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LT3640

3640f

Typical applicaTion

DescripTion

Dual Monolithic Buck

Regulator with Power-On

Reset and Watchdog Timer

The LT

3640 is a dual channel, current mode monolithic

buck switching regulator with a power-on reset and a

watchdog timer. Both regulators are synchronized to a

single oscillator with an adjustable frequency (350kHz to

2.5MHz). At light loads, both regulators operate in low

ripple Burst Mode

to maintain high efﬁciency and low

output ripple.

The high voltage channel is a nonsynchronous buck with

an internal 2.4A top switch that operates from an input

of 4V to 35V; a 36.5V OVLO protects the device to 55V.

The low voltage channel operates from an input of 2.5V to

5.5V. Internal synchronous power switches provide high

efﬁciency without the need of external Schottky diode.

Both channels have cycle-by-cycle current limit, providing

protection against shorted outputs.

The power-on reset and watchdog timeout periods are

both adjustable using external capacitors. The window

mode watchdog timer ﬂags when the µP pulses group

too close together or too far apart.

The LT3640 is available in a 28-pin 4mm × 5mm QFN

package and 28-pin TSSOP package. Both packages have

an exposed pad for low thermal resistance.

2MHz 3.3V/0.8A and 1.8V/0.8A Step Down Regulators

FeaTures

applicaTions

High Voltage Buck Regulator:

4V to 35V Operating Range

1.3A Output Current

OVLO Protects Input to 55V

Low Voltage Synchronous Buck Regulator:

2.5V to 5.5V Input Voltage Range

1.1A Output Current

Synchronizable, Adjustable 350kHz to 2.5MHz

Switching Frequency

Programmable Power-On Reset Timer

Programmable Window Mode Watchdog Timer

Typical Quiescent Current: 290µA

Short-Circuit Robust

Programmable Soft-Start

Low Shutdown Current: I

< 1µA

Available in Thermally Enhanced 28-Lead

(4mm × 5mm) QFN and 28-Lead TSSOP Packages

Industrial Power Supplies

Automotive Electronic Control Units

3640 TA01a

SYNC

PGOOD

WDE

1nF

1.5nF

32.4k

OUT2

1.8V/0.8A

OUT1

3.3V/0.8A

22µF

10µF

100k

RST2

EN2

WDO

WDI

FB2

CWDT

CPOR RT GND SS2 SS1

RST1

SW2

SW1EN/UVLO V

5V TO 35V

LT3640

BST

IN2

FB1

1µH

100k

OUT1

µP

100k

49.9k

80.6k

49.9k

3.3µH

0.22µF

22µF

HV Channel Efﬁciency,

2MHz, V

OUT1

= 3.3V

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

and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

LV Channel Efﬁciency,

2MHz, V

OUT2

= 1.8V

OUT1

CURRENT (A)

EFFICIENCY (%)

0.2 0.4 0.80.6 1.0

1.2

3640 TA01b

= 12V

OUT2

CURRENT (A)

EFFICIENCY (%)

0.2 0.4 0.80.6 1

3640 TA01c

IN2

= 3.3V

Summary of content (24 pages)

