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LTC3864

3864f

TYPICAL APPLICATION

FEATURES

DESCRIPTION

60V Low I

Step-Down

DC/DC Controller with

100% Duty Cycle Capability

The LTC

3864 is a robust, high voltage step-down DC/DC

controller optimized for automotive and industrial applica-

tions. It drives a P-channel power MOSFET switch allowing

100% duty cycle operation. The wide input and output volt-

age ranges cover a multitude of applications. This device

has been verified with the failure mode and effects analysis

(FMEA) procedure for operation during failure conditions.

The LTC3864 offers excellent light load efficiency, draw-

ing only 40µA quiescent current in a user programmable

Burst Mode operation. Its peak current mode, constant

frequency PWM architecture provides for good control of

switching frequency and output current limit. The switch-

ing frequency can be programmed from 50kHz to 850kHz

with an external resistor and can be synchronized to an

external clock from 75kHz to 750kHz.

The LTC3864 offers programmable soft-start or output

tracking. Safety features include overvoltage protection,

overcurrent and short-circuit protection including fre-

quency foldback and a power good output signal.

The LTC3864 is available in thermally enhanced 12-Pin

MSOP and 3mm × 4mm DFN packages.

5.2V to 60V Input, 5V/2A Output, 350kHz Step-Down Converter

APPLICATIONS

Wide Operating V

Range: 3.5V to 60V

Wide V

OUT

Range: 0.8V to V

Low Operating I

= 40µA

Very Low Dropout Operation: 100% Duty Cycle

Strong High Voltage MOSFET Gate Driver

Constant Frequency Current Mode Architecture

Verified FMEA for Adjacent Pin Open/Short

Selectable High Efficiency Burst Mode

Operation or

Pulse-Skipping Mode at Light Loads

Programmable Fixed Frequency: 50kHz to 850kHz

Phase-Lockable Frequency: 75kHz to 750kHz

Accurate Current Limit

Programmable Soft-Start or Voltage Tracking

Internal Soft-Start Guarantees Smooth Start-Up

Power Good Output Voltage Monitor

Low Shutdown I

= 7µA

Available in Small 12-Pin Thermally Enhanced MSOP

and DFN Packages

Industrial and Automotive Power Supplies

Telecom Power Supplies

Distributed Power Systems

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

trademarks and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents including 5731694.

Efficiency

350kHz

100k

25mΩ

10µH

CAP

0.1µF

10µF

PGND

LTC3864

3864 TA01a

ITH

FREQ

SGND

RUN

PLLIN/MODE

SENSE

GATE

PGOOD

9.09k

422k

47µF

×2

OUT

80.6k

5.2V TO 60V

3.3nF

OUT

FOLLOWS V

WHEN 3.5V ≤ V

≤ 5.2V

LOAD CURRENT (A)

0.01

EFFICIENCY (%)

100

0.1 1

3864 TA01b

PULSE-SKIPPING

Burst Mode

OPERATION

= 12V

OUT

= 5V

Summary of content (28 pages)

