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LT3688

3688f

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Dual 800mA Step-Down

Switching Regulator with

Power-On Reset and

Watchdog Timer

The LT

3688 is an adjustable frequency (350kHz to

2.2MHz) dual monolithic step-down switching regulator

with two power-on reset timers and a watchdog timer.

The regulator operates off inputs up to 36V. Low ripple

Burst Mode

operation maintains high efﬁ ciency at low

output current while keeping output ripple below 25mV in

a typical application, with input quiescent current of just

115A. Shutdown circuitry reduces input supply current

to less than 1A while EN/UVLO is pulled low.

The reset and watchdog timeout periods are both adjust-

able using external capacitors. Tight accuracy speciﬁ ca-

tions and glitch immunity ensure reliable reset operation

without false triggering. The open collector RST pins will

pull down if the monitored output voltage drops 10%

below the programmed value. The LT3688 watchdog timer

monitors for watchdog falling edges grouped too close

together or too far apart.

The LT3688 is available in 24-Pin TSSOP and 4mm ×

4mm QFN packages, each with an exposed pad for low

thermal resistance.

5V and 3.3V Regulator with Power-On Reset and Watchdog Timers

Wide Input Range:

Operation from 3.8V to 36V

Low Ripple (<25mV

P-P

) Burst Mode Operation:

= 115μA at 12V

to 3.3V and 5V

Programmable, Defeatable Window Watchdog Timer

Two Independently Programmable Power-On-Reset

Timers

Synchronizable, Adjustable 350kHz-2.2MHz

Switching Frequency

Two 800mA Output Switching Regulators with Internal

Power Switches

Programmable Input Undervoltage Lockout with

Hysteresis

Thermally Enhanced 24-Pin TSSOP and 4mm × 4mm

QFN Packages

Automotive Electronic Control Units

Industrial Power Supplies

High-Reliability µProcessor Systems

EN/UVLO V

LT3688

GND

BIAS

BST1

0.22µF

4.7µF

OUT1

800mA

6V TO 36V

22µF

22pF

18µH

12µH

523k

100k

316k

100k

1nF

µP

0.22µF

SW1

DA1

BST2

SW2

DA2

FB1

RUN/SS1

WDI

WDO

RST1

RST2

I/O

RESET

FB2

RUN/SS2

WDT

POR1

POR2

SYNC

22µF

1nF

3688 TA01a

4.7nF

110k

= 500kHz

OUT2

3.3V

800mA

22pF

Efﬁ ciency

3688 TA01b

0.0001 10.001 0.01 0.1

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (mW)

10.0

100.0

1000.0

1.0

0.01

0.1

10000.0

= 12V

OUT

= 3.3V

L = 4.7µH

f = 800kHz

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.

Summary of content (28 pages)

