input Voltage. This pin supplies current to the internal circuitry of the LT3742. This pin must be locally bypassed with a capacitor. UVLO (Pin 3): Undervoltage Lockout. Do not leave this pin open ; connect it to VIN if not used.
PAGE 7
LT3742 Block Diagram SWB BIAS 25 GND VIN 2 VIN 3 UVLO GATE DRIVE BIAS BOOST REGULATOR 1.25V + – VIN 5 4 BIAS UNDERVOLTAGE LOCKOUT THERMAL SHUTDOWN 3µA UVLO VREF INTERNAL SUPPLY ENABLE COMPARATOR – + + ≈0.5V 1.25V 0.88V 0.80V 0.
PAGE 8
LT3742 Operation The LT3742 is a dual, constant frequency, current mode DC/DC step-down controller. The two controllers in each device share some common circuitry including protection circuitry, the internal bias supply, voltage reference, master oscillator and the gate drive boost regulator. The Block Diagram shows the shared common circuitry and the independent circuitry for both DC/DC controllers.
PAGE 9
LT3742 Applications Information Soft-Start and Shutdown The RUN/SS (Run/Soft-Start) pins are used to enable each controller independently, and to provide a user-programmable soft-start function that reduces the peak input current and prevents output voltage overshoot during start-up. To disable either controller, pull its RUN/SS pin to ground with an open-drain or open-collector device. If both RUN/SS pins are pulled to ground, the LT3742 is placed in shutdown mode, and quiescent current is reduced to 20µA.
PAGE 10
LT3742 Applications Information Power Good Indicators Output Sequencing and Tracking The PG pin is the open-collector output of an internal window comparator that is pulled low whenever the FB pin is not within ±10% of the 800mV internal reference voltage. Tie the PG pin to any supply less than 30V with a pull-up resistor that will supply less than 200µA. This pin will be open when the LT3742 is placed in shutdown mode regardless of the voltage at the FB pin.
PAGE 11
LT3742 Applications Information characteristic to notice about this method is that if the output of controller 1 goes out of regulation enough to trip the power good comparator, controller 2 will be disabled. tie it to VIN if not used. A separate, less accurate, internal undervoltage lockout will disable the LT3742 when VIN is less than 2.5V. The UVLO resistor values are chosen to give the desired minimum operating voltage (VIN(MIN)) and the desired amount of hysteresis (VHYST).
PAGE 12
LT3742 Applications Information The maximum input voltage is determined by the absolute maximum ratings of the VIN and BIAS pins (30V and 40V, respectively) and by the minimum duty cycle, DCMIN = 15%. ⎛V +V ⎞ VIN(MAX) = ⎜ OUT D ⎟ + VSW – VD ⎝ DCMIN ⎠ The formula above calculates the maximum input voltage that allows the part to regulate without pulse-skipping, and is mainly a concern for applications with output voltages lower than 3.3V. For example, for a 2.5V output, the maximum input voltage is: ⎛ 2.
PAGE 13
LT3742 Applications Information operation would reduce the RMS input capacitor current from ~1.8ARMS to ~0.8ARMS. While this is an impressive reduction by itself, remember that power losses are proportional to IRMS2, meaning that the actual power wasted due to the input capacitor is reduced by a factor of ~4. Figure 5 shows the reduction in RMS ripple current for a typical application. The reduced input ripple current also means that less power is lost in the input power path.
PAGE 14
LT3742 Applications Information value calculated above, and then recheck the amount of ripple current: IRIPPLE V – VOUT VOUT 1 = IN • • L VIN 500kHz The DC resistance (DCR) of the inductor can have a significant impact on total system efficiency, as it causes an I2RDCR power loss. Consider inductance value, DCR, and current rating when choosing an inductor. Table 1 shows several recommended inductor vendors. Each offers numerous devices in a wide variety of values, current ratings, and package sizes.
PAGE 15
LT3742 Applications Information (QG). A few simple guidelines will make the selection process easier. The maximum drain current must be higher than the maximum rated current, IRATED, calculated on the previous page. Note that the ID specification is largely temperature dependent (lower ID at higher ambient temperatures), so most data sheets provide a graph or table of ID versus temperature to show this.
PAGE 16
LT3742 Applications Information voltages, it is also 3× higher at high input voltages when compared to the low CRSS device. Total gate charge, QG, is closely related to CRSS. Low gate charge corresponds to a small value of CRSS. Many manufacturers have MOSFETs advertised as “low gate charge” devices (which means they are low CRSS devices) that are specifically designed for low transition loss, and are ideal for high input voltage applications.
PAGE 17
LT3742 Applications Information LT3742’s control loop. Output ripple can be estimated with the following equation: The output voltage for each controller is programmed with a resistor divider between the output and the FB pin. Always use 1% resistors (or better) for the best output voltage accuracy. The value of RA should be 8k or less, and the value of R1 should be chosen according to: ⎛ ⎞ 1 VRIPPLE = ΔIL ⎜ + ESR ⎟ ⎝ 8 • fSW • COUT ⎠ ⎛V ⎞ RB = RA • ⎜ OUT – 1⎟ ⎝ 0.
