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LTC3891

3891fa

TYPICAL APPLICATION

FEATURES DESCRIPTION

Low I

, 60V Synchronous

Step-Down Controller

The LTC

3891 is a high performance step-down switching

regulator DC/DC controller that drives an all N-channel

synchronous power MOSFET stage. A constant fre-

quency current mode architecture allows a phase-lockable

frequency of up to 750kHz.

The 50μA no-load quiescent current extends operating run

time 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

LTC3891 features a precision 0.8V reference and power

good output indicator. A wide 4V to 60V input supply range

encompasses a wide range of intermediate bus voltages

and battery chemistries. The output voltage of the LTC3891

can be programmed between 0.8V to 24V.

The TRACK/SS pin ramps the output voltages during

start-up. Current foldback limits MOSFET heat dissipation

during short-circuit conditions. The PLLIN/MODE pin se-

lects among Burst Mode operation, pulse-skipping mode,

or continuous conduction mode at light loads.

Efficiency and Power Loss vs

Output Current

APPLICATIONS

Wide V

Range: 4V to 60V (65V Abs Max)

Low Operating I

: 50μA

Wide Output Voltage Range: 0.8V ≤ V

OUT

≤ 24V

SENSE

or DCR Current Sensing

Phase-Lockable Frequency (75kHz to 750kHz)

Programmable Fixed Frequency (50kHz to 900kHz)

Selectable Continuous, Pulse-Skipping or Low Ripple

Burst Mode

Operation at Light Load

Selectable Current Limit

Very Low Dropout Operation: 99% Duty Cycle

Adjustable Output Voltage Soft-Start or Tracking

Power Good Output Voltage Monitor

Output Overvoltage Protection

Low Shutdown I

: < 14μA

Internal LDO Powers Gate Drive from V

or EXTV

No Current Foldback During Start-Up

Small 20-Pin 3mm × 4mm QFN and TSSOP Packages

Automotive Always-On Systems

Battery Powered Digital Devices

Distributed DC Power Systems

L, LT, LTC, LTM, OPTI-LOOP, Burst Mode, Linear Technology and the Linear logo are registered

trademarks of Linear Technology Corporation. All other trademarks are the property of their

respective owners. Patents, including 5481178, 5705919, 6611131, 6498466, 6580258,

7230497.

0.1µF

22µF

4V TO 60V

41.2k

10k

2200pF

100k

31.6k

3891 TA01a

2.2µF

100pF

0.1µF

FREQ

ITH

SGND

LTC3891

INTV

4.7µH

8mΩ

INTV

BOOST

SENSE

–

100k

150µF

OUT

3.3V

TRACK/SS

PGOOD

OUTPUT CURRENT (A)

EFFICIENCY (%)

POWER LOSS (mW)

100

10.10.010.0010.0001 10

3891 TA01b

20 1

100

1000

10000

0.1

= 12V

OUT

= 3.3V

High Efficiency 3.3V Step-Down Converter

Summary of content (32 pages)

