Datasheet

ManualsBrandsLinear Technology ManualsDC / DC Adjustable Switching RegulatorsSwitching Regulator IC Step-Down QFN- 41.2V

LT8614

Rev. E

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TYPICAL APPLICATION

FEATURES DESCRIPTION

42V, 4A Synchronous

Step-Down Silent Switcher

with 2.5µA Quiescent Current

The LT

8614 step-down regulator features Silent Switcher

architecture designed to minimize EMI emissions while

delivering high efficiency at frequencies up to 3MHz.

Assembled in a 3mm × 4mm QFN, the monolithic con-

struction with integrated power switches and inclusion of

all necessary circuitry yields a solution with a minimal PCB

footprint. An ultralow 2.5µA quiescent current—with the

output in full regulation— enables applications requiring

highest efficiency at very small load currents. Transient

response remains excellent and output voltage ripple is

below 10mV

P-P

at any load, from zero to full current.

The LT8614 allows high V

to low V

OUT

conversion at high

frequency with a fast minimum top switch on-time of 30ns.

Operation is safe in overload even with a saturated inductor.

Essential features are included and easy to use: An open-

drain PG pin signals when the output is in regulation. The

SYNC pin allows clock synchronization and choice of Burst

Mode operation or pulse-skipping mode. Soft-start and

tracking functionality is accessed via the TR/SS pin. An

accurate enable threshold can be set using the EN/UV pin

and a resistor at the RT pin programs switch frequency.

5V 4A Step-Down Converter

12V

to 5V

OUT

Efficiency

APPLICATIONS

Silent Switcher

Architecture

Ultralow EMI Emissions

High Efficiency at High Frequency

Up to 96% Efficiency at 1MHz, 12V

to 5V

OUT

Up to 94% Efficiency at 2MHz, 12V

to 5V

OUT

Wide Input Voltage Range: 3.4V to 42V

Ultralow Quiescent Current Burst Mode

Operation:

2.5μA I

Regulating 12V

to 3.3V

OUT

Output Ripple < 10mV

P-P

Fast Minimum Switch On-Time: 30ns

Low Dropout Under All Conditions: 125mV at 1A

Safely Tolerates Inductor Saturation in Overload

Adjustable and Synchronizable: 200kHz to 3MHz

Peak Current Mode Operation

Accurate 1V Enable Pin Threshold

Internal Compensation

Output Soft-Start and Tracking

Small 18-Lead 3mm × 4mm QFN

Automotive and Industrial Supplies

General Purpose Step-Down

GSM Power Supplies

IN2

IN1

EN/UV

LT8614

8614 TA01a

BST

SYNC/MODE SW

TR/SS BIAS

INTV

RT GND

1µF

0.1µF

4.7pF

47µF

OUT

1µF

4.7µF

5.8V TO 42V

10nF

41.2k

1µF

4.7µH

243k

GND2GND1

= 1MHz

LOAD CURRENT (A)

EFFICIENCY (%)

4.0

8614 TA01b

1.0

2.0

3.0

0.5

1.5

2.5

3.5

100

1MHz

2MHz

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Summary of content (24 pages)

