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LT3995

3995f

For more information www.linear.com/LT3995

TYPICAL APPLICATION

FEATURES DESCRIPTION

60V, 3A, 2MHz Step-Down

Switching Regulator with

2.7µA Quiescent Current

The LT

3995 is an adjustable frequency monolithic buck

switching regulator that accepts a wide input voltage range

up to 60V. Low quiescent current design consumes only

2.7µA of supply current while regulating with no load. Low

ripple Burst Mode operation maintains high efficiency at

low output currents while keeping the output ripple below

15mV in a typical application. The LT3995 can supply up

to 3A of load current and has current limit foldback to

limit power dissipation during short circuit. A low dropout

voltage of 500mV is maintained when the input voltage

drops below the programmed output voltage, such as

during automotive cold crank.

An internally compensated current mode topology is used

for fast transient response and good loop stability. A high

efficiency 85mΩ switch is included on the die along with a

boost Schottky diode and the necessary oscillator, control,

and logic circuitry. An accurate 1.02V threshold enable pin

can be driven directly from a microcontroller or used as a

programmable undervoltage lockout. A capacitor on the

SS pin provides a controlled inrush current (soft-start).

A power good flag signals when V

OUT

reaches 91.6% of

the programmed output voltage. The LT3995 is available

in a small 16-lead MSOP package with exposed pad for

low thermal resistance.

No-Load Supply Current

3.3V Step-Down Converter

APPLICATIONS

Ultralow Quiescent Current:

2.7µA I

at 12V

to 3.3V

OUT

Low Ripple Burst Mode

Operation

Output Ripple < 15mV

P-P

Wide Input Range: Operation from 4.3V to 60V

3A Maximum Output Current

Excellent Start-Up and Dropout Performance

Adjustable Switching Frequency: 200kHz to 2MHz

Synchronizable Between 250kHz to 2MHz

Accurate Programmable Undervoltage Lockout

Low Shutdown Current: I

= 700nA

Power Good Flag

Soft-Start Capability

Thermal Shutdown Protection

Current Limit Foldback with SS Override

Saturating Switch Design: 85mΩ On Resistance

Small, Thermally Enhanced 16-Lead MSOP Package

Automotive Battery Regulation

Portable Products

Industrial Supplies

L, LT, LTC, LTM, 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.

EN BOOSTOFF ON

4.3V TO 60V

0.47µF

47µF

1210

×2

PDS560

3995 TA01a

10nF

10µF

576k182k

f = 300kHz

OUT

3.3V

8.2µH

LT3995

OUT

SYNC GND

10pF

INPUT VOLTAGE (V)

1.0

INPUT CURRENT (µA)

1.5

2.5

3.0

3.5

30 35 40 5045 55

4.5

3995 TA01b

2.0

5 10 15 20 25 60

4.0

IN REGULATION

Summary of content (24 pages)