PAGE 1
LTC3864 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability DESCRIPTION FEATURES n n n n n n n n n n n n n n n n Wide Operating VIN Range: 3.5V to 60V Wide VOUT Range: 0.
PAGE 2
LTC3864 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN).......................... –0.3V to 65V VIN-VSENSE Voltage....................................... –0.3V to 6V VIN-VCAP Voltage......................................... –0.3V to 10V RUN Voltage............................................... –0.3V to 65V PGOOD, PLLIN/MODE Voltages.................... –0.3V to 6V SS, ITH, FREQ, VFB Voltages......................... –0.3V to 5V Operating Junction Temperature Range (Notes 2, 3) LTC3864E,I ....
PAGE 3
LTC3864 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply VIN Input Voltage Operating Range VUVLO Undervoltage Lockout IQ Input DC Supply Current 3.5 (VIN-VCAP) Ramping Up Threshold (VIN-VCAP) Ramping Down Threshold Hysteresis l l 3.25 3.00 60 V 3.50 3.25 0.25 3.8 3.
PAGE 4
LTC3864 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS RUP Gate Pull-Up Resistance Gate High 2 Ω RDN Gate Pull-Down Resistance Gate Low 0.9 Ω 0.
PAGE 5
LTC3864 TYPICAL PERFORMANCE CHARACTERISTICS Pulse-Skipping Mode Operation Waveforms TA = 25°C, unless otherwise noted.
PAGE 6
LTC3864 TYPICAL PERFORMANCE CHARACTERISTICS Overcurrent Protection SHORTCIRCUIT TRIGGER 1A 1A SHORT-CIRCUIT REGION VIN 20V/DIV VOUT 5V/DIV SOFT RECOVERY FROM SHORT IL 1A/DIV VOUT 500mV/DIV IL 2A/DIV VOUT DROOPS DUE TO REACHING CURRENT LIMIT 20ms/DIV VIN = 12V, VOUT = 5V FIGURE 8 CIRCUIT Pulse-Skipping Mode Input Current Over Input Voltage (No Load) 950 VIN = 12V, VOUT = 5V ILOAD = 0A FIGURE 8 CIRCUIT 65 2ms/DIV VIN = 12V, SURGE TO 48V VOUT = 5V ILOAD = 200mA, FIGURE 8 CIRCUIT 3864 G11 IVIN (µA
PAGE 7
LTC3864 TYPICAL PERFORMANCE CHARACTERISTICS Free Running Frequency Over Input Voltage Free Running Frequency Over Temperature FREQ = OPEN 550 100 500 f (kHz) 500 f (kHz) 120 FREQUENCY FOLDBACK % FREQ = OPEN 550 450 450 400 400 FREQ = 0V 350 0 20 10 30 VIN (V) FREQ = 0V 350 40 50 300 –75 60 25 75 125 TEMPERATURE (°C) –25 0.1 –0.3 –0.4 –3.0 5 10 IGATE (mA) 15 –0.5 20 0 5 10 IGATE (mA) 15 20 92 25 75 125 TEMPERATURE (°C) 70 60 50 40 30 20 10 175 3864 G25 0 0.4 0.8 1.
PAGE 8
LTC3864 PIN FUNCTIONS PLLIN/MODE (Pin 1): External Reference Clock Input and Burst Mode Enable/Disable. When an external clock is applied to this pin, the internal phase-locked loop will synchronize the turn-on edge of the gate drive signal with the rising edge of the external clock. When no external clock is applied, this input determines the operation during light loading. Floating this pin selects low IQ (40μA) Burst Mode operation. Pulling to ground selects pulse-skipping mode operation.
PAGE 9
LTC3864 FUNCTIONAL DIAGRAM VIN UVLO + – 0.4µA CIN VIN 3.25V RSENSE SENSE RUN RUN 1.26V + – – LOGIC CONTROL DRV GATE MP L Q S R CLOCK PLLIN/MODE 20µA FREQ MODE/CLOCK DETECT + PLL SYSTEM – VCO ICMP VOUT COUT IN Burst Mode OPERATION LDO OUT VIN – 8V O.425V – 10µA + + – SGND VOUT SLOPE COMPENSATION RPGD OV PGOOD + UV PGND SS 0.8V CSS EA (Gm = 1.8mS) O.88V – DELAY 100µs RFB2 VFB + – D1 + + RFREQ CCAF CAP O.
PAGE 10
LTC3864 OPERATION Main Control Loop (Refer to Functional Diagram) Shutdown and Soft-Start The LTC3864 uses a peak current-mode control architecture to regulate the output in an asynchronous step-down DC/DC switching regulator. The VFB input is compared to an internal reference by a transconductance error amplifier (EA). The internal reference can be either a fixed 0.8V reference VREF or the voltage input on the SS pin. In normal operation VFB regulates to the internal 0.8V reference voltage.
PAGE 11
