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LTC3854

3854fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Small Footprint, Wide

Range Synchronous

Step-Down DC/DC Controller

The LTC3854

is a high performance synchronous step-

down switching DC/DC controller that drives an all N-

channel synchronous power MOSFET stage.

The LTC3854 features a 400kHz constant frequency current

mode architecture. The LTC3854 operates from a 4.5V to

38V (40V absolute maximum) input voltage range and

regulates the output voltage from 0.8V to 5.5V.

The RUN/SS pin provides both soft-start and enable

features. OPTI-LOOP compensation allows the transient

response to be optimized over a wide range of output ca-

pacitance and ESR values. Current foldback limits MOSFET

dissipation during short circuit conditions. Current foldback

functions are disabled during soft-start.

The LTC3854 has a minimum on-time at 75ns, making it

well suited for high step-down ratios. The strong onboard

MOSFET drivers allow the use of high power external

MOSFETs to produce output currents up to 20A.

High Efﬁciency Synchronous Step-Down Converter

Wide Operating V

Range: 4.5V to 38V

SENSE

or DCR Current Sensing

±1% 0.8V Reference Accuracy Over Temperature

400kHz Switching Frequency

Dual N-channel MOSFET Synchronous Drive

Very Low Dropout Operation: 97% Duty Cycle

Starts Up Into Pre-Biased Output

Adjustable Output Voltage Soft-Start

Output Current Foldback Limiting

(Disabled During Soft-Start)

Output Overvoltage Protection

5V LDO for External Gate Drive

OPTI-LOOP

Compensation Minimizes C

OUT

Low Shutdown I

: 15µA

Tiny Thermally Enhanced 12-Pin 2mm × 3mm DFN

and MSOP Packages

Automotive Systems

Telecom Systems

Industrial Equipment

Distributed DC Power Systems

3854 TA01

LTC3854

0.1µF0.1µF

47µF

50V

4.7µF

100pF

2200pF

4.7µH

AT 8.2mΩ DCR

150µF

OUT

6V TO 38V

42.2k

8.06k

10k

BOOST

INTV

GND

RUN/SS

ITH

SENSE

–

SENSE

0.22µF

5.49k

4.99k

Efﬁciency and Power Loss

vs Load Current

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

2.0

1.6

0.8

1.2

0.4

3 4 5 6

3854 TA01b

EFFICIENCY

= 12V

POWER LOSS

L, LT, LTC, LTM, Linear Technology, the Linear logo 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 5705919, 6498466, 5408150, 6222231.

Summary of content (28 pages)