PAGE 1
LT3995 60V, 3A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current FEATURES DESCRIPTION Ultralow Quiescent Current: 2.7µA IQ at 12VIN to 3.3VOUT n Low Ripple Burst Mode® Operation Output Ripple < 15mVP-P n Wide Input Range: Operation from 4.
PAGE 2
LT3995 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN Voltage (Note 3)............................................60V BOOST Pin Voltage....................................................75V BOOST Pin Above SW Pin..........................................30V FB, RT, SYNC, SS Voltage............................................6V PG Voltage.................................................................30V OUT Voltage...............................................................
PAGE 3
LT3995 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
PAGE 4
LT3995 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency at 5VOUT TA = 25°C, unless otherwise noted. Efficiency at 3.3VOUT Efficiency at 5VOUT 95 85 90 90 80 80 85 75 70 80 75 70 65 12V 24V 36V 48V 60 L: MSS1260-682ML fSW = 500kHz 55 50 0.5 0 1.5 2 1 LOAD CURRENT (A) 70 65 60 55 12V 24V 36V 48V 50 L: MSS1260-822ML fSW = 300kHz 45 40 3 2.5 EFFICIENCY (%) 100 EFFICIENCY (%) 90 EFFICIENCY (%) 100 0 0.5 1.5 2 1 LOAD CURRENT (A) 60 50 40 12V 24V 36V 48V 10 0 0.01 0.
PAGE 5
LT3995 TYPICAL PERFORMANCE CHARACTERISTICS Thermal Derating LIMITED BY MAXIMUM JUNCTION TEMPERATURE θJA = 40°C/W 2.5 2.0 H-GRADE 12VIN 24VIN 36VIN 48VIN 1.0 60VIN 0.5 VOUT = 3.3V fSW = 300kHz 2.5in × 2.5in 4-LAYER BOARD 0 100 75 0 25 50 1.5 150 125 Switch Current Limit 7.0 LIMITED BY MAXIMUM JUNCTION TEMPERATURE θJA = 40°C/W 3.0 LOAD CURRENT (A) LOAD CURRENT (A) 3.0 Thermal Derating 3.5 6.5 2.5 CURRENT LIMIT (A) 3.5 TA = 25°C, unless otherwise noted. 2.0 H-GRADE 12VIN 24VIN 36VIN 48VIN 1.
PAGE 6
LT3995 TYPICAL PERFORMANCE CHARACTERISTICS LOAD = 2A 200 LOAD = 1A 175 150 125 100 –55 –25 65 35 95 5 TEMPERATURE (°C) 125 300 720 660 600 540 480 65 35 95 5 TEMPERATURE (°C) 125 RT = 78.7k 600 EN Threshold 1.09 400 300 200 EN RISING 1.08 1.07 EN THRESHOLD (V) 500 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 SWITCHING FREQUENCY (MHz) 3995 G20 5 INPUT VOLTAGE (V) SWITCHING FREQUENCY (kHz) 100 0 155 6 4 3 2 1.06 1.05 1.04 1.03 1 100 0 0.2 0.8 0.6 0.4 FB PIN VOLTAGE (V) 0 –55 –25 1.
PAGE 7
LT3995 TYPICAL PERFORMANCE CHARACTERISTICS SS Pin Current Burst Frequency 2.6 500 VOUT = 5V fSW = 500kHz L = 10µH 400 300 VOUT = 3.3V fSW = 300kHz L = 8.2µH 200 100 50 Boost Capacitor Charger 160 VSS = 0.5V 2.4 140 2.2 120 OUT PIN CURRENT (mA) VIN = 12V SS PIN CURRENT (µA) SWITCHING FREQUENCY (kHz) 600 2.0 1.8 1.6 1.4 1.2 0 20 40 60 80 LOAD CURRENT (mA) 100 1.0 –55 –25 120 100 80 60 40 95 65 35 TEMPERATURE (°C) 5 125 155 0 0 1.6 580 DROPOUT THRESHOLD (mV) 600 1.4 1.0 0.
PAGE 8
LT3995 TYPICAL PERFORMANCE CHARACTERISTICS Load Transient: 0.5A to 2.5A Load Transient: 10mA to 2A IL 1A/DIV IL 1A/DIV VOUT 200mV/DIV VOUT 200mV/DIV VIN = 48V VOUT = 3.3V COUT = 47µF ×2 20µs/DIV TA = 25°C, unless otherwise noted. 3995 G36 VIN = 48V VOUT = 3.3V COUT = 47µF ×2 20µs/DIV 3995 G37 PIN FUNCTIONS FB (Pin 1): The LT3995 regulates the FB pin to 1.197V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and the output.
PAGE 9
LT3995 BLOCK DIAGRAM OUT VIN C1 INTERNAL 1.197V REF EN 1.02V + – + SHDN RT 0.5V – + SLOPE COMP OSCILLATOR 200kHz TO 2MHz RT SYNC + – + – VIN SWITCH LATCH BOOST R S C3 Q SW Burst Mode DETECT PG ERROR AMP + – + – 1.097V VC L1 VOUT D1 C2 VC CLAMP 1.8µA SS SHDN C4 OPT FB GND R2 R1 3995 BD C5 3995f For more information www.linear.
PAGE 10
LT3995 OPERATION The LT3995 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node.
PAGE 11
LT3995 APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current To enhance efficiency at light loads, the LT3995 operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current. In Burst Mode operation the LT3995 delivers single pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. When in sleep mode the LT3995 consumes 1.
PAGE 12
LT3995 APPLICATIONS INFORMATION clock is 11µA. Holding the SYNC pin DC high yields no advantages in terms of output ripple or minimum load to full frequency, so is not recommended. FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: RT = 51.1 ( fSW ) 1.09 – 9.27 where fSW is the desired switching frequency in MHz and RT is in kΩ. Operating Frequency Trade-Offs  V  R1= R2  OUT – 1  1.
PAGE 13
