Max756CSD

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MAX7 5 6 / MAX7 5 7

3.3V/5V/Adjustable-Output,Step-Up DC-DC Converters

________________________________________________________________ Maxim Integrated Products 1

Call toll free 1-800-998-8800 for free samples or literature.

19-0113; Rev. 2; 1/95

_______________General Description The MAX756/MAX757 are CMOS step-up DC-DC switch-ing regulators for small, low input voltage or battery-pow-ered systems. The MAX756 accepts a positive inputvoltage down to 0.7V and converts it to a higher pin-selectable output voltage of 3.3V or 5V. The MAX757 isan adjustable version that accepts an input voltage downto 0.7V and generates a higher adjustable output voltagein the range from 2.7V to 5.5V. Typical full-load efficienciesfor the MAX756/MAX757 are greater than 87%.

The MAX756/MAX757 provide three improvements overprevious devices. Physical size is reducedthe highswitching frequencies (up to 0.5MHz) made possible byMOSFET power transistors allow for tiny (

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Supply Voltage (OUT to GND) ....................................-0.3V, +7VSwitch Voltage (LX to GND)........................................-0.3V, +7VAuxiliary Pin Voltages (SHDN, LBI, LBO, REF,

3/5, FB to GND)........................................-0.3V, (V OUT + 0.3V)Reference Current (I REF ) ....................................................2.5mAContinuous Power Dissipation (T A = +70C)

Plastic DIP (derate 9.09mW/C above +70C) .............727mWSO (derate 5.88mW/C above +70C) ..........................471mW

Operating Temperature Ranges:MAX75_C_ _ ........................................................0C to +70CMAX75_E_ _ ......................................................-40C to +85C

Junction Temperature......................................................+150CStorage Temperature Range............................... -65to +160CLead Temperature (soldering, 10sec) ........................... +300C

ELECTRICAL CHARACTERISTICS(Circuits of Figure 1 and Typical Operating Circuit, V IN = 2.5V, I LOAD = 0mA, T A = T MIN to T MAX , unless otherwise noted.)

Stresses beyond those listed under Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

ABSOLUTE MAXIMUM RATINGS

Note 1: Supply current from the 3.3V output is measured with an ammeter between the 3.3V output and OUT pin. This currentcorrelates directly with actual battery supply current, but is reduced in value according to the step-up ratio and efficiency.

Note 2: Minimum value is production tested. Maximum value is guaranteed by design and is not production tested.

LBO Output Leakage Current

SHDN, 3/5, FB, LBI Input Current

V2.7 5.5MAX757, I LOAD = 0mA (Note 2)Output Voltage Range

V1.22 1.25 1.28MAX757FB Voltage

nA100LBI = 1.25V, FB = 1.25V, SHDN = 0V or 3V,3/5 = 0V or 3V

V1.6SHDN, 3/5 Input Voltage High

V0.4SHDN, 3/5 Input Voltage Low

Shutdown Quiescent Current(Note 1)

Battery Quiescent CurrentMeasured at V IN in Figure 1

V1.22 1.25 1.28With falling edgeLBI Input Threshold

mV25LBI Input Hysteresis

V0.4ISINK = 2mALBO Output Voltage Low

A1LBO = 5V

A20 40SHDN = 0V, LBI = 1.25V, 3/5 = 3V, V OUT = 3.47V,FB = 1.3V (MAX757 only)

AQuiescent Supply Current in3.3V Mode (Note 1)

MAX757, V OUT = 5V, 0mA < I LOAD < 200mA

MAX756, 3/5 = 0V, 0mA < I LOAD < 200mA

MAX756, 3/5 = 3V, 0mA < I LOAD < 300mA

%0.8 2.03/5 = 3V, -20A < REF load < 250A, C REF = 0.22FReference-Voltage Regulation

V1.23 1.25 1.27No REF load, C REF = 0.1F

60ILOAD = 0mA, 3/5= 3V, LBI = 1.25V, V OUT = 3.47V,FB = 1.3V (MAX757 only)

A

V

60Output set for 3.3V

1.1 1.8ILOAD = 10mAMinimum Start-Up Supply Voltage

4.8 5.0 5.2

2V < V IN < 3V

Reference Voltage

3.17 3.30 3.43 V

4.8 5.0 5.2

Output Voltage

UNITSMIN TYP MAXCONDITIONSPARAMETER

V0.7ILOAD = 20mAMinimum Operating SupplyVoltage (once started)

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MINIMUM START-UP INPUT VOLTAGEvs. LOAD CURRENT

M A X 7 5 6 - 7

LOADCURRENT(mA)