PAGE 1
LT3688 Dual 800mA Step-Down Switching Regulator with Power-On Reset and Watchdog Timer DESCRIPTION FEATURES n n n n n n n n Wide Input Range: Operation from 3.8V to 36V Low Ripple (<25mVP-P) Burst Mode Operation: IQ = 115μA at 12VIN to 3.3V and 5V Programmable, Defeatable Window Watchdog Timer Two Independently Programmable Power-On-Reset Timers Synchronizable, Adjustable 350kHz-2.
PAGE 2
LT3688 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UVLO, CONFIG Voltage (Note 2)...................36V BST Voltage ..............................................................55V BST above SW Voltage .............................................30V BIAS Voltage .............................................................30V WDE, WDI, RST, WDO Voltage ...................................6V FB, RT , SYNC, RUN/SS Voltage ..................................6V CWDT , CPOR Voltage ................................
PAGE 3
LT3688 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 3) SYMBOL PARAMETER CONDITIONS l VIN Undervoltage Lockout MIN TYP MAX 3 3.5 3.8 UNITS V Quiescent Current from VIN VEN/UVLO = 0.3V VBIAS = 5V, Not Switching VBIAS = 0V, Not Switching l 0.01 65 235 1 105 310 μA μA μA Quiescent Current from BIAS VEN/UVLO = 0.
PAGE 4
LT3688 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 3) SYMBOL PARAMETER CONDITIONS MIN TYP RST Pull-Up Current (Note 6) –1.5 –2.5 μA WDO Pull-Up Current (Note 6) –1.5 –2.
PAGE 5
LT3688 TYPICAL PERFORMANCE CHARACTERISTICS No-Load Supply Current (Temperature) No-Load Supply Current (Input) VOUT1 = 5V VOUT2 = 3.3V 120 SUPPLY CURRENT (μA) SUPPLY CURRENT (μA) 140 100 80 60 40 Maximum Load Current (5V) 3000 2 CATCH DIODE: B140HB VIN = 12V 2500 VOUT1 = 5V VOUT2 = 3.3V 2000 1500 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 1000 L = 15μH L = 10μH 1 0.5 500 20 0 0 10 20 VIN (V) 30 0 –50 –25 40 0 2.0 1.4 0.5 1.
PAGE 6
LT3688 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Switching Frequency vs Temperature Switching Frequency vs RT 2.5 Minimum Switch On-Time 200 1.2 ISW = 700mA 2.0 1.5 1.0 0.5 0 0 50 100 RT (kΩ) 150 1.0 MINIMUM ON TIME (ns) SWITCHING FREQUENCY (MHz) SWITCHING FREQUENCY (MHz) RT = 49.9k 0.8 0.6 0.4 100 50 0.2 0.
PAGE 7
LT3688 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Typical Transient Duration vs Comparator Overdrive Power-On Reset Threshold vs Temperature Watchdog Window Mode Period vs Temperature 700 0.730 25 CWDT = 1000pF 600 20 0.715 500 tWDU (ms) RESET DELAY (μs) 0.720 400 300 15 10 200 5 100 0 25 50 75 100 TEMPERATURE (°C) 0 0.
PAGE 8
LT3688 PIN FUNCTIONS (QFN/TSSOP) RT (Pin 1/Pin 22): The RT pin is used to set the internal oscillator frequency. Tie a resistor from RT to GND to set the switching frequency. SYNC (Pin 2/Pin 23): Drive the SYNC pin with a logiclevel signal with positive and negative pulse widths of at least 150ns. Do not ﬂoat this pin. Tie to GND if the SYNC feature is not used. EN/UVLO (Pin 3/Pin 24): The EN/UVLO pin is used to put the LT3688 in shutdown mode. Pull the pin below 0.3V to shut down the LT3688. The 1.
PAGE 9
LT3688 PIN FUNCTIONS (QFN/TSSOP) CPOR1, CPOR2 (Pins 23, 21/Pins 20, 18): Place a capacitor between this pin and ground to set the power-on-reset timeout period. Exposed Pad (Pin 25/Pin 25): Ground. Tie the exposed pad directly to the ground plane. The exposed pad metal of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. The device must be soldered to the circuit board for proper operation.
PAGE 10
LT3688 OPERATION The LT3688 is a constant-frequency, current mode stepdown regulator with two reset timers and a watchdog timer that perform microprocessor supervisory functions. Operation can be best understood by referring to the Block Diagram. Keeping the EN/UVLO pin at ground completely shuts off the part drawing minimal current from the VIN source. To turn on the internal bandgap and the rest of the logic circuitry, raise the EN/UVLO pin above the accurate threshold of 1.25V.
