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LT3690

3690fa

Typical applicaTion

FeaTures DescripTion

36V, 4A, 1.5MHz Synchronous

Step-Down Switching Regulator

with 70µA Quiescent Current

The LT

3690 is an adjustable frequency monolithic buck

switching regulator that accepts input voltages up to 36V.

A high efficiency 90mΩ switch is included on the device

along with the boost diode and the necessary oscillator,

control, and logic circuitry. The internal synchronous power

switch of 30mΩ increases efficiency and eliminates the

need for an external Schottky catch diode. Current mode

topology is used for fast transient response and good

loop stability. Shutdown reduces input supply current to

less than 1µA. The low ripple Burst Mode maintains high

efficiency at low output currents while keeping output

ripple below 15mV in typical applications.

The LT3690 features robust operation and is easily con-

figurable. Using a resistor divider on the UVLO pin provides

a programmable undervoltage lockout. A power good flag

signals when V

OUT

reaches 90% of the programmed out-

put voltage. Protection circuitry senses the current in the

power switches to protect the LT3690 against short-circuit

conditions. Frequency foldback and thermal shutdown

provide additional protection. The LT3690 is available in

a 4mm × 6mm QFN package with exposed pads for low

thermal resistance.

L, LT , LT C , LTM , Linear Technology, the Linear logo and Burst Mode are registered trademarks

of Linear T

echnology Corporation. All other trademarks are the property of their respective owners.

3.3V Step-Down Converter

applicaTions

Wide Input Range:

– Operation from 3.9V to 36V

– Overvoltage Lockout Protects Circuits

Through 60V Transients

4A Maximum Output Current

Integrated 30mΩ N-Channel Synchronous Switch

Low Ripple (<15mV

P-P

)

Burst Mode

Operation:

= 70µA at 12V

to 3.3V

OUT

Programmable Input Undervoltage Lockout

0.8V Feedback Reference Voltage

Output Voltage: 0.8V to 20V

Programmable and Synchronizable Oscillator

(170kHz to 1.5MHz)

Soft-Startup and Output Voltage Tracking

Short-Circuit Robust

Power Good Flag

Small Thermally Enhanced 4mm × 6mm QFN Package

Automotive Systems

Industrial Supplies

Distributed Supply Regulation

Efficiency and Power Loss

3690 TA01a

LT3690

GND

680pF

UVLO

CCINT

SYNC

BST

BIAS

4.5V TO 36V

0.47µF

ƒ = 600kHz

3.3V

22k

32.4k

102k

316k

3.3µH

0.68µF

100µF

10µF

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

100

2.5

0.5

2.0

1.5

1.0

2 31 2.5 3.51.50.5

3690 TA01b

OUT

= 5V

OUT

= 3.3V

OUT

= 5V

OUT

= 3.3V

= 12V

L = 4.7µH

ƒ = 600kHz

Summary of content (30 pages)