PAGE 1
LTC3854 Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n Wide Operating VIN Range: 4.5V to 38V RSENSE or DCR Current Sensing ±1% 0.
PAGE 2
LTC3854 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN)......................... 40V to –0.3V Top Side Driver Voltage (BOOST)................ 46V to –0.3V Switch Voltage (SW)................................... 40V to –5.0V INTVCC, BOOST-SW..................................... 6V to –0.3V SENSE+, SENSE– .......................................... 6V to –0.3V RUN/SS......................................................... 6V to –0.3V ITH, FB Voltages........................................ 2.
PAGE 3
LTC3854 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 = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 38 V 0.8 0.808 V ±5 ±50 nA Main Control Loop VIN Operating Input Voltage Range VFB Regulated Feedback Voltage (Note 4); ITH Voltage = 1.2V 4.
PAGE 4
LTC3854 ELECTRICAL CHARACTERISTICS Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3854E is guaranteed to meet performance speciﬁcations from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls.
PAGE 5
LTC3854 TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms at High Duty Cycle, No Load Switching Waveforms at No Load VOUT = 2.5V VOUT = 5V TG TG BG BG IL IL 1µs/DIV Quiescent Current vs VIN Quiescent Current vs Temperature 1.30 1.14 1.13 1.12 1.11 1.10 1.15 1.10 1.05 1.00 6 11 16 26 21 VIN (V) 31 4.90 4.85 0.90 –45 –25 36 –5 15 35 55 75 TEMPERATURE (°C) 3854 G08 4.
PAGE 6
LTC3854 TYPICAL PERFORMANCE CHARACTERISTICS Oscillator Frequency vs Temperature Undervoltage Lockout Falling Threshold vs Temperature 440 440 3.9 430 3.8 400 FREQUENCY (kHz) 410 3.6 3.5 3.4 3.3 0 20 40 60 80 TEMPERATURE (°C) 3.0 –40 –20 100 120 0 3854 G15 360 100 120 19 20 10 15 10 0 5 –10 0 0 0.5 1 1.5 VITH (V) 2 2.5 3 SHUTDOWN CURRENT (µA) 30 20 4 8 12 16 3854 G18 INTVCC Load Regulation 5.20 5.15 5.10 5.10 5.05 5.05 INTVCC (V) 4.95 4.85 4.
PAGE 7
LTC3854 PIN FUNCTIONS FB (Pin 1): Error Amplifier Feedback Input. This pin receives the remotely-sensed feedback voltage from an external resistor divider across the output. ITH (Pin 2): Error Amplifier Output and Switching Regulator Compensation Point. The current comparator trip point increases with this control voltage. RUN/SS (Pin 3): Run Control, Soft-Start. If the voltage on this pin is held below 0.4V, the part is in shutdown.
PAGE 8
LTC3854 FUNCTIONAL DIAGRAM VIN 1.2V 0.88V 0.80V BANDGAP/ LDO INTVCC BOOST MAIN SWITCH DRIVER OSCILLATOR INTVCC UVLO TG DRIVE CONTROL AND ANTI-SHOOT THROUGH SW SYNCHRONOUS SWITCH DRIVER BG ON GND CLK S Q + R SLOPE COMP GENERATOR SENSE– SENSE+ V TO I CONVERTER VSENSE → ICS OV – CURRENT SENSE COMPARATOR VFB 0.88V TG_OFF + + RUN ICMP – – + 1.2V SS – – 0.4V 1.25µA SD + ITH ERROR AMP VITH + + – LEVEL SHIFT RUN/SS 0.
PAGE 9
LTC3854 OPERATION Main Control Loop Shutdown and Start-Up (RUN/SS) The LTC3854 is a constant-frequency, peak current mode step-down controller. During normal operation, the top MOSFET is turned on when the clock sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA.
PAGE 10
LTC3854 APPLICATIONS INFORMATION The LTC3854 can be configured to use either DCR (inductor winding resistance) sensing or low value resistor sensing. The choice of the two current sensing schemes is largely a design tradeoff between cost, power consumption, and accuracy. DCR sensing is becoming popular because it eliminates expensive current sensing resistors and is more power efficient, especially in high current applications.
PAGE 11
LTC3854 APPLICATIONS INFORMATION VIN CIN VIN RUN/SS M1 TG CB CSS ITH CC CC2 BOOST LTC3854 DBOOST RC RFB1 SW FB RFB2 INDUCTOR INTVCC L DCR VOUT CVINT SENSE– BG SENSE+ GND M2 R1 C1 COUT R2 3854 F02 Figure 2. Buck Regulator Using DCR Current Sense capacitor is equal to the voltage drop across the inductor DCR • R2/(R1+R2). R2 may be used to scale the voltage across the same terminals when the DCR is greater than the target sense resistance.
PAGE 12
LTC3854 APPLICATIONS INFORMATION Inductor Core Selection Once the value for L is determined, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or molypermalloy cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses decrease.
PAGE 13
LTC3854 APPLICATIONS INFORMATION An optional Schottky diode connected from GND (anode) to the SW node (cathode) conducts during the dead time between the conduction of the two power MOSFETs. This prevents the body diode of the bottom MOSFET from turning on, storing charge during the dead time and requiring a reverse recovery period that could cost as much as 3% in efficiency at high VIN. A 1A to 3A Schottky is generally a good size due to the relatively small average current.
PAGE 14
LTC3854 APPLICATIONS INFORMATION capacitor CB needs to be at least 100 times that of the total input capacitance of the topside MOSFET. 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. If there is no change in input current, then there is no change in efficiency.
PAGE 15
LTC3854 APPLICATIONS INFORMATION The required ESR due to a load current step is: ESR ≤ ∆V ∆I where ∆I is the change in current from full load to zero load (or minimum load) and ∆V is the allowed voltage deviation (not including any droop due to finite capacitance). The amount of capacitance needed is determined by the maximum energy stored in the inductor. The capacitance must be sufficient to absorb the change in inductor current when a high current to low current transition occurs.
PAGE 16
