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LT3740

3740fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Wide Operating Range,

Valley Mode, No R

SENSE

™

Synchronous Step-Down Controller

The LT

3740 is a synchronous step-down switching

regulator controller that drives N-channel power MOSFET

stages. The controller uses valley current mode architecture

to achieve very low duty cycles with excellent transient

response without requiring a sense resistor.

The LT3740 includes an internal step-up converter to

provide a bias 7.8V higher than the input voltage for the

drive. This enables the part to work from an input voltage

as low as 2.2V.

The XREF pin is an external reference input that allows the

user to override the internal 0.8V feedback reference with

any lower value, allowing full control of the output voltage

during operation, output voltage tracking or soft-start.

The LT3740 has three current limit levels that can be

chosen by connecting the RANGE pin to ground, open,

and input respectively.

High Efﬁ ciency Step-Down Converter

Wide V

Range: 2.2V to 22V

Internal Boost Provides 6V Gate Drive

For V

Down to 2.2V

No Sensing Resistor Required

Dual N-Channel MOSFET Synchronous Drive

Valley Current Mode Control

Optimized for High Step-Down Ratio

Power Good Output Voltage Monitor

0.8V Reference

Three Pin-Selected Current Limit Levels

Constant Switching Frequency: 300kHz

Programmable Soft-Start

Output Voltage Tracking

Available in 16-Pin 5mm × 3mm DFN

Notebook and Palmtop Computers, PDA

Portable Instruments

Distributed Power Systems

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.

Efﬁ ciency vs Load Current

SWB

SHDN

XREF

RANGE

BGDP

BIAS

TGATE

BGATE

GND

LT3740

–

PGND

3740 TA01a

0.9µH

1µF

0.22µF

OUT

1.8V

10A

3V to 12V

HAT2168H

HAT2165H

100µF

s 3

10µF

39pF

22µH

20k

15k

1nF

22pF

80.6k

105k

15k

1Ω

B320A

1Ω

LOAD CURRENT (A)

EFFICIENCY (%)

3740 TA01b

462

= 3V

= 5V

= 12V

OUT

= 1.8V

Summary of content (20 pages)