PAGE 1
LT3640 Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer Description Features n n n n n n n n n n n High Voltage Buck Regulator: 4V to 35V Operating Range 1.3A Output Current OVLO Protects Input to 55V Low Voltage Synchronous Buck Regulator: 2.5V to 5.5V Input Voltage Range 1.1A Output Current Synchronizable, Adjustable 350kHz to 2.
PAGE 2
LT3640 Absolute Maximum Ratings (Note 1) VIN, EN/UVLO Voltage (Note 7)..................................55V WDE Voltage..............................................................30V BST Above SW, SW1 Voltage........................ –0.3V to 6V SW1 Above SW Voltage................................ –0.3V to 6V VIN2, SYNC, EN2, PGOOD, WDI, WDO, RST1, RST2, Voltages........................ –0.3V to 6V SS1, SS2, FB1, FB2, RT, CWDT, CPOR Voltages………............................... –0.3V to 2.5V SW2 Voltage...
PAGE 3
LT3640 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VIN2 = 3.3V, EN/UVLO = 12V, EN2 = 3.3V, unless otherwise noted. PARAMETER TYP MAX VIN Undervoltage Lockout Threshold l 3.6 4 V VIN Undervoltage Release Threshold l 3.8 4.2 V VIN Overvoltage Lockout Threshold l 35 36.5 38 V VIN Overvoltage Release Threshold l 34 35.5 37 V 0.1 275 1 375 µA µA 1.26 1.
PAGE 4
LT3640 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VIN2 = 3.3V, EN/UVLO = 12V, EN2 = 3.3V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX SS1, SS2 Charge Current SS1 = 0.5V, SS2 = 0.5V 1.4 1.9 2.5 µA SS1 to FB1 Offset Voltage SS1 = 0.6V 5 30 mV SS2 to FB2 Offset Voltage SS2 = 0.
PAGE 5
LT3640 Typical Performance Characteristics HV Channel Efficiency (2MHz, VOUT1 = 5V) HV Channel Efficiency (2MHz, VOUT1 = 3.3V) 90 90 80 VIN = 24V 75 85 VIN = 24V VIN = 16V 80 75 0 0.2 0.4 0.6 0.8 1.0 VOUT1 CURRENT (A) 70 1.2 0.35 0 0.2 0.4 0.6 0.8 1.0 VOUT1 CURRENT (A) VIN QUIESCENT CURRENT (mA) EFFICIENCY (%) VIN2 = 5V 80 75 0 0.2 0.4 0.6 VOUT2 CURRENT (A) 0.8 0.20 0.15 0.10 0.05 0.00 1.0 0 10 20 1.35 1.
PAGE 6
LT3640 Typical Performance Characteristics Switching Frequency vs Temperature FB2 Voltage vs SS2 0.52 SWITCHING FREQUENCY (MHz) FB2 VOLTAGE (mV) 600 500 400 300 200 100 0 200 400 6000 800 2.5 RT = 182k 0.51 0.50 0.49 0.48 –50 1000 HV Channel Current Limit vs Duty Cycle SW1 PEAK CURRENT LIMIT (A) 700 0 TA = 25°C, unless otherwise noted. 50 0 100 450 1.5 1.0 0.5 60 80 0.40 350 0.35 300 PMOS 250 200 NMOS 150 0.5 0 1 0 5 VIN = 16V VIN = 24V 0.4 15 0.6 0.8 1.0 1.
PAGE 7
LT3640 Typical Performance Characteristics TA = 25°C, unless otherwise noted. SW1 10V/DIV SW1 10V/DIV IL1 0.5A/DIV IL1 0.5A/DIV SW2 5V/DIV SW2 5V/DIV IL2 0.5A/DIV IL2 0.5A/DIV 200ns/DIV VIN2 = VOUT1 VIN1 = 12V VOUT1 = 3.3V/0.5A VOUT2 = 1.8V/0.5A 3640 G18 500ns/DIV VIN2 = VOUT1 VIN1 = 12V VOUT1 = 3.3V/25mA VOUT2 = 1.
PAGE 8
LT3640 Pin Functions (FE/QFN) FB2 (Pin 1/Pin 26): The low voltage converter regulates the FB2 pin to 600mV. Connect the feedback resistor divider tap to this pin to set output voltage. PGOOD (Pin 2/Pin 27): Open-drain logic output that starts to sink current when FB2 is in regulation. EN/UVLO (Pin 3/Pin 28): Pull this pin below 0.3V to shut down the LT3640. The 1.
PAGE 9
LT3640 Block Diagram CIN VIN 2µA BST EN/ UVLO 100k + A4 – 5.5V – A3 + ENABLE CBST R S Q1 Q SW DRIVER + – VREF 1.
PAGE 10
LT3640 Timing Diagrams Power-On Reset Timing FB tUV RST tRST Watchdog Timing WDI WDO tDLY t < tWDL tRST t < tWDU tWDL < t < tWDU tWDU tRST 3640 TD Operation The LT3640 is a dual channel, constant-frequency, current mode monolithic buck switching regulator with power-on reset and watchdog timer. Both channels are synchronized to a single oscillator with frequency set by RT. Operation can be best understood by referring to the Block Diagram.
PAGE 11
LT3640 Operation when the VIN2 pin voltage is above 2.3V, the EN2 pin is pulled high and the FB1 pin voltage is above 1.165V. The internal top power MOSFET is turned on each cycle at the beginning of each oscillator cycle, and turned off when the current flowing through the top MOSFET reaches a level determined by the voltage at the VC2 node. An error amplifier measures the output voltage through an external resistor divider tied to the FB2 pin and servos the VC2 node.
PAGE 12
LT3640 Applications Information Setting the Output Voltages The internal reference voltage is 1.265V for the high voltage channel, and 600mV for the low voltage channel. The output voltages are set by resistor dividers according to the following formulas:  V  R2 = R1 •  OUT1 − 1  1 . 265V  V  R4 = R3 •  OUT2 − 1  0 . 6V  Use 1% resistors in the resistor dividers. To avoid noise problems, R1 should be 100k or less, and R3 should be 50k or less. Reference designators refer to the Block Diagram.