PAGE 1
LT3690 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator with 70µA Quiescent Current Description Features n n n n n n n n n n n n Wide Input Range: – Operation from 3.9V to 36V – Overvoltage Lockout Protects Circuits Through 60V Transients 4A Maximum Output Current Integrated 30mΩ N-Channel Synchronous Switch Low Ripple (<15mVP-P) Burst Mode® Operation: IQ = 70µA at 12VIN to 3.3VOUT Programmable Input Undervoltage Lockout 0.8V Feedback Reference Voltage Output Voltage: 0.
PAGE 2
LT3690 Absolute Maximum Ratings Pin Configuration (Note 1) TOP VIEW SW 1 SW 2 SW 3 26 SW 25 SW 27 SW 24 SW SW 4 23 SW SYNC 5 22 BST GND 6 21 GND RT 7 VC 8 FB 9 18 PG GND 10 17 EN 20 VCCINT GND 16 19 BIAS VIN 15 VIN 14 VIN 13 SS 11 28 GND UVLO 12 EN, UVLO, VIN Voltage (Note 2).................................60V BST Voltage ..............................................................55V BST Voltage Above SW Voltage ................................
PAGE 3
LT3690 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7). PARAMETER CONDITIONS VIN Fixed Undervoltage Lockout VIN Overvoltage Lockout OVLO Quiescent Current from VIN MIN TYP MAX 3.0 3.9 V 36 38.2 40 V 0.1 1 µA 35 60 µA 110 150 µA 0.1 1 µA 70 110 µA l VIN Rising l VEN = 0.2V VBIAS = 3V, VFB = 0.
PAGE 4
LT3690 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7). PARAMETER CONDITIONS PG Threshold as Percentage of VFB VFB Rising MIN TYP MAX 88 90 92 PG Hysteresis 12 PG Sink Current VPG = 0.3V PG Leakage VPG = 5V l 100 % mV 500 0.
PAGE 5
LT3690 Typical Performance Characteristics Efficiency and Power Loss 100 3.0 90 2.5 90 2.5 80 2.0 80 2.0 70 1.5 70 1.5 60 VOUT = 3.3V 1.0 L = 3.3µH ƒ = 500kHz VIN = 12V 0.5 VIN = 24V VIN = 34V 0 3 3.5 4 60 50 40 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) VOUT = 5V L = 4.7µH 1.0 ƒ = 600kHz VIN = 12V 0.5 VIN = 24V VIN = 34V 0 3 3.5 4 EFFICIENCY (%) 3.0 50 40 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) 3690 G01 Efficiency and Power Loss 3690 G02 Efficiency and Power Loss 100 3.0 100 160 3.
PAGE 6
LT3690 Typical Performance Characteristics Switch Current Limit vs Duty Cycle TA = 25°C, unless otherwise noted. Switch Current Limit vs Temperature 0.6 8.0 8 VFB > 0.75V 6 VFB = 0V 5 7.0 HS SWITCH VOLTAGE DROP (V) 7 HS SWITCH CURRENT LIMIT (A) HS SWITCH CURRENT LIMIT (A) 7.5 DUTY CYCLE = 10% 6.5 6.0 DUTY CYCLE = 90% 5.5 5.0 4.5 4.0 Switch Voltage Drop vs ISW 0.5 0.4 0.3 0.2 0.1 3.5 4 0 20 40 60 DUTY CYCLE (%) 80 3.0 –50 –25 100 0 3690 G08 8 100 1.
PAGE 7
LT3690 Typical Performance Characteristics EN Threshold Voltage vs Temperature 2.5 8 2.0 6 4 2 0 5 10 15 20 25 EN PIN VOLTAGE (V) 30 UVLO Threshold Voltage vs Temperature 2.5 UVLO THRESHOLD VOLTAGE (V) 10 EN THRESHOLD VOLTAGE (V) EN PIN CURRENT (µA) EN Pin Current vs VEN 0 TA = 25°C, unless otherwise noted. 1.5 1.0 0.5 0 –50 –25 35 0 2.0 1.5 1.0 0.5 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 3690 G18 3690 G17 UVLO Pin Current vs Temperature (VUVLO = 1.
PAGE 8
LT3690 Typical Performance Characteristics Switching Frequency vs Temperature MINIMUM SWITCH ON-TIME (ns) 610 590 570 160 120 80 40 –1.6 –1.8 –2.0 –2.2 –2.4 –2.6 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 3690 G26 VCCINT vs VIN 0 25 50 75 100 125 150 TEMPERATURE (°C) 3690 G27 –2.8 –50 –25 VVCCINT Current Limit 0 5 4 –10 4 3 2 1 0 0 1 2 3 4 5 6 7 VIN PIN VOLTAGE (V) 8 9 10 –20 –30 3690 G29 VSW 5V/DIV IL 0.
PAGE 9
LT3690 Pin Functions SW (Pins 1-4, 23-26, Exposed Pad Pin 27): The SW pin is the emitter output of the internal highside NPN power switch (HS) and the drain output of the internal lowside power N-channel switch (LS). Connect this pin to the inductor and boost capacitor. This pin is driven up to the VIN voltage by the HS switch during the on-time of the PWM duty cycle. The inductor current drives the SW pin negative during the off-time.
PAGE 10