LTC3864 OPERATION the internal sleep signal goes high, enabling sleep mode. The ITH pin is then disconnected from the output of the error amplifier and held at 0.45V. In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current to 40µA while the load current is supplied by the output capacitor. As the output voltage and hence the feedback voltage decreases, the error amplifier’s output will rise.
PAGE 12
LTC3864 APPLICATIONS INFORMATION The LTC3864 is a current mode, constant frequency PWM controller for an asynchronous step-down DC/DC regulator with a P-channel power MOSFET acting as the main switch and a Schottky power diode acting as the commutating (catch) diode. The input range extends from 3.5V to 60V. The output range can be programmed from 0.8V to all the way up to VIN. The LTC3864 can transition from regulation to 100% duty cycle when the input voltage drops below the programmed output voltage.
PAGE 13
LTC3864 APPLICATIONS INFORMATION The free-running switching frequency can be programmed from 50kHz to 850kHz by connecting a resistor from FREQ to signal ground. The resulting switching frequency as a function of resistance on FREQ pin is shown in Figure 2. 1000 900 FREQUENCY (kHz) 800 700 A reasonable starting point for ripple current is 40% of IOUT(MAX) at nominal VIN. The largest ripple current occurs at the highest VIN.
PAGE 14
LTC3864 APPLICATIONS INFORMATION The peak inductor current limit is equal to:  95mV  IL(PEAK) ≅   RSENSE  This inductor current limit would translate to an output current limit based on the inductor ripple: I LIMIT ≅ 95mV ∆IL – RSENSE 2 The SENSE pin is a high impedance input with a maximum leakage of ±2µA. Since the LTC3864 is a peak current mode controller, noise on the SENSE pin can create pulse width jitter. Careful attention must be paid to the layout of RSENSE.
PAGE 15
LTC3864 APPLICATIONS INFORMATION generated by forcing a constant current out of the gate of a common-source connected P-MOSFET that is loaded with a resistor, and then plotting the gate voltage versus time. The initial slope is the effect of the gate-to-source and gate-to-drain capacitances. The flat portion of the curve is the result of the Miller multiplication effect of the drainto-gate capacitance as the drain voltage rises across the resistor load.
PAGE 16
LTC3864 APPLICATIONS INFORMATION Since ∆IL increases with input voltage, the output ripple is highest at maximum input voltage. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the necessary RMS current rating. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, specialty polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages.
PAGE 17
LTC3864 APPLICATIONS INFORMATION The current limit must be chosen to ensure that ILIMIT(MIN) > IOUT(MAX) under all operating conditions. The minimum current limit value should be greater than the inductor current required to produce maximum output power at worst case efficiency. Worst-case efficiency typically occurs at the highest VIN. Short-circuit fault protection is assured by the combination of current limit and frequency foldback. When the output feedback voltage VFB drops below 0.
PAGE 18
LTC3864 APPLICATIONS INFORMATION internal soft-start feature is active in all operational cases. If a short-circuit condition occurs which causes the output to drop significantly, the internal soft-start will assure a soft recovery when the fault condition is removed. The internal soft-start assures a clean soft ramp-up from any fault condition that causes the output to droop, guaranteeing a maximum ramp rate in soft-start, short-circuit fault release, or output recovery from drop out.
PAGE 19