PAGE 1
LTC3891 Low IQ, 60V Synchronous Step-Down Controller FEATURES n n n n n n n n n n n n n n n n DESCRIPTION Wide VIN Range: 4V to 60V (65V Abs Max) Low Operating IQ: 50μA Wide Output Voltage Range: 0.
PAGE 2
LTC3891 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN).......................... –0.3V to 65V Topside Driver Voltage (BOOST)..................–0.3V to 71V Switch Voltage (SW)...................................... –5V to 65V (BOOST-SW)................................................. –0.3V to 6V RUN.............................................................. –0.3V to 8V Maximum Current Sourced into Pin from Source > 8V.......................................................
PAGE 3
LTC3891 ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3891EFE#PBF LTC3891EFE#TRPBF LTC3891FE 20-Lead Plastic TSSOP –40°C to 125°C LTC3891IFE#PBF LTC3891IFE#TRPBF LTC3891FE 20-Lead Plastic TSSOP –40°C to 125°C LTC3891HFE#PBF LTC3891HFE#TRPBF LTC3891FE 20-Lead Plastic TSSOP –40°C to 150°C LTC3891MPFE#PBF LTC3891MPFE#TRPBF LTC3891FE 20-Lead Plastic TSSOP –55°C to 150°C Consult LTC Marketing for parts specified with wider operati
PAGE 4
LTC3891 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2), VIN = 12V, VRUN = 5V, EXTVCC = 0V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Gate Driver TG Pull-Up On-Resistance Pull-Down On-Resistance 2.5 1.5 Ω Ω BG Pull-Up On-Resistance Pull-Down On-Resistance 2.4 1.
PAGE 5
LTC3891 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 LTC3891 is tested under pulsed load conditions such that TJ ≈ TA. The LTC3891E is guaranteed to meet performance specifications from 0°C to 85°C.
PAGE 6
LTC3891 TYPICAL PERFORMANCE CHARACTERISTICS Load Step Burst Mode Operation Load Step Pulse-Skipping Mode Load Step Forced Continuous Mode VOUT 100mV/DIV ACCOUPLED VOUT 100mV/DIV ACCOUPLED VOUT 100mV/DIV ACCOUPLED IL 2A/DIV IL 2A/DIV IL 2A/DIV 50µs/DIV LOAD STEP = 100mA TO 3A VIN = 12V VOUT = 3.3V FIGURE 12 CIRCUIT 3891 G04 50µs/DIV LOAD STEP = 100mA TO 3A VIN = 12V VOUT = 3.
PAGE 7
LTC3891 TYPICAL PERFORMANCE CHARACTERISTICS MAXIMUM CURRENT SENSE VOLTAGE (mV) 800 5% DUTY CYCLE 700 60 PULSE-SKIPPING MODE 40 Burst Mode OPERATION 20 ILIM = GND 0 ILIM = INTVCC ILIM = FLOAT –20 0 0.2 0.4 0.6 0.8 VITH (V) 1.0 1.2 600 500 400 300 200 100 0 FORCED CONTINUOUS MODE –100 1.4 0 5 10 15 25 20 VSENSE COMMON MODE VOLTAGE (V) 3891 G13 75 60 ILIM = INTVCC 50 40 30 ILIM = GND 20 10 0 100 200 300 400 500 600 FEEDBACK VOLTAGE (MV) VIN = 12V 65 60 55 50 45 40 20 12.
PAGE 8
LTC3891 TYPICAL PERFORMANCE CHARACTERISTICS SENSE– Pin Input Bias Current vs Temperature Shutdown Current vs Input Voltage 700 SENSE– CURRENT (µA) SHUTDOWN CURRENT (µA) VOUT > INTVCC + 0.5V 600 500 400 300 200 100 VOUT < INTVCC – 0.5V 0 –100 –75 –50 –25 30 600 25 550 20 500 FREQUENCY (kHz) 800 15 10 5 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 356 FALLING 3.7 3.
PAGE 9
LTC3891 PIN FUNCTIONS (QFN/eTSSOP) PLLIN/MODE (Pin 1/Pin 3): External Synchronization Input to Phase Detector and Forced Continuous Mode Input. When an external clock is applied to this pin, the phase-locked loop will force the rising TG signal to be synchronized with the rising edge of the external clock, and the regulator operates in forced continuous mode. When not synchronizing to an external clock, this input determines how the LTC3891 operates at light loads.
PAGE 10
LTC3891 PIN FUNCTIONS TRACK/SS (Pin 19/Pin 1): External Tracking and SoftStart Input. The LTC3891 regulates the VFB voltage to the smaller of 0.8V or the voltage on the TRACK/SS pin. An internal 10μA pull-up current source is connected to this pin. A capacitor to ground at this pin sets the ramp time to final regulated output voltage. Alternatively, a resistor divider on another voltage supply connected to this pin allows the LTC3891 output to track another supply during start-up.