LT3995 APPLICATIONS INFORMATION than the absolute maximum ratings of the VIN pin. It can be calculated using the following equation: VIN(OP-MAX) = VOUT + VD –V +V fSW • tON(MIN) D SW where tON(MIN) is the minimum switch on-time. A lower switching frequency can be used to extend normal operation to higher input voltages. The circuit will tolerate inputs above the maximum operating input voltage and up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency.
PAGE 14
LT3995 APPLICATIONS INFORMATION To keep the boost capacitor charged regardless of load during dropout conditions, a minimum dropout voltage is enforced. When the OUT pin is tied to the output, the LT3995 regulates the output such that: VIN – VOUT > VDROPOUT(MIN) where VDROPOUT(MIN) is 500mV. The 500mV dropout voltage limits the duty cycle and forces the switch to turn off regularly to charge the boost capacitor.
PAGE 15
LT3995 APPLICATIONS INFORMATION When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor: ∆IL = (1–DC) • ( VOUT + VD ) L • fSW where fSW is the switching frequency of the LT3995, DC is the duty cycle and L is the value of the inductor. Therefore, the maximum output current that the LT3995 will deliver depends on the switch current limit, the inductor value, and the input and output voltages.
PAGE 16
LT3995 APPLICATIONS INFORMATION Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3995 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7μF capacitor is capable of this task, but only if it is placed close to the LT3995 (see the PCB Layout section).
PAGE 17
LT3995 APPLICATIONS INFORMATION robust for a wide input voltage range. A diode with even higher current rating can be selected for the worst-case scenario of overload, where the max diode current can then increase to the typical peak switch current. Short circuit is not the worst-case condition due to current limit foldback. Peak reverse voltage is equal to the regulator input voltage. For inputs up to 60V, a 60V diode is adequate. An additional consideration is reverse leakage current.
PAGE 18
LT3995 APPLICATIONS INFORMATION VIN VIN BOOST SW VIN VIN LT3995 GND BOOST SW VIN VIN LT3995 OUT VOUT GND (4a) For 3.2V ≤ VOUT ≤ 16V BOOST SW LT3995 OUT VOUT GND (4b) For 2.5V ≤ VOUT ≤ 3.2V OUT (4c) For VOUT < 2.5V, VIN < 30V VS VIN VIN BOOST VS SW VIN BOOST VIN LT3995 SW LT3995 OUT GND VOUT VOUT GND OUT VOUT 3995 F04 (4d) For VOUT < 2.5V, 3.1V ≤ VS ≤ 16V (4e) For VOUT > 16V, 3.1V ≤ VS ≤ 16V Figure 4.
PAGE 19
LT3995 APPLICATIONS INFORMATION VIN R3 LT3995 1.02V EN + – IL 1A/DIV SHDN VOUT 3.3V/DIV R4 LT3995 F06 VSS 0.5V/DIV Figure 6. Undervoltage Lockout 1ms/DIV tions where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops.
PAGE 20
LT3995 APPLICATIONS INFORMATION output current at the frequency set by the RT resistor (see Inductor Selection section). The slope compensation is set by the RT value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage and output voltage.
PAGE 21
LT3995 APPLICATIONS INFORMATION For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 10 shows a sample component placement with trace, ground plane and via locations, which serves as a good PCB layout example. Note that large, switched currents flow in the LT3995’s VIN and SW pins, the catch diode (D1), and the input capacitor (C1). The loop formed by these components should be as small as possible.
PAGE 22
LT3995 APPLICATIONS INFORMATION Also keep in mind that the leakage current of the power Schottky diode goes up exponentially with junction temperature. When the power switch is off, the power Schottky diode is in parallel with the power converter’s output filter stage. As a result, an increase in a diode’s leakage current results in an effective increase in the load, and a corresponding increase in the input quiescent current.
PAGE 23
LT3995 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E) 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) 8 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 0.305 ±0.038 (.0120 ±.0015) TYP 16 0.50 (.0197) BSC 4.039 ±0.102 (.159 ±.
PAGE 24
LT3995 TYPICAL APPLICATION 5V, 2MHz Step-Down Converter with Power Good VIN 5.9V TO 16V TRANSIENT TO 60V) VIN OFF ON BOOST EN SW PDS360 LT3995 4.7µF RT SYNC FB GND 14.7k 150k PGOOD PG OUT SS 10nF 0.47µF 2.2µH VOUT 5V 3A 1M 10pF 47µF 1210 316k 3995 TA04 f = 2MHz RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3975 42V, 2.5A, 2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.7µA VIN: 4.3V to 40V, VOUT(MIN) = 1.2V, IQ = 2.