PAGE 1
LT8614 42V, 4A Synchronous Step-Down Silent Switcher with 2.5µA Quiescent Current DESCRIPTION FEATURES Silent Switcher® Architecture nn Ultralow EMI Emissions nn High Efficiency at High Frequency nn Up to 96% Efficiency at 1MHz, 12V to 5V IN OUT nn Up to 94% Efficiency at 2MHz, 12V to 5V IN OUT nn Wide Input Voltage Range: 3.4V to 42V nn Ultralow Quiescent Current Burst Mode® Operation: nn 2.5μA I Regulating 12V to 3.
PAGE 2
LT8614 PIN CONFIGURATION VIN, EN/UV, PG...........................................................42V BIAS...........................................................................30V BST Pin Above SW Pin................................................4V FB, TR/SS, RT, INTVCC ................................................4V SYNC/MODE Voltage ..................................................6V Operating Junction Temperature Range (Note 2) LT8614E..............................................
PAGE 3
LT8614 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A VIN = 6V, ILOAD = 0.5A l Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 0.5A l Feedback Pin Input Current VFB = 1V –20 INTVCC Voltage ILOAD = 0mA, VBIAS = 0V ILOAD = 0mA, VBIAS = 3.
PAGE 4
LT8614 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency at 3.3VOUT Efficiency at 5VOUT Efficiency at 5VOUT 100 95 95 90 90 90 80 85 70 EFFICIENCY (%) 75 70 fSW = 1MHz L = 4.7µH VIN = 12V VIN = 24V VIN = 36V 65 60 55 50 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 80 75 70 fSW = 1MHz L = 2.2µH VIN = 12V VIN = 24V VIN = 36V 65 60 55 50 4 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 8614 G01 98 94 60 50 40 fSW = 1MHz L = 4.7µH VIN = 12V VIN = 24V VIN = 36V 1 10 100 LOAD CURRENT (mA) 0.
PAGE 5
LT8614 TYPICAL PERFORMANCE CHARACTERISTICS No-Load Supply Current Line Regulation 0.10 5.0 VOUT = 5V ILOAD = 1A 0.08 0.02 0 –0.02 –0.04 3.5 3.0 2.5 2.0 1.5 –0.06 1.0 –0.08 0.5 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 0 45 20 INPUT CURRENT (µA) INPUT CURRENT (µA) 0 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 8614 G10 40 0 –55 45 Top FET Current Limit 8.5 9.0 7.0 6.5 6.0 CURRENT LIMIT (A) 7.0 8.5 5% DC 8.0 7.5 7.0 0.2 0.4 0.6 DUTY CYCLE 6.0 –50 –25 1.0 0.
PAGE 6
LT8614 TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency 700 730 600 500 400 300 200 RT = 60.4k FRONT PAGE APPLICATION VIN = 12V 1000 VOUT = 5V 720 710 700 690 680 670 100 0 Burst Frequency 1200 SWITCHING FREQUENCY (kHz) 740 SWITCHING FREQUENCY (kHz) DROPOUT VOLTAGE (mV) Dropout Voltage 800 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 660 –50 –25 4 0 SWITCHING FREQUENCY (kHz) 20 600 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 1.0 500 400 300 0 45 0.2 0 0.2 0.4 0.
PAGE 7
LT8614 TYPICAL PERFORMANCE CHARACTERISTICS RT Programmed Switching Frequency VIN UVLO 250 225 6.0 3.4 5.5 150 125 100 75 BIAS PIN CURRENT (mA) 3.2 175 INPUT VOLTAGE (V) RT PIN RESISTOR (kΩ) 200 Bias Pin Current 3.6 3.0 2.8 2.6 2.4 50 0 0.2 0.6 1.4 1.8 2.2 2.6 1 SWITCHING FREQUENCY (MHz) 3 2.0 –55 –25 95 65 35 TEMPERATURE (°C) 5 125 8614 G27 BIAS PIN CURRENT (mA) 12 10 4.0 3.5 155 2.
PAGE 8
LT8614 TYPICAL PERFORMANCE CHARACTERISTICS Start-Up Dropout Performance Start-Up Dropout Performance VIN VIN 2V/DIV VOUT VOUT 2V/DIV VIN VIN 2V/DIV VOUT VOUT 2V/DIV 100ms/DIV 2.
PAGE 9
LT8614 PIN FUNCTIONS BIAS (Pin 1): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.1V. For output voltages of 3.3V to 30V this pin should be tied to VOUT. If this pin is tied to a supply other than VOUT use a 1µF local bypass capacitor on this pin. If no supply is available, tie to GND. GND2 (10, 11): Power Switch Ground. These pins are the return path of the internal bottom side power switch and must be tied together.
PAGE 10
LT8614 PIN FUNCTIONS FB (Pin 20): The LT8614 regulates the FB pin to 0.970V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and VOUT. Typically, this capacitor is 4.7pF to 22pF. SYNC/MODE (Pin 17): External Clock Synchronization Input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization to an external frequency. Apply a DC voltage of 3V or higher or tie to INTVCC for pulse-skipping mode.
PAGE 11
LT8614 OPERATION The LT8614 is a monolithic, constant frequency, current mode step-down DC/DC converter. An oscillator, with frequency set using a resistor on the RT pin, turns on the internal top power switch at the beginning of each clock cycle. Current in the inductor then increases until the top switch current comparator trips and turns off the top power switch. The peak inductor current at which the top switch turns off is controlled by the voltage on the internal VC node.
PAGE 12
