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LTC3834

3834fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

30µA I

Synchronous

Step-Down Controller

High Efﬁ ciency Synchronous Step-Down Converter

Automotive Systems

Telecom Systems

Battery-Operated Digital Devices

Distributed DC Power Systems

L, LT, LTC, LTM, Burst Mode, PolyPhase 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 5408150, 5481178, 5705919, 5929620, 6304066,

6498466, 6580258, 6611131.

Wide Output Voltage Range: 0.8V ≤ V

OUT

≤ 10V

Low Operating Quiescent Current: 30μA

OPTI-LOOP

Compensation Minimizes C

OUT

±1% Output Voltage Accuracy

Wide V

Range: 4V to 36V

Phase-Lockable Fixed Frequency 140kHz to 650kHz

Dual N-Channel MOSFET Synchronous Drive

Very Low Dropout Operation: 99% Duty Cycle

Adjustable Output Voltage Soft-Start or Tracking

Output Current Foldback Limiting

Power Good Output Voltage Monitor

Clock Output for PolyPhase

Applications

Output Overvoltage Protection

Low Shutdown I

: 4μA

Internal LDO Powers Gate Drive from V

or V

OUT

Selectable Continuous, Pulse Skipping or

Burst Mode

Operation at Light Loads

Small 20-Lead TSSOP or 4mm × 5mm QFN Package

The LTC

3834 is a high performance step-down switching

regulator controller that drives an all N-channel synchro-

nous power MOSFET stage. A constant-frequency current

mode architecture allows a phase-lockable frequency of

up to 650kHz.

The 30μA no-load quiescent current extends operating

life in battery powered systems. OPTI-LOOP compensa-

tion allows the transient response to be optimized over

a wide range of output capacitance and ESR values. The

LTC3834 features a precision 0.8V reference and a power

good output indicator. The 4V to 36V input supply range

encompasses a wide range of battery chemistries.

The TRACK/SS pin ramps the output voltage during

start-up. Current foldback limits MOSFET heat dissipa-

tion during short-circuit conditions. A reduced feature set

version of the part (LTC3834-1) is available in a smaller,

lower pin count package.

Comparison of LTC3834 and LTC3834-1

PART #

CLKOUT/

PHASMD EXTV

PGOOD PACKAGES

LTC3834 Yes Yes Yes

FE20/4mm × 5mm QFN

LTC3834-1 No No No

GN16/3mm × 5mm DFN

OUT

3.3V

150μF

0.01μF

4V TO 36V

SGND

PLLIN/MODE

SENSE

–

CLKOUT

PLLLPF

RUN

PGOOD

TRACK/SS

INTV

EXTV

PGND

LTC3834

3.3μH

0.012Ω

68.1k

3834 TA01

150pF

560pF

54.2k

215k

4.7μF

0.22μF

BOOST

10μF

OUTPUT CURRENT (A)

EFFICIENCY (%)

POWER LOSS (mW)

0.0001 0.01 1

3834 TA01b

0.000001

1000

100

0.1

10000

100

Summary of content (28 pages)

