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LT3437

3437fc

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High Voltage Power Conversion

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14V and 42V Automotive Systems

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Industrial Power Systems

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Distributed Power Systems

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Battery-Powered Systems

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Powered Ethernet

High Voltage 500mA, 200kHz

Step-Down Switching Regulator

with 100µA Quiescent Current

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Wide Input Range: 3.3V to 60V

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Load Dump (Input Transient) Protection to 80V

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500mA Peak Switch Current

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Burst Mode

Operation: 100µA Quiescent Current**

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Low Shutdown Current: I

< 1µA

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Defeatable Burst Mode Operation

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200kHz Switching Frequency

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Saturating Switch Design: 0.8Ω On-Resistance

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Peak Switch Current Maintained Over

Full Duty Cycle Range*

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1.25V Feedback Reference Voltage

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Easily Synchronizable

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Soft-Start Capability

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Small 10-Pin Thermally Enhanced DFN Package

The LT

3437 is a 200kHz monolithic buck switching

regulator that accepts input voltages up to 80V. A high

efficiency 500mA, 0.8Ω switch is included on the die along

with all the necessary oscillator, control and logic cir-

cuitry. Current mode topology is used for fast transient

response and good loop stability.

Innovative design techniques along with a new high volt-

age process achieve high efficiency over a wide input

range. Efficiency is maintained over a wide output current

range by employing Burst Mode operation at low currents,

utilizing the output to bias the internal circuitry, and by

using a supply boost capacitor to fully saturate the power

switch. Burst Mode operation can be defeated by a logic

high signal on the SYNC pin which results in lower light

load ripple at the expense of light load efficiency. Patented

circuitry maintains peak switch current over the full duty

cycle range.* Shutdown reduces input supply current to

less than 1µA. External synchronization can be imple-

mented by driving the SYNC pin with logic-level inputs. A

single capacitor from the C

pin to the output provides a

controlled output voltage ramp (soft-start).

The LT3437 is available in a low profile (0.75mm) 3mm ×

3mm 10-pin DFN package or a 16-Pin TSSOP Package,

both with exposed pad leadframes for low thermal

resistance.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode

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

property of their respective owners. *Protected by U.S. Patents, including 6498466.

**See Burst Mode Operation section for conditions.

SHDN

BOOST

BIAS

LT3437

2.2µF

100V

CER

330pF

0.1µF

100µH

BAS21

SYNC

GND

100µF

6.3V

TANT

OUT

3.3V

400mA

4.5V TO 80V*

10MQ100N

1500pF

25k

165k

100k

3437 TA01

27pF

*FOR INPUT VOLTAGES ABOVE 60V RESTRICTIONS APPLY

14V to 3.3V Step-Down Converter with

100µA No Load Quiescent Current

Supply Current vs Input Voltage

INPUT VOLTAGE (V)

SUPPLY CURRENT (µA)

100

10 20 30 40

3437 TA02

200

120

140

160

180

8060 70

Input Voltage

Transient Response

LOAD DUMP COLD CRANK

20V/DIV

OUT

20mV/DIV

AC COUPLED

3437 TA03

50ms/DIV

OUT

250mA

Summary of content (28 pages)