PAGE 11
LTC3891 OPERATION Main Control Loop Shutdown and Start-Up (RUN, TRACK/SS Pins) The LTC3891 uses a constant frequency, current mode step-down architecture. During normal operation, the external top MOSFET is turned on when the clock for that channel 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 12
LTC3891 OPERATION Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Forced Continuous Mode) (PLLIN/MODE Pin) The LTC3891 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse-skipping mode, or forced continuous conduction mode at low load currents. To select Burst Mode operation, tie the PLLIN/MODE pin to SGND. To select forced continuous operation, tie the PLLIN/MODE pin to INTVCC.
PAGE 13
LTC3891 OPERATION LTC3891’s phase detector adjusts the voltage (through an internal lowpass filter) of the VCO input to align the turn-on of the controller’s external top MOSFET to the rising edge of the synchronizing signal. The VCO input voltage is prebiased to the operating frequency set by the FREQ pin before the external clock is applied.
PAGE 14
LTC3891 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3891 application circuit. LTC3891 can be configured to use either DCR (inductor resistance) sensing or low value resistor sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy. DCR sensing is becoming popular because it saves expensive current sensing resistors and is more power efficient, especially in high current applications.
PAGE 15
LTC3891 APPLICATIONS INFORMATION Low Value Resistor Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX) determined by the ILIM setting. The current comparator threshold voltage 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 16
LTC3891 APPLICATIONS INFORMATION The equivalent resistance R1 || R2 is scaled to the temperature inductance and maximum DCR: R1|| R2 = L (DCR at 20°C) • C1 The sense resistor values are: R1= R1|| R2 R1• RD ; R2 = RD 1– RD The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at the maximum input voltage: PLOSS R1= ( VIN(MAX) – VOUT ) • VOUT R1 Ensure that R1 has a power rating higher than this value.
PAGE 17
LTC3891 APPLICATIONS INFORMATION Power MOSFET and Schottky Diode (Optional) Selection Two external power MOSFETs must be selected for the LTC3891 controller: 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. This voltage is typically 5.1V during start-up (see EXTVCC Pin Connection). Consequently, logic-level threshold MOSFETs must be used in most applications.
PAGE 18
LTC3891 APPLICATIONS INFORMATION maximum RMS current of one channel must be used. The maximum RMS capacitor current is given by: CIN Required IRMS ≈ 1/2 IMAX  VOUT ) ( VIN – VOUT ) (1) (  VIN This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours of life.
PAGE 19
LTC3891 APPLICATIONS INFORMATION The rising and falling UVLO thresholds are calculated using the RUN pin thresholds:  R  VUVLO(RISING) = 1.21V 1+ B   RA   R  VUVLO(FALLING) = 1.16V 1+ B   RA  The resistor values should be carefully chosen such that the absolute maximum ratings of the RUN pin do not get violated over the entire VIN voltage range. pin of the slave supply (VOUT), as shown in Figure 7.
PAGE 20
LTC3891 APPLICATIONS INFORMATION INTVCC Regulators The LTC3891 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 depending on the connection of the EXTVCC pin. INTVCC powers the gate drivers and much of the LTC3891’s internal circuitry. The VIN LDO and the EXTVCC LDO regulate INTVCC to 5.1V.
PAGE 21
LTC3891 APPLICATIONS INFORMATION 4. EXTVCC Connected to an Output-Derived Boost Network. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage that has been boosted to greater than 4.7V. This can be done with the capacitive charge pump shown in Figure 8. Ensure that EXTVCC < VIN. CIN BAT85 VIN BAT85 MTOP NDS7002 TG LTC3891 EXTVCC L SW RSENSE BAT85 VOUT MBOT BG PGND COUT 3891 F08 Figure 8.