LT3690 Block Diagram VIN 14 CIN 15 16 17 VIN CURRENT SENSE VIN VIN + – 13 GND 19 EN 2.5V REFERENCE 2µA 12 5 7 BST 0.8V 22 0.72V SLEEP VIN MONITOR OVLO TEMPERATURE MONITOR UVLO CBST TSD HS SWITCH UVLO SYNC OSCILLATOR 0.17MHz TO 1.5MHz SYNC – + SLOPE COMP R S FF Q SWITCH CONTROL + – CSS TSD OVLO UVLO R1 9 R2 10 POWER GOOD + – 0.
PAGE 11
LT3690 Operation The LT3690 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal high side (HS) power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the RS flip-flop and HS switch off when this current reaches a level determined by the voltage at VC.
PAGE 12
LT3690 Applications Information FB Resistor Network Operating Frequency Trade-Offs The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: Selection of the operating frequency is a trade-off between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used.
PAGE 13
LT3690 Applications Information Input Voltage Range The minimum input voltage is determined by either the LT3690’s minimum operating voltage of 3.9V (VBIAS > 3V) or by its maximum duty cycle (see equation in the Operating Frequency Trade-offs section). The minimum input voltage due to duty cycle limitation is: VIN(MIN) = VOUT + VLS − VLS + VSW 1− ƒSW • tOFF(MIN) where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off-time (210ns).
PAGE 14
LT3690 Applications Information Inductor Selection and Maximum Output Current A good first choice for the inductor value is: ( ) L = VOUT + VLS • 0.67MHz ƒSW where VLS is the voltage drop of the low side switch (0.12V), ƒSW is in MHz, and L is in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.03Ω.
PAGE 15
LT3690 Applications Information One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals. Then use these equations to check that the LT3690 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinuous operation occurs when IOUT is less than ΔIL/2.
PAGE 16
LT3690 Applications Information Since the LT3690 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Frequency Compensation The LT3690 uses current mode control to regulate the output. This simplifies loop compensation.
PAGE 17
LT3690 Applications Information Low-Ripple Burst Mode and Pulse-Skipping Mode The LT3690 is capable of operating in either low ripple Burst Mode operation or pulse-skipping mode, which is selected using the SYNC pin. See the Synchronization and Mode section for details. To enhance efficiency at light loads, the LT3690 can be operated in low ripple Burst Mode operation that keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current.
PAGE 18
LT3690 Applications Information The switching will only resume once the low side switch current has fallen below the 5A limit. This way, the comparator regulates the valley current of the inductor to 5A during short-circuit. With properly chosen external components, this will ensure that the part will survive a short-circuit event. VCCINT Considerations The linear voltage regulator requires a capacitor of 0.47µF to deliver the peak current for the gate driver of the low side N-channel transistor.
PAGE 19
LT3690 Applications Information 6.0 TO START 5.0 4.5 4.0 VOUT = 5V L = 4.7µH ƒ = 600kHz TO START 7.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 5.5 7.5 VOUT = 3.3V L = 4.7µH ƒ = 600kHz 6.5 6.0 5.5 TO RUN TO RUN 3.5 3.0 5.0 1 10 100 1000 LOAD CURRENT (mA) 10000 4.5 1 10 100 1000 LOAD CURRENT (mA) 3690 F06a 10000 3690 F06b Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit is also limited by the boost circuit.
PAGE 20
LT3690 Applications Information LT3690 SS OUT1 5V VEN 2V/DIV 0.1µF VOUT1 2V/DIV LT3690 SS OUT2 3.3V 0.047µF VOUT2 2V/DIV 3690 F08a 5ms/DIV 3690 F08b 5ms/DIV 3690 F08d (8a) Independent Start-Up LT3690 SS OUT1 5V 0.22µF VOUT1 2V/DIV LT3690 SS OUT2 VEN 2V/DIV 3.3V VOUT2 2V/DIV 3690 F08c (8b) Ratiometric Start-Up Figure 8.
PAGE 21