LTC3854 APPLICATIONS INFORMATION sense voltage is progressively lowered from its maximum programmed value to 25% of the maximum value. Foldback current limiting is disabled during soft-start. large currents when VIN is in the normal operating range yet still provide an adequate safety margin and good overall performance in dropout with a slow ramping VIN.
PAGE 17
LTC3854 APPLICATIONS INFORMATION I2R losses. For example, if each RDS(ON) = 10mΩ, DCR = 10mΩ, RSENSE = 5mΩ then the total resistance is 25mΩ. This results in losses ranging from 2% to 8% as the output current increases from 3A to 15A for a 5V output, or a 3% to 12% loss for a 3.3V output. Efficiency varies as the inverse square of VOUT for the same external components and output power level.
PAGE 18
LTC3854 APPLICATIONS INFORMATION sudden step change in output voltage if the load switch resistance is low and it is driven quickly. If the ratio of CLOAD to COUT is greater than 1:50, the switch rise time should be controlled so that the load rise time is limited to approximately 25 • CLOAD. Thus a 10μF capacitor would require a 250μs rise time, limiting the charging current to about 200mA.
PAGE 19
LTC3854 APPLICATIONS INFORMATION 3) 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 LTC3854. Ensure accurate current sensing with Kelvin connections as shown in Figure 5. Series resistance can be added to the SENSE lines to increase noise rejection.
PAGE 20
LTC3854 APPLICATIONS INFORMATION Design Example Consider the design of a 1.2V, 15A buck regulator with a VIN range of 4.5V to 28V using a DCR sensing scheme. Inductor Selection Assuming an inductor ripple of 40% of IOUT, L can be calculated for the worst case of VIN = VIN(MAX). LMIN =   V 1 • VOUT  1− OUT  ∆IL • fSW  VIN(MAX)  1  1.2V  • 1.2V •  1−  0.40 • 15A • 400kHz 20V  LMIN = 0.47µH LMIN = Next, determine the DCR of the inductor.
PAGE 21
LTC3854 APPLICATIONS INFORMATION However, the amount of capacitance needed is determined not only by the allowed ripple in steady state but by the maximum energy stored in the inductor. The capacitance must be sufficient in value to absorb the change in inductor current when a high current to low current transient occurs. The minimum capacitance to assure the inductor’s energy is adequately absorbed during a 5A load step for a maximum overshoot of 2% is: L • ∆IL 2 COUT ≥ 2 • ∆VOUT • VOUT 0.56µH • (5A)2 0.
PAGE 22
LTC3854 APPLICATIONS INFORMATION Choosing an RJK0330DPB for the bottom FET will provide: BVDSS = 30V RDS(ON) = 3.9mΩ maximum at 25°C, VGS = 4.5V Given a typical QG of 8nC for the RJK0305DPB and 27nC for the RJK0330DPB and the 400kHz switching frequency, the current supplied by INTVCC will be: QG = 27nC, typical, at VGS = 4.5V PSYNC = Some airflow may be required for higher ambient temperatures. A maximum MOSFET junction temperature of 110°C at worst case ambient generally provides adequate margin.
PAGE 23
LTC3854 TYPICAL APPLICATIONS VIN M1 LTC3854 TG 0.1µF RUN/SS ITH 2200pF 3.9k 47µF 25V ×2 0.1µF BOOST L 0.56µH SW 100pF VIN 4.5V TO 14V 180µF 25V RSENSE 2mΩ D1 + INTVCC 20k 4.7µF FB 17.4k SENSE– BG SENSE+ GND M2 VOUT 1.5V 15A COUT1 330µF ×2 COUT2 47µF ×2 1nF 47Ω 47Ω D1 = CMDSH-3 M1 = RENESAS RJK0305DPB M2 = RENESAS RJK0330DPB L = TOKO FDA1055-R56M COUT1 = SANYO 2R5TPE330M9 COUT2 = MURATA GRM31CR60J476K 3854 TA02 Figure 7. 1.
PAGE 24
LTC3854 TYPICAL APPLICATIONS VIN LTC3854 0.1µF RUN/SS 2200pF 15k 0.1µF SW 100pF 47µF 25V ×2 L 0.22µH, 1.8mΩ DCR BOOST ITH 180µF 25V M1 TG VIN 4.5V TO 14V D1 INTVCC + 0.1µF 1.21k 4.7µF 32.4k FB COUT2 47µF ×2 M2 BG SENSE– 8.06k SENSE+ VOUT 1V 20A COUT1 330µF ×2 GND 3854 TA03 D1 = CMDSH-3 M1, M2 = VISHAY Si7866ADP L = SUMIDA CDEP104NP-0R2NC COUT1 = SANYO 2RSTPE330M9 COUT2 = MURATA GRM31CR60J476K Figure 8. 1.0V/20A DCR Sense Application VIN M1 LTC3854 TG 0.
PAGE 25
LTC3854 PACKAGE DESCRIPTION DDB Package 12-Lead Plastic DFN (Reference LTC DWG # 05-08-1723 Rev Ø) 0.64 ±0.05 (2 SIDES) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 2.39 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ±0.10 (2 SIDES) R = 0.05 TYP R = 0.115 TYP 7 0.40 ± 0.10 12 2.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.64 ± 0.10 (2 SIDES) 6 0.23 ± 0.05 0 – 0.05 PIN 1 R = 0.
PAGE 26
LTC3854 PACKAGE DESCRIPTION MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev D) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ± 0.102 (.112 ± .004) 5.23 (.206) MIN 2.845 ± 0.102 (.112 ± .004) 0.889 ± 0.127 (.035 ± .005) 6 1 1.651 ± 0.102 3.20 – 3.45 (.065 ± .004) (.126 – .136) 0.12 REF 12 0.65 0.42 ± 0.038 (.0256) (.0165 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 4.039 ± 0.102 (.159 ± .
PAGE 27
LTC3854 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 10/09 Edits to Typical Application Updated Efficiency Graph Edit to Electrical Characteristics and Notes Text Changes to Pin Functions B 2/11 1 1 3, 4 7 Change to Functional Diagram 8 Updated Related Parts Table 28 Updated TG RDOWN and BG RDOWN Typical values 3 Updated MSE package from Rev B to Rev D 26 3854fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
PAGE 28
LTC3854 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3851A/ LTC3851A-1 No RSENSE™ Wide VIN Range Synchronous Step-Down DC/DC Controller Phase-Lockable Fixed Operating Frequency, 250kHz to 750kHz, 4V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 5.25V, MSOP-16E, 3mm × 3mm QFN-16, SSOP-16 LTC3878 No RSENSE Constant On-Time Synchronous Step-Down DC/DC Controller Very Fast Transient Response, tON(MIN) = 43ns, 4V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 0.