PAGE 22
LTC3891 APPLICATIONS INFORMATION The LTC3891 has an internal phase-locked loop (PLL) comprised of a phase frequency detector, a lowpass filter, and a voltage-controlled oscillator (VCO). This allows the turn-on of the top MOSFET 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 23
LTC3891 APPLICATIONS INFORMATION charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that: tON(MIN) < VOUT VIN ( f ) If the duty cycle falls below what can be accommodated by the minimum on-time, the controller 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 LTC3891 is approximately 95ns.
PAGE 24
LTC3891 APPLICATIONS INFORMATION Other hidden losses such as copper trace and internal battery resistances can account for an additional 5% to 10% efficiency degradation in portable systems. It is very important to include these system level losses during the design phase. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency.
PAGE 25
LTC3891 APPLICATIONS INFORMATION assumption. The highest value of ripple current occurs at the maximum input voltage. Tie the FREQ pin to GND, generating 350kHz operation. The minimum inductance for 30% ripple current is: ΔIL =  VOUT  V 1– OUT  ( f) (L)  VIN(NOM)  A 4.7μH inductor will produce 29% ripple current. The peak inductor current will be the maximum DC value plus one half the ripple current, or 5.73A.
PAGE 26
LTC3891 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 IC. Ensure accurate current sensing with Kelvin connections at the SENSE resistor. 4. Is the INTVCC decoupling capacitor connected close to the IC, between the INTVCC and the power ground pins? This capacitor carries the MOSFET drivers’ current peaks.
PAGE 27
LTC3891 APPLICATIONS INFORMATION LTC3891 TRACK/SS ILIM VIN VIN PGND CIN GND + FREQ PLLIN/MODE EXTVCC SGND INTVCC 1µF CERAMIC CINTVCC COUT + SGND BG RUN SENSE– C1* M1 BOOST SENSE+ M2 SW R1* VFB TG ITH PGOOD L1 ROUT PGOOD D1* RSENSE VOUT VPULL-UP 3891 F10 *R1, C1 AND D1 ARE OPTIONAL Figure 10. Recommended Printed Circuit Layout Diagram SW VIN RIN CIN D1 L1 RSENSE VOUT COUT RL1 3891 F11 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH.
PAGE 28
LTC3891 APPLICATIONS INFORMATION VIN 4V TO 60V CIN 22µF VIN 100k INTVCC LTC3891 PGOOD INTVCC 2.2µF PGND RUN ILIM D1 EXTVCC PLLIN/MODE 41.2k 2200pF BOOST L1 4.7µH RSENSE 8mΩ COUT 150µF MBOT BG ITH 100pF 0.1µF SW FREQ 10k MTOP TG VOUT 3.3V 5A SENSE+ 0.1µF TRACK/SS SGND 1nF SENSE– 100k VFB SGND 31.6k MTOP, MBOT: Si7850DP L1 COILCRAFT SER1360-472KL COUT: SANYO 6TPE470M D1: DFLS1100 3891 F12 Figure 12. High Efficiency 3.3V Step-Down Converter VIN 12.
PAGE 29
LTC3891 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UDC Package 20-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG # 05-08-1742 Rev Ø) 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 1.50 REF 2.65 ±0.05 1.65 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF 3.10 ±0.05 4.50 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ±0.10 0.75 ±0.05 1.50 REF 19 R = 0.05 TYP PIN 1 NOTCH R = 0.
PAGE 30
LTC3891 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev I) 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 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.
PAGE 31
LTC3891 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 10/12 Added INTVCC to Absolute Maximum Ratings 2 Updated FE package 30 3891fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 32
LTC3891 TYPICAL APPLICATION VIN 9V TO 60V CIN 22µF VIN 100k INTVCC PGOOD LTC3891 RUN INTVCC 2.2µF PGND ILIM VOUT EXTVCC PLLIN/MODE 470pF FREQ 34.8k ITH D1 TG BOOST MTOP 0.1µF L1 8µH SW RSENSE 10mΩ COUT 330µF MBOT BG VOUT 8.5V 3A SENSE+ 0.1µF TRACK/SS SGND SGND SENSE– 1nF 100k VFB 10.5k 3891 F14 MTOP, MBOT: Si7850DP L1 COILCRAFT SER1360-802KL COUT: SANYO 10TPE330M D1: DFLS1100 Figure 14. High Efficiency 8.