PAGE 1
LT3740 Wide Operating Range, Valley Mode, No RSENSE™ Synchronous Step-Down Controller DESCRIPTION FEATURES n n n n n n n n n n n n n Wide VIN Range: 2.2V to 22V Internal Boost Provides 6V Gate Drive For VIN Down to 2.2V No Sensing Resistor Required Dual N-Channel MOSFET Synchronous Drive Valley Current Mode Control Optimized for High Step-Down Ratio Power Good Output Voltage Monitor 0.
PAGE 2
LT3740 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) SN–, BGATE, VC, FB, XREF, PGOOD Voltages .............10V VIN , SHDN, SW, Range Voltages ...............................22V BIAS, TGATE, BGDP, SN+ Voltages ............................32V TOP VIEW SWB Voltage ............................................................36V Maximum Junction Temperature........................... 125°C Operating Temperature Range (Note 2)....–40°C to 85°C Storage Temperature Range...................
PAGE 3
LT3740 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 noted. (Note 2) PARAMETER CONDITIONS MIN SHDN Voltage to Enable Device ● SHDN Voltage to Disable Device ● TYP MAX UNITS 1.1 V 0.5 V TGATE On Voltage 5.5 V TGATE Off Voltage 0.2 V BGATE On Voltage 5.5 V BGATE Off Voltage 0.
PAGE 4
LT3740 TYPICAL PERFORMANCE CHARACTERISTICS XREF Pin Start-Up (TA = 25°C, unless otherwise noted) Current Limit vs Bottom Gate On-Time Load Regulation 200 0 VIN = 10V VOUT = 2.5V RS = 12mΩ –0.1 IL 2A/DIV 180 CURRENT SENSING LIMIT (mV) SS 0.5V/DIV DVOUT (%) –0.2 VO 2V/DIV –0.3 RANGE = VIN –0.4 –0.5 RANGE = GND –0.6 VIN = 10V, VOUT = 2.5V ILOAD = 3A –0.7 –0.
PAGE 5
LT3740 TYPICAL PERFORMANCE CHARACTERISTICS BIAS-VIN to Enable Controller vs Temperature 7.50 390 7.45 380 7.40 370 7.35 360 350 340 7.25 7.20 7.15 320 7.10 310 7.05 –25 0 25 50 TEMPERATURE (°C) 75 100 3.0 7.30 330 300 –50 Undervoltage Lockout Threshold vs Temperature UNDERVOLTAGE LOCKOUT VIN (V) 400 BIAS-VIN (V) ERROR AMP (μs) Error Ampliﬁer Transconductance vs Temperature (TA = 25°C, unless otherwise noted) 7.00 –50 –25 0 25 50 TEMPERATURE (°C) 3740 G12 75 100 3740 G13 2.
PAGE 6
LT3740 PIN FUNCTIONS VC (Pin 16): Error Ampliﬁer Compensation Pin. Connect the external compensation RC to this pin. The current comparator threshold increases with the voltage of this pin. Exposed Pad (Pin 17): Ground. Must be soldered to PCB ground. BLOCK DIAGRAM VIN SWB 10 9 + 7.8V + Gm – – A2 R Q1 Q S + – VC 16 Σ A3 + SHDN 13 RAMP GENERATOR + – + XREF 14 1.2MHz OSCILLATOR 8 BIAS 7 TGATE 6 SW 4 BGDP 3 BGATE 2 PGND – 5 SN+ + 1 SN– – gm A1 SWITCH LOGIC + VREF 0.
PAGE 7
LT3740 OPERATION The LT3740 is a constant-frequency, valley current mode controller for DC/DC step-down converters. At the start of each oscillator cycle, the switch logic is set, which turns on the bottom MOSFET. After a 500ns blanking time, the bottom MOSFET current is sensed and added to a stabilizing ramp, and the resulting sum is fed into the PWM comparator A1. When this voltage goes below the voltage at VC pin, the switch logic is reset, which turns off the bottom MOSFET, and turns on the top MOSFET.
PAGE 8
LT3740 APPLICATIONS INFORMATION Current Sensing Range Reverse Current Limit Inductor current is determined by measuring the voltage across a sense resistance – either the on-resistance of the bottom MOSFET or an external sensing resistor. The maximum current sense threshold has three steps that are selected by the RANGE pin. The current sense threshold voltage without slope compensation is shown in Table 1. This is the value for high duty cycle operation.
PAGE 9
LT3740 APPLICATIONS INFORMATION The ρT term is a normalization factor (unity at 25°C) accounting for the signiﬁcant variation in on-resistance with temperature, typically about 0.4%/°C as shown in Figure 1. For a maximum junction temperature of 100°C, using a value ρT = 1.3 is reasonable. of the circuit. One of the solution circuits is shown in Figure 2. The Zener diode and the small MOSFET limit the SHDN voltage to be about 6V below VIN. This shuts down the LT3740 for VIN lower than 7V.
PAGE 10