PAGE 13
LT3640 Applications Information The maximum VIN should not exceed the absolute maximum rating. For fixed frequency operation, the maximum VIN is: V +V  VIN(MAX ) =  OUT1 D  − VD + VCE  DCMIN  Note that the high voltage buck will still regulate at an input voltage that exceeds VIN(MAX) (up to 35V). However, the switching frequency will be lowered to satisfy the equation (Figure 1). Once the input voltage reaches 36.
PAGE 14
LT3640 Applications Information To guarantee sufficient output current, peak inductor current must be lower than the switch current limit (ILIM). The largest inductor current ripple occurs at the highest VIN. To guarantee current capacity, use VIN(MAX) in the above formula. The inductance for the low voltage channel is: L2 = 1.
PAGE 15
LT3640 Applications Information Output Capacitors and Output Ripple The output capacitor has two essential functions. In steady state, it determines the output voltage ripple. In transient, it stores energy in order to satisfy transient loads and stabilize the control loop. Ceramic capacitors have low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: 150 COUT1 = VOUT • fS where fS is in MHz, and COUT is the recommended output capacitance in µF.
PAGE 16
LT3640 Applications Information BST and SW Pin Considerations The high voltage channel requires an external capacitor between the BST and SW pins and an external boost diode from a voltage source to the BST pin. In most cases, a 0.22µF capacitor will work well. The (BST-SW) voltage cannot exceed 5.5V, and must be more than 2.3V for best efficiency. Connect the boost diode to any voltage between 2.7V and 5.5V. The VIN2 pin is the best choice if the low voltage channel is used.
PAGE 17
LT3640 Applications Information In the event of VIN undervoltage lockout, VIN overvoltage lockout or the EN/UVLO pin being driven below 1.26V, the soft-start latch is set, triggering a start-up sequence. A latch is set to discharge the SS2 pin at power-up. After the FB1 pin reaches 1.165V, the VIN2 voltage is above 2.3V, the EN2 pin is enabled, and the SS2 pin is below 100mV, the latch is reset. The internal 2µA current source starts to charge the SS2 pin.
PAGE 18
LT3640 Applications Information Reverse Protection In battery charging applications or in battery back-up systems, the output will be held high when the input to the LT3640 is absent. If the VIN pin is floated and the LT3640 is enabled, the LT3640’s internal circuitry will pull its quiescent current through the SW1 pin or the SW2 pin. This is fine if the system can tolerate a few mA in this state. If the LT3640 is disabled, the SW1 pin and the SW2 pin current will drop to essentially zero.
PAGE 19
LT3640 Applications Information pull-ups eliminate the need for external pull-ups when the rise time of these pins is not critical. The open-drain configuration allows wired-OR connections. The two power-on reset timers share one oscillator.
PAGE 20
LT3640 Applications Information Figure 9a shows the power-on reset timing. Having FB1 or FB2 high starts the CPOR oscillator. After t RST, the corresponding RST is released. When both RST1 and RST2 are released, the CWDT oscillator starts. Figure 9b shows the watchdog waveform with the WDI period between tWDL and tWDU. The WDI falling edge resets the CWDT oscillator. The CPOR oscillator is disabled and WDO remains high. Figure 9c shows the watchdog waveform with the WDI period longer than tWDU.
PAGE 21
LT3640 Typical Applications 2MHz 3.3V/1.3A and 1.8V/1A Buck Regulators VIN 5V TO 35V 0.22µF 4.7µF 100k D2 301k L1 3.3µH EN/UVLO VIN SW BST SW1 SYNC DA FB1 WDE EN2 LT3640 PGOOD 80.6k D1 49.9k VIN2 4.7µF L2 1µH RST1 RST2 SW2 100k WDO WDI L1: VISHAY IHLP-2020 L2: VISHAY IHLP-1616 D1: DIODES B240A D2: CENTRAL SEMI CMDSH-4E VOUT1 3.3V/1.3A 22µF VIN2 2.5V TO 5.5V VOUT2 1.8V/1.1A 22µF FB2 CWDT CPOR RT 1.5nF GND SS2 49.9k 1nF 32.4k 1.5nF SS1 1nF 3640 TA02 2MHz 5V/0.8A and 1.
PAGE 22
LT3640 Package Description FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1.
PAGE 23
LT3640 Package Description UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) 0.75 ± 0.05 PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP R = 0.05 TYP 27 28 0.40 ± 0.
PAGE 24
LT3640 Typical Application 2MHz 3.3V/0.8A and 0.8V/1.2A Buck Regulators 0.22µF VIN 4V TO 35V 4.7µF 3.3µH EN/UVLO VIN SW BST SW1 80.6k DA SYNC VOUT1 3.3V/0.8A 22µF FB1 WDE 49.9k EN2 LT3640 PGOOD VIN2 RST1 0.47µH RST2 SW2 16.5k WDO WDI VOUT2 0.8V/1.2A 22µF FB2 CWDT CPOR RT 1.5nF 1.5nF GND SS2 32.4k SS1 49.9k 1nF 1nF 3640 TA05 Related Parts PART NUMBER DESCRIPTION COMMENTS LT3689 36V, 60V Transient Protection, 800mA, 2.