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LTC3642

3642fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

High Efﬁciency, High Voltage

50mA Synchronous

Step-Down Converter

The LTC

3642 is a high efﬁciency, high voltage step-down

DC/DC converter with internal high side and synchronous

power switches that draws only 12μA typical DC sup-

ply current at no load while maintaining output voltage

regulation.

The LTC3642 can supply up to 50mA load current and

features a programmable peak current limit that provides

a simple method for optimizing efﬁciency in lower current

applications. The LTC3642’s combination of Burst Mode

operation, integrated power switches, low quiescent cur-

rent, and programmable peak current limit provides high

efﬁciency over a broad range of load currents.

With its wide 4.5V to 45V input range and internal

overvoltage monitor capable of protecting the part through

60V surges, the LTC3642 is a robust converter suited for

regulating a wide variety of power sources. Additionally,

the LTC3642 includes a precise run threshold and soft-start

feature to guarantee that the power system start-up is

well-controlled in any environment.

The LTC3642 is available in the thermally enhanced

3mm × 3mm DFN and MS8E packages.

Efﬁciency and Power Loss vs Load Current

Wide Input Voltage Range: 4.5V to 45V

Tolerant of 60V Input Transients

Internal High Side and Low Side Power Switches

No Compensation Required

50mA Output Current

Low Dropout Operation: 100% Duty Cycle

Low Quiescent Current: 12µA

0.8V Feedback Voltage Reference

Adjustable Peak Current Limit

Internal and External Soft-Start

Precise RUN Pin Threshold with Adjustable

Hysteresis

3.3V, 5V and Adjustable Output Versions

Only Three External Components Required for Fixed

Output Versions

Low Proﬁle (0.75mm) 3mm × 3mm DFN and

Thermally-Enhanced MS8E Packages

4mA to 20mA Current Loops

Industrial Control Supplies

Distributed Power Systems

Portable Instruments

Battery-Operated Devices

Automotive Power Systems

5V, 50mA Step-Down Converter

LTC3642-5

RUN

HYST

3642 TA01a

5V TO 45V

1µF 10µF

OUT

50mA

OUT

SET

GND

150µH

LOAD CURRENT (mA)

0.1

EFFICIENCY (%)

POWER LOSS (mW)

100

1 10 100

3642 TA01b

100

0.1

= 10V

EFFICIENCY

POWER LOSS

L, LT, LTC, LTM, Burst Mode, Linear Technology, 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.

Summary of content (22 pages)