S T A R T - U P I N P U T V O L T A G E ( V )

10 100 10001

1.8

1.6

1.4

1.2

1.0

0.8

3.3VMODE

400.1 10 1000

EFFICIENCY vs. LOAD CURRENT3.3V OUTPUT MODE

M A X 7 5 6 - 1

LOADCURRENT(mA)

E F F I C I E N C Y ( % )

60

80

90

50

70

1 100

VIN =2.0V

VIN =1.2V

400.1 10 1000

EFFICIENCY vs. LOAD CURRENT5V OUTPUT MODE

M A X 7 5 6 - 2

LOADCURRENT(mA)

E F F I C I E N C Y ( % )

60

80

90

50

70

1 100

VIN =3.3V

VIN =2.5V

VIN =1.25V

800

00 2

MAXIMUM OUTPUT CURRENTvs. INPUT VOLTAGE

200

600

M A X 7 5 6 - 3

INPUTVOLTAGE(V)

M A X I M U M O U T P U T C U R R E N T ( m A )

4

400

100

300

500

700

1 3 5

3.3VMODE5VMODE

1M

1010 10m 1

SWITCHING FREQUENCYvs. LOAD CURRENT

100

M A X 7 5 6 - 4

LOADCURRENT(A)

S W I T C H I N G F R E Q U E N C Y ( H z )

1k

10k

100k

100 1m 100m

5VMODE

3.3VMODE

VIN =2.5V

02

QUIESCENT CURRENTvs. INPUT VOLTAGE

M A X 7 5 6 - 5

INPUTVOLTAGE(V)

Q U I E S C E N T C U R R E N T ( A )

4

100

200

300

400

500

1 3 5

VOUT =3.3V

VOUT =5V

CURRENTMEASUREDATV IN

50

01 2 5

SHUTDOWN QUIESCENT CURRENTvs. INPUT VOLTAGE

20

M A X 7 5 6 - 6

INPUTVOLTAGE(V)

S H U T D O W N Q U I E S C E N T C U R R E N T ( A )

4

40

10

30

3

CURRENTMEASUREDATV IN

10

00

REFERENCE VOLTAGELOAD REGULATION

2

8

M A X 7 5 6 - 8

LOADCURRENT( A)

V R E F L O A D R E G U L A T I O N ( m V )

6

4

50 100 150 200 250

VOUT =3.3V

__________________________________________Typical Operating Characteristics (Circuit of Figure 1, T A = +25C, unless otherwise noted.)

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______________________________________________________________Pin Description

NAME FUNCTION

1 SHDN

2 3/5 Selects the main output voltage setting; 5V when low, 3.3V when high.

FB

3 REF

4 LBO

5 LBI

6 OUT

7 GND Power Ground. Must be low impedance; solder directly to ground plane.

8 LX 1A, 0.5 N-Channel Power MOSFET Drain

1

2

3

4

5

6

7

8

PINMAX756 MAX757

Shutdown Input disables SMPS when low, but the voltage reference and low-battery com-parator remain active.

Feedback Input for adjustable output operation. Connect to an external voltage dividerbetween OUT and GND.

1.25V Reference Voltage Output. Bypass with 0.22F to GND (0.1F if there is no externalreference load). Maximum load capability is 250A source, 20A sink.

Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage atLBI drops below +1.25V.

Low-Battery Input. When the voltage on LBI drops below +1.25V, LBO sinks current.Connect to V IN if not used.

Connect OUT to the regulator output. It provides bootstrapped power to both devices,and also senses the output voltage for the MAX756.

OUTPUTVOLTAGE50mV/div

VIN = 2.5VHORIZONTAL= 50 s/div5VMode

LOAD-TRANSIENT RESPONSE

OUTPUTCURRENT

0mAto200mA

_____________________________Typical Operating Characteristics (continued) (Circuit of Figure 1, T A = +25C, unless otherwise noted.)

VSHDN2V/div

VIN = 2.5VHORIZONTAL= 5ms/div5VMode

START-UP DELAY

VOUT2V/div

3V

0V

5V

0V

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_______________Detailed Description

Operating Principle The MAX756/MAX757 combine a switch-mode regulatorwith an N-channel MOSFET, precision voltage reference,and power-fail detector in a single monolithic device.The MOSFET is a sense-FET type for best efficiency,and has a very low gate threshold voltage to ensurestart-up under low-battery voltage conditions (1.1V typ).