PAGE 18
LT3742 Applications Information Bias Supply Considerations The LT3742 uses an internal boost regulator to provide a bias rail for enhancement of the external MOSFETs. This bias rail is regulated to VIN + 7V and must be in regulation before either controller is allowed to start switching. As this is a high speed switching regulator, standard procedures must be followed regarding placement of the external components.
PAGE 19
LT3742 Applications Information VIN VOUT1 VIN SHDN RUN1 SYSTEM GROUND SHDN VIN RUN2 VOUT2 3742 F08 VIAS TO LOCAL GROUND PLANE OUTLINE OF LOCAL GROUND PLANE Figure 8.
PAGE 20
LT3742 Typical Applications Supercap Charger Plus a DC/DC Buck Converter L3 22µH VIN 5.5V TO 30V 2 R7 357k C5 1µF C6 4.7µF 3 VIN UVLO 5 4 SWB BIAS R8 124k LT3742 VIN VOUT1 5V 4A C1 6.8µF RS1 0.010Ω 1 M1 L1 4.7µH 24 21 R1 1.05k C2 150µF 20 19 R2 200Ω SENSE1+ SENSE1– SENSE2+ SENSE2– FB1 FB2 7 16 C7 1nF CC1 680pF M2 L2 47µH RS2 0.030Ω C3 6.8µF VIN SUPERCAP CHARGER OUTPUT 10 11 C4 47µF 12 150mF D2 17 M3 SW2 SW1 6 D1 PG1 RUN1 G2 G1 18 RC1 51k D1, D2: DIODES INC.
PAGE 21
LT3742 Typical Applications 8V and 5V Dual Step-Down Converter L3 22µH VIN 14V 2 R1 45.3k C5 1µF C6 2.2µF 3 VIN UVLO 5 4 SWB BIAS R1 20k LT3742 VIN VOUT1 8V 4A C1 10µF RS1 0.01Ω 1 M1 L1 6.5µH 24 21 R1 1.8k C2 47µF 20 19 R2 200Ω SENSE1+ SENSE1– SENSE2+ SENSE2– FB1 FB2 M2 L2 6.5µH 7 16 C7 1nF CC1 1000pF RS2 0.01Ω C3 10µF 18 RC1 30k PG1 PG2 RUN/SS1 VC1 C2, C4: MURATA GRM32ER71A476K D1, D2: DIODES INC.
PAGE 22
LT3742 Typical Applications 5V and 3.3V Dual Step-Down Converter L3 22µH VIN 14V 348k 1µF 2.2µF SWB BIAS VIN UVLO 130k LT3742 VIN VIN 10µF VOUT1 3.3V 4A 10mΩ G1 L1 3.3µH G2 SW2 SENSE2+ SENSE2– FB2 SW1 SENSE1+ SENSE1– FB1 619Ω 220µF L2 4.7µH 10µF 10mΩ 1.05k 150µF 200Ω D2 200Ω PG1 1nF 680pF 51k PG1 RUN/SS1 VC1 PG2 RUN/SS2 VC2 PG2 51k GND 68pF VOUT2 5V 4A D1, D2: DIODES INC.
PAGE 23
LT3742 Typical Applications High Current, Low Ripple 12V Step-Down Converter L3 10µH VIN 24V R7 124k C5 1µF C6 4.7µF SWB BIAS VIN UVLO R8 20.0k LT3742 VIN C1 10µF RS1 0.010Ω VOUT1 12V 8A G1 M1 G2 L2 8.2µH L1 8.2µH SW2 SENSE2+ SENSE2– FB2 SW1 SENSE1+ SENSE1– D1 PG1 C7 1nF CC1 680pF RC1 51k PG1 FB1 RUN/SS1 VC1 RS2 0.010Ω PG2 RUN/SS2 VC2 PG2 GND COUTA 100µF 20V COUTA-COUTD: KEMET T495E107K020E060 D1, D2: DIODES INC.
PAGE 24
LT3742 Package Description UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 ±0.05 4.50 ± 0.05 2.45 ± 0.05 3.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 0.75 ± 0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 23 24 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 1 2 2.45 ± 0.10 (4-SIDES) (UF24) QFN 0105 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC NOTE: 1.
PAGE 25
LT3742 Revision History REV DATE DESCRIPTION A 5/11 Revised Conditions in the Electrical Characteristics section. PAGE NUMBER 2, 3 Revised the title of curve G04 in the Typical Performance Characteristics section. 4 Updated the PG1, PG2 pin description in the Pin Functions section. 6 Updated values in the Block Diagram, Operation, and Applications Information sections. 7-12 3742fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
PAGE 26
LT3742 Related Parts PART NUMBER DESCRIPTION COMMENTS LTC1625/LTC1775 No RSENSE™ Current Mode Synchronous Step-Down Controllers 97% Efficiency, No Sense Resistor, 16-Pin SSOP LTC1735 High Efficiency Synchronous Step-Down Switching Regulator Output Fault Protection, 16-Pin SSOP LTC1778 No RSENSE Wide Input Range Synchronous Step-Down Controller Up to 97% Efficiency, 4V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ (0.9)(VIN), IOUT Up to 20A LT3430/LT3431 Monolithic 3A, 200kHz/500kHz Step-Down Regulators 5.