PAGE 1
LTC3834 30µA IQ Synchronous Step-Down Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n n n Wide Output Voltage Range: 0.
PAGE 2
LTC3834 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN) ......................... 36V to –0.3V Topside Driver Voltage (BOOST) ................ 42V to –0.3V Switch Voltage (SW) ..................................... 36V to –5V INTVCC, (BOOST-SW), CLKOUT, PGOOD .. 8.5V to –0.3V RUN, TRACK/SS ......................................... 7V to –0.3V SENSE+, SENSE– Voltages ........................ 11V to –0.3V PLLIN/MODE, PHASMD, PLLLPF ......... INTVCC to –0.3V EXTVCC ...............................
PAGE 3
LTC3834 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 12V, VRUN = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.792 0.800 0.808 V –5 –50 nA 0.002 0.02 %/V 0.1 –0.1 0.5 –0.5 % % Main Control Loops VFB Regulated Feedback Voltage (Note 4); ITH Voltage = 1.
PAGE 4
LTC3834 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 12V, VRUN = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN VPGL PGOOD Voltage Low IPGOOD = 2mA IPGOOD PGOOD Leakage Current VPGOOD = 5V VPG PGOOD Trip Level VFB with Respect to Set Regulated Voltage VFB Ramping Negative VFB Ramping Positive TYP MAX UNITS 0.1 0.
PAGE 5
LTC3834 TYPICAL PERFORMANCE CHARACTERISTICS Load Step (Forced Continuous Mode) Load Step (Burst Mode Operation) VOUT 100mV/DIV AC COUPLED Load Step (Pulse Skipping Mode) VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 2A/DIV IL 2A/DIV IL 2A/DIV 20μs/DIV VOUT = 3.3V FIGURE 11 CIRCUIT TA = 25°C, unless otherwise noted. 20μs/DIV VOUT = 3.3V FIGURE 11 CIRCUIT 3834 G04 Inductor Current at Light Load Soft Start-Up 3834 G06 20μs/DIV VOUT = 3.
PAGE 6
LTC3834 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Current Sense Voltage vs ITH Voltage Cycle SENSE Pins Total Input Bias Current Maximum Current Sense Threshold vs Duty 80 6O 30 0 40 20 0 –30 –60 –90 –120 –150 –180 –210 –20 –240 –40 –270 –300 10% DUTY CYCLE 0 0.2 1.0 0.4 0.6 0.8 ITH PIN VOLTAGE (V) MAXIMUM CURRENT SENSE VOLTAGE (mV) 60 FORCED CONTINUOUS Burst Mode OPERATION (RISING) Burst Mode OPERATION (FALLING) PULSE SKIPPING INPUT BIAS CURRENT (μA) CURRENT SENSE THRESHOLD (mV) 100 1.
PAGE 7
LTC3834 TYPICAL PERFORMANCE CHARACTERISTICS SENSE Pins Total Input Bias Current vs Temperature 60 806 0 INPUT CURRENT (μA) 804 802 800 798 796 10 VOUT = 3.3V –30 –60 –90 –120 –150 –180 VOUT = 0V 8 6 4 –210 2 –240 794 0 15 30 45 60 TEMPERATURE (°C) 75 90 –270 –300 –45 –30 –15 0 15 30 45 60 TEMPERATURE (°C) 3834 G21 75 90 0 5 10 15 20 25 INPUT VOLTAGE (V) 3834 G22 Oscillator Frequency vs Temperature 30 35 3834 G23 Undervoltage Lockout Threshold vs Temperature 4.2 800 4.
PAGE 8
LTC3834 PIN FUNCTIONS (FE/UFD) CLKOUT (Pin 1/Pin 19): Open-Drain Output Clock Signal available to daisy-chain other controller ICs for additional MOSFET driver stages/phases. PLLLPF (Pin 2/Pin 20): The phase-locked loop’s lowpass ﬁlter is tied to this pin when synchronizing to an external clock. Alternatively, tie this pin to GND, VIN or leave ﬂoating to select 250kHz, 530kHz or 400kHz switching frequency. ITH (Pin 3/Pin 1): Error Ampliﬁer Outputs and Switching Regulator Compensation Points.
PAGE 9
LTC3834 PIN FUNCTIONS (FE/UFD) operation. Tying this pin to a voltage greater than 0.9V and less than INTVCC selects pulse-skipping operation. Exposed Pad (Pin 21/Pin 21): SGND. Must be soldered to the PCB. PHASMD (Pin 20/Pin 18): Control Input to Phase Selector which determines the phase relationships between TG and the CLKOUT signal. FUNCTIONAL DIAGRAM PLLIN/ MODE FIN PHASE DET INTVCC VIN PHASMD BOOST DB RLP PLLLPF CLP DROP OUT DET CLK OSCILLATOR INTVCC 10k CLKOUT – 0.