PAGE 1
LTC3642 High Efficiency, High Voltage 50mA Synchronous Step-Down Converter DESCRIPTION FEATURES n n n n n n n n n n n n n n Wide Input Voltage Range: 4.5V to 45V Tolerant of 60V Input Transients Internal High Side and Low Side Power Switches No Compensation Required 50mA Output Current Low Dropout Operation: 100% Duty Cycle Low Quiescent Current: 12µA 0.8V Feedback Voltage Reference Adjustable Peak Current Limit Internal and External Soft-Start Precise RUN Pin Threshold with Adjustable Hysteresis 3.
PAGE 2
LTC3642 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Supply Voltage...................................... –0.3V to 60V SW Voltage (DC)............................ –0.3V to (VIN + 0.3V) RUN Voltage............................................... –0.3V to 60V HYST, ISET, SS Voltages................................ –0.3V to 6V VFB................................................................ –0.3V to 6V VOUT (Fixed Output Versions)........................ –0.3V to 6V Operating Junction Temperature Range (Note 2)...
PAGE 3
LTC3642 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are for TA = 25°C (Note 2). VIN = 10V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply (VIN) VIN Input Voltage Operating Range 4.
PAGE 4
LTC3642 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current FIGURE 11 CIRCUIT ISET OPEN VOUT = 5V 95 VIN = 10V VIN = 15V 85 80 VIN = 45V VIN = 24V VIN = 36V 75 70 5.03 0.10 0 1 10 LOAD CURRENT (mA) 100 –0.30 5 15 10 20 25 30 35 INPUT VOLTAGE (V) 0.799 110 5.6 5.4 5.2 5.0 4.8 4.6 4.
PAGE 5
LTC3642 TYPICAL PERFORMANCE CHARACTERISTICS Switch On-Resistance vs Input Voltage TA = 25°C, unless otherwise noted. Switch On-Resistance vs Temperature 5 4.5 Switch Leakage Current vs Temperature 0.6 VIN = 10V 3.5 TOP 3.0 2.5 2.0 BOTTOM 1.5 1.0 4 SWITCH LEAKAGE CURRENT (µA) SWITCH ON-RESISTANCE (Ω) SWITCH ON-RESISTANCE (Ω) 4.0 TOP 3 BOTTOM 2 1 VIN = 45V 0.5 0.4 SW = 0V 0.3 0.2 SW = 45V 0.1 0.
PAGE 6
LTC3642 PIN FUNCTIONS SW (Pin 1): Switch Node Connection to Inductor. This pin connects to the drains of the internal power MOSFET switches. VIN (Pin 2): Main Supply Pin. A ceramic bypass capacitor should be tied between this pin and GND (Pin 8). ISET (Pin 3): Peak Current Set Input. A resistor from this pin to ground sets the peak current trip threshold. Leave floating for the maximum peak current (115mA). Short this pin to ground for the minimum peak current (25mA).
PAGE 7
LTC3642 BLOCK DIAGRAM VIN 1µA ISET 2 C2 3 PEAK CURRENT COMPARATOR – + RUN + 5 1.2V LOGIC AND SHOOTTHROUGH PREVENTION – SW L1 VOUT 1 C1 HYST 7 + REVERSE CURRENT COMPARATOR FEEDBACK COMPARATOR GND GND 8 9 – VOLTAGE REFERENCE + + – 0.800V 5µA SS 4 R1 R2 PART NUMBER R1 R2 LTC3642 LTC3642-3.3 LTC3642-5 0 2.5M 4.
PAGE 8
LTC3642 OPERATION (Refer to Block Diagram) The LTC3642 is a step-down DC/DC converter with internal power switches that uses Burst Mode control, combining low quiescent current with high switching frequency, which results in high efficiency across a wide range of load currents.
PAGE 9
LTC3642 OPERATION (Refer to Block Diagram) inductor current from 25mA during its 1ms ramp time to the set peak current threshold. The external ramp on the SS pin does not limit the peak inductor current during start-up; however, placing a capacitor from the ISET pin to ground does provide this capability. Peak Inductor Current Programming The offset of the peak current comparator nominally provides a peak inductor current of 115mA.
PAGE 10
LTC3642 APPLICATIONS INFORMATION The basic LTC3642 application circuit is shown on the front page of this data sheet. External component selection is determined by the maximum load current requirement and begins with the selection of the peak current programming resistor, RISET. The inductor value L can then be determined, followed by capacitors CIN and COUT.
PAGE 11
LTC3642 APPLICATIONS INFORMATION 700 VOUT = 5V ISET OPEN SWITCHING FREQUENCY (kHz) 600 500 L = 47µH L = 68µH 400 L = 100µH 300 L = 150µH 200 L = 220µH 100 L = 470µH 0 where board area is not a limiting factor, inductors with larger cores can be used, which extends the recommended range of Figure 4 to larger values. 10 5 15 20 25 30 35 INPUT VOLTAGE (V) Inductor Core Selection 40 45 3642 F02 Figure 2. Switching Frequency for VOUT = 5V 500 VOUT = 3.