LT8614 APPLICATIONS INFORMATION Low EMI PCB Layout Note that large, switched currents flow in the LT8614 VIN1, VIN2, GND1, and GND2 pins and the input capacitors (CIN1, CIN2). The loops formed by the input capacitors should be as small as possible by placing the capacitors adjacent to the VIN1/2 and GND1/2 pins. Capacitors with small case size such as 0603 are optimal due to lowest parasitic inductance.
PAGE 13
LT8614 APPLICATIONS INFORMATION To enhance efficiency at light loads, the LT8614 operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current and minimizing output voltage ripple. In Burst Mode operation the LT8614 delivers single small pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. While in sleep mode the LT8614 consumes 1.7μA.
PAGE 14
LT8614 APPLICATIONS INFORMATION If low input quiescent current and good light-load efficiency are desired, use large resistor values for the FB resistor divider. The current flowing in the divider acts as a load current, and will increase the no-load input current to the converter, which is approximately: VSW 5V/DIV IL 500mA/DIV VOUT 10mV/DIV 5µs/DIV FRONT PAGE APPLICATION 12VIN TO 5VOUT AT 10mA VSYNC = 0V ⎛ V ⎞ ⎛ V ⎞ ⎛ 1⎞ IQ = 1.7µA + ⎜ OUT ⎟ ⎜ OUT ⎟ ⎜ ⎟ ⎝ R1+R2 ⎠ ⎝ VIN ⎠ ⎝ n ⎠ 8614 F03 Figure 3.
PAGE 15
LT8614 APPLICATIONS INFORMATION For transient operation, VIN may go as high as the absolute maximum rating of 42V regardless of the RT value, however the LT8614 will reduce switching frequency as necessary to maintain control of inductor current to assure safe operation. Table 1. SW Frequency vs RT Value fSW (MHz) RT (kΩ) 0.2 232 0.3 150 0.4 110 0.5 88.7 0.6 71.5 0.7 60.4 0.8 52.3 1.0 41.2 1.2 33.2 1.4 28.0 1.6 23.7 1.8 20.5 2.0 18.2 2.2 15.8 3.0 10.
PAGE 16
LT8614 APPLICATIONS INFORMATION To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than the maximum expected output load of the application.
PAGE 17
LT8614 APPLICATIONS INFORMATION A third, larger ceramic capacitor of 2.2µF or larger should be placed close to VIN1 or VIN2. See layout section for more detail. X7R or X5R capacitors are recommended for best performance across temperature and input voltage variations. Note that larger input capacitance is required when a lower switching frequency is used.
PAGE 18
LT8614 APPLICATIONS INFORMATION voltages where the problems might occur. This threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: ⎛ R3 ⎞ VIN(EN) = ⎜ +1⎟ •1.0V ⎝ R4 ⎠ (10) where the LT8614 will remain off until VIN is above VIN(EN). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN).
PAGE 19
LT8614 APPLICATIONS INFORMATION input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 500kHz. 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 20
LT8614 TYPICAL APPLICATIONS 5V 4A Step-Down Converter VIN 5.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 BST SYNC/MODE TR/SS 0.1µF 4.7µH VOUT 5V 4A SW BIAS 1µF 4.7pF 1M FB INTVCC 41.2k 1µF 0603 RT 243k GND fSW = 1MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA08 3.3V, 4A Step-Down Converter VIN 4.1V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 SYNC/MODE TR/SS BST 0.1µF 4.7µH VOUT 3.3V 4A SW BIAS 1µF 41.2k 1µF 0603 4.
PAGE 21
LT8614 TYPICAL APPLICATIONS 2MHz 5V, 4A Step-Down Converter VIN 5.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 SYNC/MODE TR/SS BST 0.1µF 2.2µH VOUT 5V 4A SW BIAS 1µF 4.7pF 1M FB INTVCC 18.2k 1µF 0603 RT 243k GND fSW = 2MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA03 2MHz 3.3V, 4A Step-Down Converter VIN 4.1V TO 42V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 GND2 LT8614 SYNC/MODE TR/SS BST 0.1µF 1.5µH 4.7pF INTVCC RT VOUT 3.
PAGE 22
LT8614 PACKAGE DESCRIPTION UDC Package Variation: UDC20(18) Package 20(18)-LeadUDC Plastic QFN (3mm × 4mm) Variation: (Reference LTC DWGUDC20(18) # 05-08-1956 Rev C) 20(18)-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG 05-08-1956 Exposed Pad#Variation AARev C) Exposed Pad Variation AA 0.055 BSC 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 1.50 REF 0.770 BSC 0.220 ±0.05 0.356 ±0.05 0.400 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF 3.10 ±0.05 4.50 ±0.
PAGE 23
LT8614 REVISION HISTORY REV DATE DESCRIPTION A 03/14 Clarified package description. PAGE NUMBER 2 Clarified Applications Information. 13 Clarified applications components. 20, 21, 24 Clarified rev of package drawing. 22 Clarified Related Parts list.
PAGE 24
LT8614 TYPICAL APPLICATIONS 12V, 4A Step-Down Converter VIN 12.8V TO 42V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 1µF 0603 GND2 LT8614 BST SYNC/MODE TR/SS 0.1µF 8.2µH 1µF 4.7pF 1M FB INTVCC 41.2k VOUT 12V 4A SW BIAS RT 88.7k GND fSW = 1MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA04 2MHz 1.8V, 4A Step-Down Converter VIN 3.4V TO 22V (42V TRANSIENT) 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 LT8614 SYNC/MODE TR/SS BST SW BIAS 1µF 18.