PAGE 10
LTC3834 OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3834 uses a constant-frequency, current mode step-down architecture. During normal operation, the external 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 trips and resets the latch is controlled by the voltage on the ITH pin, which is the output of the error ampliﬁer EA.
PAGE 11
LTC3834 OPERATION (Refer to Functional Diagram) When the RUN pin is pulled low to disable the LTC3834, or when VIN drops below its undervoltage lockout threshold of 3.7V, the TRACK/SS pin is pulled low by an internal MOSFET. When in undervoltage lockout, the controller is disabled and the external MOSFETs are held off.
PAGE 12
LTC3834 OPERATION Frequency Selection and Phase-Locked Loop (PLLLPF and PLLIN/MODE Pins) The selection of switching frequency is a trade-off between efﬁciency and component size. Low frequency operation increases efﬁciency by reducing MOSFET switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The switching frequency of the LTC3834’s controllers can be selected using the PLLLPF pin.
PAGE 13
LTC3834 APPLICATIONS INFORMATION RSENSE Selection For Output Current RSENSE is chosen based on the required output current. The current comparator has a maximum threshold of 100mV/RSENSE and an input common mode range of SGND to 10V. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-to-peak ripple current, ΔIL.
PAGE 14
LTC3834 APPLICATIONS INFORMATION exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Power MOSFET and Schottky Diode (Optional) Selection Two external power MOSFETs must be selected for the LTC3834: one N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the INTVCC voltage.
PAGE 15
LTC3834 APPLICATIONS INFORMATION CIN and COUT Selection VOUT In continuous mode, the source current of the top MOSFET is a square wave of duty cycle (VOUT)/(VIN). To prevent large voltage transients, a low ESR capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement is satisﬁed, the capacitance is adequate for ﬁltering.
PAGE 16
LTC3834 APPLICATIONS INFORMATION according to the voltage on the TRACK/SS pin, allowing VOUT to rise smoothly from 0V to its ﬁnal regulated value. The total soft-start time will be approximately: Tracking and Soft-Start (TRACK/SS Pin) The start-up of VOUT is controlled by the voltage on the TRACK/SS pin. When the voltage on the TRACK/SS pin is less than the internal 0.8V reference, the LTC3834 regulates the VFB pin voltage to the voltage on the TRACK/SS pin instead of 0.8V.
PAGE 17
LTC3834 APPLICATIONS INFORMATION INTVCC Regulators The LTC3834 features two separate internal P-channel low dropout linear regulators (LDO) that supply power at the INTVCC pin from either the VIN supply pin or the EXTVCC pin, respectively, depending on the connection of the EXTVCC pin. INTVCC powers the gate drivers and much of the LTC3834’s internal circuitry. The VIN LDO regulates the voltage at the INTVCC pin to 5.25V and the EXTVCC LDO regulates it to 7.5V.
PAGE 18
LTC3834 APPLICATIONS INFORMATION Topside MOSFET Driver Supply (CB, DB) Fault Conditions: Current Limit and Current Foldback External bootstrap capacitors, CB, connected to the BOOST pins supply the gate drive voltages for the topside MOSFET. Capacitor CB in the Functional Diagram is charged though external diode DB from INTVCC when the SW pin is low. When the topside MOSFET is to be turned on, the driver places the CB voltage across the gate-source of the desired MOSFET.
PAGE 19