LTC3864 APPLICATIONS INFORMATION Minimum On-Time Considerations The minimum on-time, tON(MIN), is the smallest time duration that the LTC3864 is capable of turning on the power MOSFET, and is typically 220ns. It is determined by internal timing delays and the gate charge required to turn on the MOSFET.
PAGE 20
LTC3864 APPLICATIONS INFORMATION but also provides a test point for the step-down regulator ’s DC-coupled and AC-filtered closed-loop response. The DC step, rise time and settling at this test point truly reflects the closed-loop response. Assuming a predominantly second order system, phase margin and/or damping factor can be estimated using the percentage of overshoot seen at this pin. The bandwidth can also be estimated by examining the rise time at this pin.
PAGE 21
LTC3864 APPLICATIONS INFORMATION Next, set the RSENSE resistor value to ensure that the converter can deliver a maximum output current of 2.0A with sufficient margin to account for component variations and worst-case operating conditions. Using a 30% margin factor: The FREQ pin is tied to signal ground in order to program the switching frequency to 350kHz.
PAGE 22
LTC3864 APPLICATIONS INFORMATION resulting junction temperature for the MOSFET can be calculated at TA = 70°C, VIN(MAX) = 55V and IOUT(MAX) = 2A: 5V (2A)2 •1.5 •105mΩ + 55V  0.9Ω  2Ω + (55V)2 •(2A / 2)•100pF •  • 350kHz  8V – 3V  3V ≈ 57mW + 90mW = 147mW PPMOS = TJ = 70°C+147mW • 60°C/W ≈ 80°C The calculations can be repeated for VIN(MIN) = 5V: 5V (2A)2 •1.5 •105mΩ + 5V 2Ω   0.9Ω • 350kHz (5.2V)2 •100pF •  +  5.
PAGE 23
LTC3864 APPLICATIONS INFORMATION the inductor currents without large copper losses. Place the sense resistor and source of P-channel MOSFET as close as possible to the (+) plate of CIN capacitor(s) that provides the bulk of the AC current (these are normally the ceramic capacitors), and connect the anode of the Schottky diode as close as possible to the (–) terminal of the same CIN capacitor(s).
PAGE 24
LTC3864 APPLICATIONS INFORMATION Table 2 FAILURE MODE None RECOVERY WHEN FAULT IS REMOVED? BEHAVIOR VOUT IOUT IVIN f 5V 1A 453mA 350kHz N/A Normal Operation. 5V 1A 453mA 350kHz OK Pin already left open in normal application, so no difference. Pin Open Open Pin 1 (PLLIN/MODE) Open Pin 2 (FREQ) 5V 1A 453mA 535kHz OK Frequency jumps to default open value. Open Pin 3 (GND) 5V 1A 453mA 350kHz OK Exposed pad still provides GND connection to device.
PAGE 25
LTC3864 TYPICAL APPLICATIONS 24V to 60V Input, 24V/1A Output at 750kHz Efficiency CCAP 0.1µF CIN2 2.2µF + RUN VIN MODE/PLLN CVIN 0.1µF VIN 24V TO 60V 90 80 EFFICIENCY (%) CAP CIN1 33µF 63V 100 RSENSE 50mΩ SENSE CITH1 6.8nF RITH 30.1k SS ITH MP GATE LTC3864 CITH2 100pF RFREQ 97.
PAGE 26
LTC3864 TYPICAL APPLICATIONS 12V to 58V Input, 12V/2A Output at 535kHz CCAP 0.1µF CIN2 4.7µF CAP CIN1 33µF 63V 90 CVIN 0.1µF MODE/PLLN RSENSE 30mΩ SENSE CITH1 3300pF SS RITH 11.3k LTC3864 MP GATE L1 22µH ITH CITH2 100pF D1 FREQ RPGD2 549k PGND CIN1: SANYO 63ME33AX D1: DIODES INC SBR3U100LP L1: TOKO 1217AS-H-220M MP: VISHAY/SILICONIX SI7465DP 10µF ×2 VOUT* 12V 2A VFB 70 PULSE-SKIPPING 60 VIN = 48V VOUT = 12V 50 0.
PAGE 27
LTC3864 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev F) 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 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 12 0.65 0.42 ±0.038 (.0256) (.0165 ±.
PAGE 28
LTC3864 TYPICAL APPLICATION 3.5V to 38V Input, 3.3V/3A Output at 300kHz CIN2 10µF ×2 CAP RUN CSS 0.1µF CITH1 10nF VIN CIN1 33µF 63V RSENSE 20mΩ LTC3864 ITH MP GATE VOUT 3.3V 47µF 3A ×2 L1 6.8µH RFREQ 42.2k SGND PGND RPGD 100k 60 PULSE-SKIPPING 40 0.01 RFB2 634k PGOOD FREQ 70 50 CITH2 100pF D1 Burst Mode OPERATION 80 SENSE SS RITH 20k CVIN 0.1µF MODE/PLLN + 90 EFFICIENCY (%) CCAP 0.1µF Efficiency VIN 3.5V TO 60V VIN = 12V VOUT = 3.3V 0.