PAGE 11
LT3688 TIMING DIAGRAMS Power-On Reset Timing VOUT VUV tUV tRST RST Watchdog Timing t < tWDL tWDTO WDI WDO 3686 TD01 tWDTO tWDU 3688f 11
PAGE 12
LT3688 TIMING DIAGRAMS VOUT1 VUV tRST1 RST1 VUV VOUT2 tRST2 RST2 tWDU WDO WDI STARTUP TIMING (VCONFIG = HIGH) VOUT1 VUV VUV VOUT2 tRST1 RST1 RST2 tWDU WDO WDI STARTUP TIMING (VCONFIG = LOW) VOUT1 VUV tRST1 RST1 VOUT2 VUV tRST2 RST2 tWDU WDO WDI STARTUP TIMING (VCONFIG = OPEN) tRST1 = PROGRAMMED RESET PERIOD (CPOR1) tRST2 = PROGRAMMED RESET PERIOD (CPOR2) tWDU = WATCHDOG WINDOW UPPER BOUNDRY VUV = RESET THRESHOLD 3688 TD 3688f 12
PAGE 13
LT3688 APPLICATIONS INFORMATION Setting the Output Voltage The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: ⎛V ⎞ R1= R2 ⎜ OUT – 1⎟ ⎝ 0.8V ⎠ For reference designators, refer to the Block Diagram. Setting the Switching Frequency The LT3688 uses a constant-frequency PWM architecture that can be programmed to switch from 350 kHz to 2.2 MHz by using a resistor tied from the RT pin to ground.
PAGE 14
LT3688 APPLICATIONS INFORMATION The LT3688 will regulate the output voltage at input voltages greater than VIN(PS). For example, an application with an output voltage of 3.3V and switching frequency of 2.1MHz has a VIN(PS) of 12.7V, as shown in Figure 1. Figure 2 shows operation at 27V. Output ripple and peak inductor current have signiﬁcantly increased. A saturating inductor may further reduce performance. In pulse skip mode, the LT3688 skips switching pulses to maintain output regulation.
PAGE 15
LT3688 APPLICATIONS INFORMATION Table 2. Inductor Vendors VENDOR Murata TDK Toko Sumida PART SERIES TYPE URL LQH55D Open SLF7045 SLF10145 Shielded Shielded www.component.tdk.com DC62CB D63CB D75C D75F Shielded Shielded Shielded Open www.toko.com CR54 CDRH74 CDRH6D38 CR75 Open Shielded Shielded Open www.sumida.com www.murata.com The optimum inductor for a given application may differ from the one indicated by this simple design guide.
PAGE 16
LT3688 APPLICATIONS INFORMATION additional bulk capacitance may be necessary. This can be provided with a lower performance electrolytic capacitor. 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 LT3688 input and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 2.
PAGE 17
LT3688 APPLICATIONS INFORMATION IAVE (A) MBR0520L 20 0.5 MBR0540 40 0.5 620 MBRM120E 20 1 530 MBRM140 40 1 550 B0530W 30 0.5 B120 20 1 500 500 Part Number VF at 1A (mV) On Semiconductor Diodes Inc. B130 30 1 B140HB 40 1 DFLS140 40 1.1 510 Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3688 due to their piezoelectric nature.
PAGE 18
LT3688 APPLICATIONS INFORMATION Low Ripple Burst Mode Operation To enhance efﬁciency at light loads, the LT3688 operates in low ripple Burst Mode operation that keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3688 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor.
PAGE 19
LT3688 APPLICATIONS INFORMATION age source. However, the full beneﬁt of the BIAS pin is not realized unless it is at least 3V. Ensure that the maximum voltage ratings of the BST and BIAS pins are not exceeded. There is one particular issue to note if sequencing is used. If the BIAS pin is tied to VOUT2, it will be low during the startup of VOUT1. This will prevent the boost circuit from working on VOUT1 until it has risen to 90% of its programmed value, increasing the required startup voltage.
PAGE 20