LTC3834 APPLICATIONS INFORMATION Phase-Locked Loop and Frequency Synchronization The LTC3834 has a phase-locked loop (PLL) comprised of an internal voltage-controlled oscillator (VCO) and a phase detector. This allows the turn-on of the top MOSFET (TG) to be locked to the rising edge of an external clock signal applied to the PLLIN/MODE pin. The phase detector is an edge sensitive digital type that provides zero degrees phase shift between the external and internal oscillators.
PAGE 20
LTC3834 APPLICATIONS INFORMATION Table 2 summarizes the different states in which the PLLLPF pin can be used. Table 2 PLLLPF PIN PLLIN/MODE PIN FREQUENCY 0V DC Voltage 250kHz Floating DC Voltage 400kHz INTVCC DC Voltage 530kHz RC Loop Filter Clock Signal Phase-Locked to External Clock Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3834 is capable of turning on the top MOSFET.
PAGE 21
LTC3834 APPLICATIONS INFORMATION a 5V output, or a 4% to 20% loss for a 3.3V output. Efﬁciency varies as the inverse square of VOUT for the same external components and output power level. The combined effects of increasingly lower output voltages and higher currents required by high performance digital systems is not doubling but quadrupling the importance of loss terms in the switching regulator system! 4.
PAGE 22
LTC3834 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. The power dissipation on the topside MOSFET can be easily estimated.
PAGE 23
LTC3834 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 9. The Figure 10 illustrates the current waveforms present in the various branches of the synchronous regulator operating in the continuous mode. Check the following in your layout: side” of the LTC3834 and occupy minimum PC trace area. 7.
PAGE 24
LTC3834 APPLICATIONS INFORMATION high current output loading at lower input voltages, look for inductive coupling between CIN, Schottky and the top MOSFET components to the sensitive current and voltage sensing traces. In addition, investigate common ground path voltage pickup between these components and the SGND pin of the IC. when the current sensing leads are hooked up backwards.
PAGE 25
LTC3834 TYPICAL APPLICATIONS High Efﬁciency 9.5V, 3A Step-Down Converter INTVCC 100k CLKOUT VIN CIN 10μF PLLLPF 100k 0.01μF TG RUN PGOOD TRACK/SS ITH 39.2k L1 7.2μH 0.015Ω SW LTC3834 100pF CB 0.22μF BOOST 560pF 105k M1 VIN 10V TO 36V DB CMDSH-3 SGND INTVCC PLLIN/MODE EXTVCC VFB VOUT 9.5V 3A COUT 150μF 4.7μF M2 BG SENSE– 432k SENSE+ 22pF PGND 3834 TA02 M1, M2: Si4840DY L1: CDEP105-7R2M COUT: SANYO 10TPD150M High Efﬁciency 12V to 1.
PAGE 26
LTC3834 TYPICAL APPLICATIONS High Efﬁciency 5V, 5A Step-Down Converter CLKOUT VIN PLLLPF TG RUN 0.01μF PGOOD TRACK/SS ITH 69.8k L1 3.3μH 0.012Ω SW DB CMDSH-3 LTC3834 150pF M1 BOOST 560pF 54k CB 0.22μF VIN CIN 5.5V TO 36V 10μF SGND INTVCC PLLIN/MODE EXTVCC VFB VOUT 5V 5A COUT 150μF 4.7μF M2 BG SENSE– 365k 39pF SENSE+ PGND 3834 TA04 M1, M2: Si4840DY L1: CDEP105-3R2M COUT: SANYO 10TPD150M High Efﬁciency 1.2V, 5A Step-Down Converter CLKOUT INTVCC GND 10k PLLLPF TG RUN 0.
PAGE 27
LTC3834 PACKAGE DESCRIPTION FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation CB 6.40 – 6.60* (.252 – .260) 3.86 (.152) 3.86 (.152) 20 1918 17 16 15 14 13 12 11 6.60 p 0.10 2.74 (.108) 4.50 p 0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 p 0.05 1.05 p0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 1.20 (.047) MAX 0o – 8o 0.65 (.0256) BSC 0.50 – 0.75 (.020 – .
PAGE 28
LTC3834 TYPICAL APPLICATION CLKOUT PLLLPF RUN PGOOD TRACK/SS 0.01μF VIN TG CB 0.22μF M1 L1 3.2μH BOOST ITH 68.1k 0.012Ω VOUT 3.3V 5A COUT 150μF DB CMDSH-3 LTC3834 150pF VIN 4V TO 36V SW 560pF 54k CIN 10μF SGND INTVCC PLLIN/MODE EXTVCC VFB BG 4.7μF M2 SENSE– 215k SENSE+ 39pF PGND 3834 F11 M1, M2: Si4840DY L1: CDEP105-2R2M COUT: SANYO 10TPD150M Figure 11.