Pulse-Frequency Modulation Control Scheme

A unique minimum off time, current-limited, pulse-frequen-

cy modulation (PFM) control scheme is a key feature ofthe MAX756/MAX757. This PFM scheme combines theadvantages of pulse-width modulation (PWM) (high outputpower and efficiency) with those of a traditional PFMpulse-skipper (ultra-low quiescent currents). There is nooscillator; at heavy loads, switching is accomplishedthrough a constant peak-current limit in the switch, whichallows the inductor current to self-oscillate between thispeak limit and some lesser value. At light loads, switchingfrequency is governed by a pair of one-shots, which set aminimum off-time (1s) and a maximum on-time (4s).The switching frequency depends on the load and theinput voltage, and can range as high as 500kHz.

The peak switch current of the internal MOSFET powerswitch is fixed at 1A 0.2A. The switch's on resistanceis typically 0.5 , resulting in a switch voltage drop(VSW ) of about 500mV under high output loads. Thevalue of V SW decreases with light current loads.Conventional PWM converters generate constant-fre-quency switching noise, whereas this architecture pro-duces variable-frequency switching noise. However,the noise does not exceed the switch current limit timesthe filter-capacitor equivalent series resistance (ESR),unlike conventional pulse-skippers.

Voltage Reference The precision voltage reference is suitable for drivingexternal loads such as an analog-to-digital converter.It has guaranteed 250A source-current and 20Asink-current capability. The reference is kept aliveeven in shutdown mode. If the reference drives anexternal load, bypass it with 0.22F to GND. If the ref-erence is unloaded, bypass it with at least 0.1F.

Control-Logic Inputs The control inputs (3/5, SHDN) are high-impedanceMOS gates protected against ESD damage by normallyreverse-biased clamp diodes. If these inputs are dri-ven from signal sources that exceed the main supply

voltage, the diode current should be limited by a seriesresistor (1M suggested). The logic input thresholdlevel is the same (approximately 1V) in both 3.3V and5V modes. Do not leave the control inputs floating.

__________________Design Procedure

Output Voltage Selection The MAX756 output voltage can be selected to 3.3V or5V under logic control, or it can be left in one mode orthe other by tying 3/5 to GND or OUT. Efficiency variesdepending upon the battery and the load, and is typi-cally better than 80% over a 2mA to 200mA load range.The device is internally bootstrapped, with powerderived from the output voltage (via OUT). When theoutput is set at 5V instead of 3.3V, the higher internalsupply voltage results in lower switch-transistor onresistance and slightly greater output power.Bootstrapping allows the battery voltage to sag to lessthan 1V once the system is started. Therefore, the bat-tery voltage range is from V OUT + V D to less than 1V(where V D is the forward drop of the Schottky rectifier).If the battery voltage exceeds the programmed outputvoltage, the output will follow the battery voltage. Inmany systems this is acceptable; however, the outputvoltage must not be forced above 7V.

The output voltage of the MAX757 is set by two resis-tors, R1 and R2 (Figure 1), which form a voltage dividerbetween the output and the FB pin. The output voltageis set by the equation:

VOUT = (V REF ) [(R2 + R1) / R2]where V REF = 1.25V.

To simplify resistor selection:

R1 = (R2) [(V OUT / V REF ) - 1]

Since the input bias current at FB has a maximumvalue of 100nA, large values (10k to 200k ) can beused for R1 and R2 with no significant loss of accuracy.For 1% error, the current through R1 should be at least100 times FBs bias current.

Low-Battery Detection The MAX756/MAX757 contain on-chip circuitry for low-battery detection. If the voltage at LBI falls below the reg-ulators internal reference voltage (1.25V), LBO (an open-drain output) sinks current to GND. The low-battery mon-itor's threshold is set by two resistors, R3 and R4 (Figure1), which forms a voltage divider between the input volt-age and the LBI pin. The threshold voltage is set by R3and R4 using the following equation:

R3 = [(V IN / V REF ) - 1] (R4)

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where V IN is the desired threshold of the low-batterydetector, R3 and R4 are the input divider resistors atLBI, and V REF is the internal 1.25V reference.

Since the LBI current is less than 100nA, large resistorvalues (typically 10k to 200k ) can be used for R3and R4 to minimize loading of the input supply.

When the voltage at LBI is below the internal threshold,LBO sinks current to GND. A pull-up resistor of 10k or more connected from LBO to V OUT can be usedwhen driving CMOS circuits. Any pull-up resistor con-nected to LBO should not be returned to a voltagesource greater than V OUT . When LBI is above thethreshold, the LBO output is off. The low-battery com-parator and reference voltage remain active when theMAX756/MAX757 is in shutdown mode.

If the low-battery comparator is not used, connect LBIto V IN and leave LBO open.