LT3688 APPLICATIONS INFORMATION will charge these pins to ~2V. The RUN/SS pins provide a soft-start function that limits peak input current to the circuit during start-up. This helps to avoid drawing more current than the input source can supply or glitching the input supply when the LT3688 is enabled. The RUN/SS pins do not provide an accurate delay to start or an accurately controlled ramp at the output voltage, both of which depend on the output capacitance and the load current.
PAGE 21
LT3688 APPLICATIONS INFORMATION Output Voltage Monitoring The LT3688 provides power supply monitoring for microprocessor-based systems. The features include power-on reset (POR) and watchdog timing. A precise internal voltage reference and glitch immune precision POR comparator circuits monitor the LT3688 output voltages. Each channel’s output voltage must be above 90% of the programmed value for RST not to be asserted (refer to the Timing Diagram).
PAGE 22
LT3688 APPLICATIONS INFORMATION If WDO is low and RST goes low, then WDO will go high. The WDE pin allows the user to turn on and off the watchdog function. Leaving this pin open is okay and will automatically enable the watchdog. It has an internal weak pull-down to ground. The WDI pin has an internal weak pull-up that keeps the WDI pin high. If watchdog is disabled, leaving this pin open is acceptable.
PAGE 23
LT3688 APPLICATIONS INFORMATION ⎛ pF ⎞ CPOR = t RST • 200 ⎜ ⎟ ⎝ ms ⎠ This equation is accurate for reset timeout periods of 1ms, or greater. To program faster timeout periods, see the Reset Timeout Period vs Capacitance graph in the Typical Characteristics section. Leaving the CPOR pin unconnected will generate a minimum reset timeout of approximately 65μs. Maximum reset timeout is limited by the largest available low leakage capacitor.
PAGE 24
LT3688 APPLICATIONS INFORMATION D4 VIN VIN keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3688 to additional ground planes within the circuit board and on the bottom side. BIAS BOOST LTC3688 VOUT SW DA EN/UVLO GND FB + High Temperature Considerations 3688 F13 Figure 13. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input.
PAGE 25
LT3688 TYPICAL APPLICATIONS VIN 6V TO 36V C1 4.7μF ON OFF L1 18μH VOUT1 5V 800mA C2 0.22μF R1 523k C6 10pF C4 22μF VIN EN/UVLO BST1 BIAS BST2 SW1 SW2 LT3688 D1 DA2 FB2 CONFIG CWDT CPOR1 CPOR2 RT SYNC WDE I/O I/O RESET WDI WDO RST1 RST2 C9 1nF C7 10pF VOUT2 3.3V 800mA C5 22μF R4 100k RUN/SS2 RUN/SS1 μP R3 316k D2 DA1 FB1 C8 1nF R2 100k L2 12μH C3 0.22μF GND C1-C5: X5R OR X7R D1, D2: DIODES INC. B140 C10 4.7nF R5 110k C12 1nF C11 4.7nF 3688 TA02 fSW = 500kHz 5V and 3.
PAGE 26
LT3688 TYPICAL APPLICATIONS VIN 6V TO 36V C1 4.7μF ON OFF L1 8.2μH VOUT1 5V 800mA C6 10μF C4 10μF C2 0.1μF VIN EN/UVLO BST1 BIAS BST2 SW1 SW2 LT3688 D1 R1 523k CONFIG WDE I/O I/O RESET WDI WDO RST1 RST2 C9 1nF C7 10pF VOUT2 3.3V 800mA C5 22μF R4 100k RUN/SS2 RUN/SS1 μP R3 316k D2 DA2 FB2 DA1 FB1 C8 1nF R2 100k L2 8.2μH C3 0.1μF GND CWDT CPOR1 CPOR2 RT SYNC C1-C5: X5R OR X7R D1, D2: DIODES INC. B140 C10 4.7nF R5 20k C12 1nF C11 4.
PAGE 27
LT3688 PACKAGE DESCRIPTION FE Package 24-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1771 Rev Ø) Variation AA 7.70 – 7.90* (.303 – .311) 3.25 (.128) 3.25 (.128) 24 23 22 21 20 19 18 17 16 15 14 13 6.60 p0.10 2.74 (.108) 4.50 p0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 p0.05 1.05 p0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 1.20 (.047) MAX 0o – 8o 0.65 (.0256) BSC 0.50 – 0.75 (.020 – .
PAGE 28
LT3688 TYPICAL APPLICATION VIN 6V TO 36V C1 4.7μF ON OFF L1 18μH VOUT1 5V 800mA C6 10pF C4 22μF C2 0.22μF R1 523k VIN EN/UVLO BST1 BIAS BST2 SW1 SW2 LT3688 D1 C8 1nF R2 100k DA2 FB2 CONFIG WDI WDO RST1 RST2 R3 340k C9 1nF R4 100k C7 10pF VOUT2 3.3V 800mA C5 22μF RUN/SS2 WDE I/O I/O RESET L2 12μH D2 DA1 FB1 RUN/SS1 μP C3 0.22μF GND CWDT CPOR1 CPOR2 RT SYNC C1-C5: X5R OR X7R D1, D2: DFLS140 C10 4.7nF R5 49.9k C11 4.