LT3740 APPLICATIONS INFORMATION Choose MOSFET Sensing or Resistor Sensing The LT3740 can use either the bottom MOSFET onresistance or an external sensing resistor for current sensing. Simplicity and high efﬁciency are the beneﬁts of using bottom MOSFET on-resistance. However, some MOSFETs have a wide on-resistance variation. As discussed previously, the gate-source voltage and the temperature also affect the MOSFET on-resistance. These factors affect the accuracy of the inductor current limit.
PAGE 11
LT3740 APPLICATIONS INFORMATION Once the value for L is known, the type of inductor must be selected. High efﬁciency converters generally cannot afford the core loss found in low cost powdered iron cores; instead use ferrite, molypermalloy or Kool Mμ® cores. A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
PAGE 12
LT3740 APPLICATIONS INFORMATION Current Limit The maximum inductor current is inherently limited in a current mode controller by the maximum sense voltage. In the LT3740, the maximum sense voltage is selected by the RANGE pin. With valley current control, the maximum sense voltage and the sense resistance determine the maximum allowed inductor valley current. The corresponding output current limit is: ILIMIT = VSN(MAX) RS + ΔIL 2 1. DC I2R losses.
PAGE 13
LT3740 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD*(ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value.
PAGE 14
LT3740 APPLICATIONS INFORMATION Selecting a standard value of 2.0μH results in a ripple current of: ΔIL = 2.5V ⎛ 2.5V ⎞ = 3.47A • ⎜ 1– (300kHz) •(2μH) ⎝ 15V ⎟⎠ Set Range = VIN. At minimum input voltage VIN = 7V, the maximum current sensing voltage is 145mV. Use an external sensing resistor of 12mΩ. The inductor current valley will be clamped to 12A. The ripple current at VIN = 7V is 2.68A, there is about 33% of margin for the 10A load current to cover the maximum current sensing voltage variation.
PAGE 15
LT3740 APPLICATIONS INFORMATION Which is adequately lower than the 35mV reverse current comparator threshold. However, a 0A to 10A load step will cause an output change of up to: CIN is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.005Ω to minimize output voltage changes due to inductor ripple current and load steps. The ripple voltage will be only: ΔVOUT(STEP) = ΔILOAD • (ESR) = (10A)(0.005Ω)= 50mV ΔVOUT(RIPPLE) = ΔIL • (ESR) = (3.47A)(0.
PAGE 16
LT3740 APPLICATIONS INFORMATION PC Board Layout Considerations ■ Minimize the parasitic inductance in the loop of CIN, MOSFETs and D1 which carries large switching current. ■ Use compact plane for switch node (SW) to improve cooling of the MOSFETs and to keep EMI low. ■ Use planes for VIN and VOUT to maintain good voltage ﬁltering and to keep power losses low. Unused areas can be ﬁlled with copper and connect to any DC node (VIN, VOUT, GND) ■ Place CB close to BIAS pin and input capacitor.
PAGE 17
LT3740 TYPICAL APPLICATIONS High Efﬁciency Step-Down Converter VIN 7V to 20V 22μH LT3740 SWB BGDP 20μF 1μF BIAS VIN 1Ω M1 HAT2168H 3.4μH TGATE SHDN 1Ω 15k VOUT 3.3V 10A SW SN+ XREF D1 B340A 1Ω 2k 100μF s3 BGATE 0.47μF 10k SN VC – M2 HAT2165H 22pF 255k PGND 16k RANGE GND FB 1500pF 80.6k 22pF 3740 TA02a Efﬁciency vs Load Current 98 VIN = 7V EFFICIENCY (%) 96 94 VIN = 15V 92 VIN = 20V 90 88 86 84 82 80 VOUT = 3.
PAGE 18
LT3740 TYPICAL APPLICATIONS 2.5V/10A 22μH 1μF LT3740 SWB VIN 4V to 15V 22μF s2 BGDP BIAS VIN 1Ω TGATE SHDN M1 HAT2168H 1Ω SW 10k 1.05μH VOUT 2.5V SN+ XREF 1Ω 1μF BGATE 10k D1 B340A M2 HAT2165H 100μF s3 47pF SN– VC 5.1k 100k PGND 7.5k RANGE GND FB 680pF 46.4k 22pF 3740 TA03a EFFICIENCY 96 94 VIN = 4V EFFICIENCY (%) 92 VIN = 15V 90 88 86 84 82 VOUT = 2.
PAGE 19
LT3740 PACKAGE DESCRIPTION DHC Package 16-Lead Plastic DFN (5mm × 3mm) (Reference LTC DWG # 05-08-1706) 0.65 p 0.05 3.50 p 0.05 1.65 p 0.05 2.20 p 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 4.40 p 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 5.00 p 0.10 (2 SIDES) R = 0.20 TYP 3.00 p 0.10 (2 SIDES) 9 0.40 p 0.10 16 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) PIN 1 NOTCH (DHC16) DFN 110 8 0.200 REF 1 0.25 p 0.05 0.50 BSC 0.75 p 0.05 4.40 p 0.
PAGE 20
LT3740 TYPICAL APPLICATION High Efﬁciency Step-Down Converter VIN 7V to 20V 22μH LT3740 SWB BGDP BIAS VIN SHDN 20μF 1μF 1Ω TGATE 1Ω SW 15k 1Ω BGATE 10k VC 16k 3.4μH VOUT 3.3V 10A SN+ XREF 0.47μF M1 HAT2168H SN– D1 B340A M2 HAT2165H 2k 100μF s3 22pF 255k PGND RANGE GND FB 1500pF 80.