PAGE 1
LT3437 High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100µA Quiescent Current DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LT®3437 is a 200kHz monolithic buck switching regulator that accepts input voltages up to 80V. A high efficiency 500mA, 0.8Ω switch is included on the die along with all the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability. Wide Input Range: 3.
PAGE 2
LT3437 U W W W ABSOLUTE AXI U RATI GS (Note 1) VIN, SHDN, BIAS, SW Operating ............................. 60V VIN, SHDN 100ms Transient, <15% Duty Cycle ....... 80V BOOST Pin Above SW ............................................ 35V BOOST Pin Voltage Operating ................................. 75V BOOST Pin 100ms Transient, <15% Duty Cycle ...... 85V SYNC, CSS, FB .......................................................... 6V Operating Junction Temperature Range LT3437EDD (Note 2) ..................
PAGE 3
LT3437 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the –40°C to 125°C temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BIAS = 5V, FB = 1.25V, CSS/SYNC = 0V unless otherwise noted. SYMBOL IVIN PARAMETER Supply Quiescent Current CONDITIONS MIN BIAS = 0V, FB = 1.15V, VC = 0.8V, SYNC = 2V BIAS = 5V, FB = 1.15V, VC = 0.8V, SYNC = 2V Minimum BIAS Voltage (Note 5) ● TYP MAX 1.35 0.475 2 1 UNITS 2.7 3.
PAGE 4
LT3437 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the –40°C to 140°C temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BIAS = 5V, FB = 1.25V, CSS/SYNC = 0V unless otherwise noted. IVIN Supply Quiescent Current BIAS = 0V, FB = 1.15V, VC = 0.8V, SYNC = 2V BIAS = 5V, FB = 1.15V, VC = 0.8V, SYNC = 2V Minimum BIAS Voltage (Note 5) ● 1.35 0.475 2 1 mA mA 2.7 3.
PAGE 5
LT3437 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency and Power Loss vs Load Current 240 450 50 250 40 200 30 150 POWER LOSS 100 FREQUENCY (kHz) 60 300 FB VOLTAGE (V) 350 230 1.28 400 POWER LOSS (mW) EFFICIENCY (%) VIN = 12V 90 VOUT = 3.3V TA = 25°C 80 70 EFFICIENCY 250 1.30 500 100 20 Oscillator Frequency vs Temperature FB Voltage vs Temperature 1.26 1.24 0 0.1 1 10 100 LOAD CURRENT (mA) 210 200 190 180 1.22 170 1.
PAGE 6
LT3437 U W TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Current Threshold vs FB Voltage Oscillator Frequency vs FB Voltage 35 Switch On Voltage (VCESAT) 600 250 SOFT-START DEFEATED 30 20 15 10 VOLTAGE (mV) FREQUENCY (kHz) ICSS (µA) 25 150 100 50 0 0 0.2 1.0 0.6 0.8 0.4 FB VOLTAGE (V) 0 1.2 0.25 0.75 0.50 FB VOLTAGE (V) 1.00 180 160 160 6.0 5.0 20 3.5 10 20 30 40 50 60 INPUT VOLTAGE (V) 70 3.3V TO START 4.5 4.0 0 5V TO RUN 5.
PAGE 7
LT3437 U W TYPICAL PERFOR A CE CHARACTERISTICS Maximum Duty Cycle vs Temperature Dropout Operation MAXIMUM DUTY CYCLE (%) 5 OUTPUT VOLTAGE (V) 94.5 VOUT = 5V ILOAD = 250mA BOOST DIODE = DIODES INC B1100 4 3 2 0.6 ILOAD = 250mA 94.0 0.5 93.5 0.4 VC VOLTAGE (V) 6 93.0 92.5 92.0 1 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) 5.5 6.
PAGE 8
LT3437 U U U PI FU CTIO S (DD/FE) SW (Pin 1/Pin 2): The SW pin is the emitter of the on-chip power NPN switch. This pin is driven up to the input pin voltage during switch on time. Inductor current drives the SW pin negative during switch off time. Negative voltage is clamped with an external schottky catch diode to prevent excessive negative voltages. NC (Pins 1, 3, 5, 7, 13, 16)(FE Package ONLY): No Connection. The NC pins are electrically isolated from the LT3437.
PAGE 9
LT3437 U U U PI FU CTIO S (DD/FE) BIAS (Pin 6/Pin 10): The BIAS pin is used to improve efficiency when operating at higher input voltages and light load current. Connecting this pin to the regulated output voltage forces most of the internal circuitry to draw its operating current from the output voltage rather than the input supply. This architecture increases efficiency especially when the input voltage is much higher than the output.
PAGE 10
LT3437 W BLOCK DIAGRA VIN INTERNAL REF UNDERVOLTAGE LOCKOUT BIAS THERMAL SHUTDOWN 2.4V SLOPE COMP Σ 200kHz OSCILLATOR + CURRENT COMP – SYNC SHDN BOOST ANTISLOPE COMP + R SHDN COMP S – SWITCH Q LATCH DRIVER CIRCUITRY SW 1.3V CSS FB SOFT-START BURST MODE DETECT FOLDBACK DETECT VC CLAMP – ERROR AMP 1.25V GND + VC PGND 3437 BD Figure 1. LT3437 Block Diagram The LT3437 is a constant frequency, current mode buck converter.
PAGE 11
LT3437 U W U U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS Table 1 The feedback (FB) pin on the LT3437 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage, and the remaining part talks about frequency foldback and soft-start features. Please read both parts before committing to a final design.
PAGE 12
LT3437 U U W U APPLICATIO S I FOR ATIO high input current surge during start-up. The soft-start function reduces input current surge by regulating switch current via the VC pin to maintain a constant voltage ramp rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from the CSS pin to the output determines the maximum output dV/dt. When the feedback voltage is below 0.4V, the VC pin will rise, resulting in an increase in switch current and output voltage.
PAGE 13
LT3437 U W U U APPLICATIO S I FOR ATIO voltage could be unacceptable. Table 2 shows some typical solid tantalum surface mount capacitors. Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E CASE SIZE AVX TPS ESR MAX (Ω) RIPPLE CURRENT (A) 0.1 to 0.3 0.7 to 1.1 0.1 to 0.3 0.7 to 1.1 0.2 0.