PAGE 12
LTC3642 APPLICATIONS INFORMATION This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based only on 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required.
PAGE 13
LTC3642 APPLICATIONS INFORMATION Output Voltage Programming For the adjustable version, the output voltage is set by an external resistive divider according to the following equation:  R1  VOUT = 0.8V • 1+   R2  controller is enabled. Figure 7 shows examples of configurations for driving the RUN pin from logic. VSUPPLY VIN 4.7M LTC3642 RUN The resistive divider allows the VFB pin to sense a fraction of the output voltage as shown in Figure 6. Output voltage adjustment range is from 0.8V to VIN.
PAGE 14
LTC3642 APPLICATIONS INFORMATION The minimum value of these thresholds is limited to the internal VIN UVLO thresholds that are shown in the Electrical Characteristics table. The current that flows through this divider will directly add to the shutdown, sleep and active current of the LTC3642, and care should be taken to minimize the impact of this current on the overall efficiency of the application circuit.
PAGE 15
LTC3642 APPLICATIONS INFORMATION The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL.
PAGE 16
LTC3642 APPLICATIONS INFORMATION The undervoltage lockout requirement on VIN can be satisfied with a resistive divider from VIN to the RUN and HYST pins. Choose R1 = 2M and calculate R2 and R3 as follows:   1.21V  • R1= 224k R2 =   – 1.21V V  IN(RISING)    1.1V  • R1– R2 = 90.8k R3 =   – 1.1V V  IN(FALLING)  3. Keep the switching node, SW, away from all sensitive small signal nodes.
PAGE 17
LTC3642 TYPICAL APPLICATIONS Efficiency vs Load Current VIN CIN 4.7µF SW LTC3642 RUN HYST ISET RSET 94 R1 4.2M VFB COUT 100µF VOUT 5V SS RSET = 750k RSET = 500k R2 800k CSS 47nF GND VIN = 10V 93 EFFICIENCY (%) VIN 5V TO 45V L1 220µH 3642 F11a CIN: TDK C5750X7R2A475MT COUT: AVX 1812D107MAT L1: TDK SLF7045T-221MR33-PF 92 ISET OPEN 91 90 89 88 Figure 11. High Efficiency 5V Regulator 10 LOAD CURRENT (mA) 1 100 3642 F11b 3.
PAGE 18
LTC3642 TYPICAL APPLICATIONS Small Size, Limited Peak Current, 10mA Regulator VIN 7V TO 45V L1 470µH CIN 1µF R3 470k R4 100k R5 33k VIN SW R1 470k LTC3642 RUN VFB COUT 10µF VOUT 5V 10mA R2 88.7k HYST SS ISET GND 3642 TA05a CIN: TDK C3225X7R1H105KT COUT: AVX 08056D106KAT2A L1: MURATA LQH32CN471K23 High Efficiency 15V, 10mA Regulator 100 L1 4700µH CIN 1µF VIN SW LTC3642 RUN ISET SS VFB HYST GND R1 3M R2 169k 3642 TA07a COUT 4.
PAGE 19
LTC3642 PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698 Rev C) 0.70 ±0.05 3.5 ±0.05 1.65 ±0.05 2.10 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED PIN 1 TOP MARK (NOTE 6) 0.200 REF 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 5 0.40 ± 0.10 8 1.65 ± 0.10 (2 SIDES) 0.75 ±0.05 4 0.25 ± 0.05 1 (DD8) DFN 0509 REV C 0.50 BSC 2.38 ±0.10 0.00 – 0.
PAGE 20
LTC3642 PACKAGE DESCRIPTION MS8E Package 8-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1662 Rev I) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 (.074) 1 1.88 ± 0.102 (.074 ± .004) 0.29 REF 1.68 (.066) 0.889 ± 0.127 (.035 ± .005) 0.05 REF 5.23 (.206) MIN DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 1.68 ± 0.102 3.20 – 3.45 (.066 ± .004) (.126 – .136) 8 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.65 (.0256) BSC 0.42 ± 0.038 (.
PAGE 21
LTC3642 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION B 6/10 Text updates in Description PAGE NUMBER 1 Updates to Absolute Maximum Ratings 2 LTC3642IMS8E-3.3E#PBF changed to LTC3642IMS8E-3.
PAGE 22
LTC3642 TYPICAL APPLICATION 5V, 50mA Regulator for Automotive Applications VBATT 12V L1 220µH CIN 1µF VIN SW LTC3642 RUN ISET SS CIN: TDK C3225X7R2A105M COUT: KEMET C1210C106K4RAC L1: COILTRONICS DRA73-221-R VFB HYST GND R1 470k COUT 10µF VOUT* 5V 50mA R2 88.7k 3642 TA06a *VOUT = VBATT FOR VBATT < 5V RELATED PARTS PART NUMBER LTC3631/LTC3631-3.