LT3690 Applications Information LT3690 SS 5V OUT1 VEN 2V/DIV 0.1µF R1 28.7k VOUT1 2V/DIV LT3690 R2 10k SS OUT2 VOUT2 2V/DIV 3.3V 5ms/DIV 3690 F09b 5ms/DIV 3690 F09d 3690 F09a (9a) Coincident Start-Up SS LT3690 OUT1 5V VEN 2V/DIV PG1 0.1µF VOUT1 2V/DIV LT3690 SS OUT2 VOUT2 2V/DIV 3.3V 0.047µF 3690 F09b (9b) Output Sequencing Figure 9.
PAGE 22
LT3690 Applications Information Output Tracking and Sequencing Output tracking and sequencing between voltage regulators can be implemented using the LT3690’s SS and PG pins. Figures 8 and 9 show several configurations for output tracking and sequencing of the LT3690 and an additional regulator. Independent soft-start for each channel is shown in Figure 8a. The output ramp time for each output is set by the soft-start capacitor as described in the soft-start section.
PAGE 23
LT3690 Applications Information limit or latch low under low source voltage conditions. The UVLO circuitry prevents the regulator from operating at source voltages where the problems might occur. An internal comparator will force the part into shutdown below the fixed VIN UVLO threshold of 3.0V. This feature can be used to prevent excessive discharge of battery-operated systems. If an adjustable UVLO threshold is required, the UVLO pin can be used. The threshold voltage of the UVLO pin comparator is 1.25V.
PAGE 24
LT3690 Applications Information PCB Layout High Temperature Considerations For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 13 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3690’s VIN, SW and GND pins and the input capacitor (CIN). The loop formed by these components should be as small as possible.
PAGE 25
LT3690 Typical Applications 5V Step-Down Converter VIN 6.3V TO 36V VIN 10µF BIAS UVLO ON OFF PG EN LT3690 SS VC 1nF 0.47µF L 4.7µH SW 536k FB VCCINT 15k 0.68µF BST RT SYNC GND 680pF VOUT 5V 4A 47µF 32.4k 102k ƒ = 600kHz 3690 TA02 3.3V Step-Down Converter VIN 4.5V TO 36V VIN 10µF BIAS UVLO ON OFF PG EN SS LT3690 VC 1nF 22k 316k FB RT SYNC GND 680pF L 3.3µH SW VCCINT 0.47µF 0.68µF BST VOUT 3.3V 4A 100µF 32.
PAGE 26
LT3690 Typical Applications 2.5V Step-Down Converter VIN 3.9V TO 36V BIAS VIN 10µF UVLO ON OFF EN BST VC 1nF L 3.3µH SW VOUT 2.5V 4A 160k FB VCCINT 0.47µF 1µF LT3690 SS 15k MBR0540 PG RT SYNC 100µF GND 1nF 32.4k 75k ƒ = 600kHz 3690 TA04 (FIXED FREQUENCY AT VIN < 21V) 1.8V Step-Down Converter AUXILIARY SUPPLY 3.3V OR 5V 1µF VIN 3.9V TO 36V 100k VIN 10µF BIAS UVLO ON OFF PG EN SS LT3690 VC 1nF 16k 0.68µF BST VOUT 1.8V 4A 18.7k FB RT SYNC GND 1nF L 4.
PAGE 27
LT3690 Typical Applications 1.2V Step-Down Converter AUXILIARY SUPPLY 3.3V OR 5V 1µF VIN 3.9V TO 36V 100k 2× 10µF VIN BIAS UVLO ON OFF PG EN BST VC 14k 0.47µF 10nF L 8.2µH 0.68µF LT3690 SS POWER GOOD VOUT 1.2V 4A SW VCCINT FB SYNC RT GND 2.2nF 23.2k + 100µF 140k 46.
PAGE 28
LT3690 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFEMA UFE Package Package 26-Lead Plastic QFN × 6mm) 26-Lead Plastic(4mm QFN (4mm × 6mm) (Reference LTC DWG # 05-08-1770 Rev A) (Reference LTC DWG # 05-08-1770 Rev A) 0.70 ±0.05 4.50 ± 0.05 2.64 ± 0.05 2.64 ± 0.05 2.31 ± 0.05 3.10 ± 0.05 0.41 ± 0.05 2.18 ± 0.05 2.50 REF 2.36 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.55 ± 0.05 3.25 ± 0.05 4.
PAGE 29
LT3690 Revision History REV DATE DESCRIPTION PAGE NUMBER A 9/11 Added H- and MP-grades. Revised BIAS pin description in Pin Functions section. 2 to 8, 13, 25 9 3690fa 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 30
LT3690 Typical Application 3.3V Step-Down Converter VIN 4.5V TO 36V BIAS VIN 10µF UVLO ON OFF PG EN SS LT3690 VC 1nF 0.47µF 316k FB RT SYNC GND 680pF L 3.3µH SW VCCINT 22k 0.68µF BST VOUT 3.3V 4A 100µF 40.2k 102k ƒ = 500kHz 3690 TA08 (FIXED FREQUENCY AT VIN < 31V) Related Parts PART NUMBER DESCRIPTION COMMENTS LT3680 36V, 3.5A, 2.4MHz High Efficiency MicroPower Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.