PAGE 14
LT3437 U W U U APPLICATIO S I FOR ATIO Discontinuous operation occurs when: VOUT 10mV/DIV 100µF TANTALUM ESR 75mΩ IOUT(DIS) ≤ VOUT 10mV/DIV 100µF CERAMIC ( VOUT VIN – VOUT ) 2(L)( f)(VIN ) For VOUT = 5V, VIN = 8V and L = 68µH: VSW 10V/DIV VIN = 12V VOUT = 3.3V ILOAD = 500mA L = 100µH 1µs/DIV 3437 F03 Figure 3. LT3437 Ripple Voltage Waveform MAXIMUM OUTPUT LOAD CURRENT Maximum load current for a buck converter is limited by the maximum switch current rating (IPK).
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LT3437 U W U U APPLICATIO S I FOR ATIO CHOOSING THE INDUCTOR For most applications the output inductor will fall in the range of 68µH to 220µH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT3437 switch, which has a 0.5A limit. Higher values also reduce output ripple voltage and reduce core loss.
PAGE 16
LT3437 U W U U APPLICATIO S I FOR ATIO Short-Circuit Considerations The LT3437 is a current mode controller. It uses the VC node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the VC node, nominally 1.5V, then acts as an output switch peak current limit. This action becomes the switch current limit specification.
PAGE 17
LT3437 U W U U APPLICATIO S I FOR ATIO 200 Burst Mode OPERATION VOUT = 3.3V 180 The minimum average input current depends on the VIN to VOUT ratio, VC frequency compensation, feedback divider network and Schottky diode leakage.
PAGE 18
LT3437 U W U U APPLICATIO S I FOR ATIO These requirements result in the use of a Schottky type diode. DC switching losses are minimized due to its low forward voltage drop, and AC behavior is benign due to its lack of a significant reverse recovery time. Schottky diodes are generally available with reverse voltage ratings of 60V, and even 100V, and are price competitive with other types. The use of so-called “ultrafast” recovery diodes is generally not recommended.
PAGE 19
LT3437 U W U U APPLICATIO S I FOR ATIO SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT LT3437 2 The SHDN pin on the LT3437 controls the operation of the IC. When the voltage on the SHDN pin is below the 1.3V shutdown threshold, the LT3437 is placed in a “zero” supply current state. Driving the SHDN pin above the shutdown threshold enables normal operation. The SHDN pin has an internal sink current with a typical value of 5µA.
PAGE 20
LT3437 U W U U APPLICATIO S I FOR ATIO LAYOUT CONSIDERATIONS As with all high frequency switchers, when considering layout, care must be taken in order to achieve optimal electrical, thermal and noise performance. For maximum efficiency, switch rise and fall times are typically in the nanosecond range. To prevent noise both radiated and conducted, the high speed switching current path, shown in Figure 8, must be kept as short as possible. This is implemented in the suggested layouts of Figure 9.
PAGE 21
LT3437 U W U U APPLICATIO S I FOR ATIO C1 D1 L1 C2 3437 F09b FE Package Topside Metal DD Package Topside Metal Figure 9.
PAGE 22
LT3437 U W U U APPLICATIO S I FOR ATIO THERMAL CALCULATIONS Power dissipation in the LT3437 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formulas show how to calculate each of these losses. These formulas assume continuous mode operation, and should not be used for calculating efficiency at light load currents.
PAGE 23
LT3437 U W U U APPLICATIO S I FOR ATIO increase in internal dissipation is of insufficient time duration to raise die temperature significantly. A second consideration is controllability. A potential limitation occurs with a high step-down ratio of VIN to VOUT, as this requires a correspondingly narrow minimum switch on time.
PAGE 24
LT3437 U U W U APPLICATIO S I FOR ATIO A zero can be added into the loop by placing a resistor (RC) at the VC pin in series with the compensation capacitor, CC, or by placing a capacitor (CFB) between the output and the FB pin. When using RC, the maximum value has two limitations. First, the combination of output capacitor ESR and RC may stop the loop rolling off altogether.
PAGE 25
LT3437 U PACKAGE DESCRIPTIO DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) R = 0.115 TYP 6 0.38 ± 0.10 10 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) PACKAGE OUTLINE 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD10) DFN 1103 5 0.25 ± 0.05 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) 1 0.75 ±0.05 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1.
PAGE 26
LT3437 U PACKAGE DESCRIPTIO FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 ±0.10 9 2.94 (.116) 4.50 ±0.10 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 ±0.05 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. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
PAGE 27
LT3437 U TYPICAL APPLICATIO 14V to 3.3V Step-Down Converter with 100µA No Load Quiescent Current BAS21 VIN 4.5V TO 80V* VIN 2.2µF 100V CER BOOST SW LT3437 VC 0.1µF 10MQ100N CSS VBIAS 330pF 1500pF 100µH 0.1µF SHDN VOUT 3.3V 400mA 165k 27pF 25k SYNC FB 100k GND *FOR INPUT VOLTAGES ABOVE 60V, SOME RESTRICTIONS MAY APPLY 100µF 6.
PAGE 28
LT3437 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1765 25V, 3A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 15µA, SO-8, TSSOP16E LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD < 25µA, TSSOP16/E LT1767 25V, 1.5A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter VIN: 3V to 25V, VOUT(MIN) = 1.