Inductor Selection The inductors should have a saturation (incremental)current rating equal to or greater than the peak switch-current limit, which is 1.2A worst-case. However, itsgenerally acceptable to bias the inductor into satura-tion by 20%, although this will reduce the efficiency.

The 22H inductor shown in the typical applications cir-cuit is sufficient for most MAX756/MAX757 applicationcircuits. Higher input voltages increase the energytransferred with each cycle, due to the reducedinput/output differential. Minimize excess ripple due toincreased energy transfer by reducing the inductorvalue (10H suggested).

The inductors DC resistance significantly affects effi-ciency. For highest efficiency, limit L1s DC resistanceto 0.03 or less. See Table 1 for a list of suggestedinductor suppliers.

Table 1. Component Suppliers

AVX USA: (207) 282-5111, FAX (207) 283-1941(800) 282-9975

CoilCraft USA: (708) 639-6400, FAX (708) 639-1969Coiltronics USA: (407) 241-7876, FAX (407) 241-9339CollmerSemiconductor USA: (214) 233-1589Motorola USA: (602) 244-3576, FAX (602) 244-4015Nichicon USA: (708) 843-7500, FAX (708) 843-2798

Japan: +81-7-5231-8461, FAX (+81-) 7-5256-4158Nihon USA: (805) 867-2555, FAX (805) 867-2556

Japan: +81-3-3494-7411, FAX (+81-) 3-3494-7414Sanyo OS-CON USA: (619) 661-6835

Japan: +81-720-70-1005, FAX (+81-720-) 70-1174Sprague USA: (603) 224-1961, FAX (603) 224-1430Sumida USA: (708) 956-0666

Japan: +81-3-3607-5111, FAX (+81-3-) 3607-5428UnitedChemi-Con USA: (708) 696-2000, FAX (708) 640-6311

Capacitor Selection

A 100F, 10V surface-mount (SMT) tantalum capacitortypically provides 50mV output ripple when steppingup from 2V to 5V at 200mA. Smaller capacitors, downto 10F, are acceptable for light loads or in applica-tions that can tolerate higher output ripple.

MAX757

REF3

LX

7

C1150 F

GND

OUT6

VIN

D11N5817 VOUT

LBO4

8

C30.1 F

L122 H

LBI5

C2100 F

SHDN1

FB2

R1

R2

R3

R4

Figure 1. Standard Application Circuit

PRODUCTIONMETHOD

INDUCTORS CAPACITORS

Surface-Mount AVXTPS series

Sprague595D series

MiniatureThrough-Hole

SumidaRCH654-220

Low-CostThrough-Hole

SumidaCD54-220 (22H)CoilCraft

DT3316-223CoiltronicsCTX20-1

Sanyo OS-CONOS-CON serieslow-ESR organicsemiconductor

CoilCraftPCH-27-223

NichiconPL serieslow-ESRelectrolyic

United Chemi-ConLXF series

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N

N

MAX756

GND

VOUT

VIN

TRIGQ

ONE-SHOT

MINIMUMOFF-TIME

ONE-SHOT

Q

F/F

S

R

MAXIMUMON-TIME

ONE-SHOT

TRIG Q

ONE-SHOT

LX

OUT

REFERENCE

REF

LBI

LBO

3/5

SHDN

Figure 2. MAX756 Block Diagram

The ESR of both bypass and filter capacitors affectsefficiency. Best performance is obtained by using spe-cialized low-ESR capacitors, or connecting two or morefilter capacitors in parallel. The smallest low-ESR SMTtantalum capacitors currently available are Sprague595D series, which are about half the size of competingproducts. Sanyo OS-CON organic semiconductorthrough-hole capacitors also exhibit very low ESR, andare especially useful for operation at cold tempera-tures. Table 1 lists suggested capacitor suppliers.

Rectifier Diode For optimum performance, a switching Schottky diode,such as the 1N5817, is recommended. 1N5817 equiv-alent diodes are also available in surface-mount pack-ages from Collmer Semiconductor in Dallas, TX, phone(214) 233-1589. The part numbers are SE014 orSE024. For low output power applications, a pn junc-tion switching diode, such as the 1N4148, will alsowork well, although efficiency will suffer due to thegreater forward voltage drop of the pn junction diode.

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3.3V/5V/Adjustable-Output Step-Up DC-DC Converters 3.3V/5V/Adjustable-Output,Step-Up DC-DC Converters

GND

GND

LBI

OUT

3/5 (MAX756)FB (MAX757)

REF

SHDN LX

LBO

0.122"(3. 10mm)

0.080"(2. 03mm)

___________________Chip Topography

________________________________________________________Package Information

L

DIM

AA1

B

C

D

E

e

H

h

L

MIN

0.0530.004

0.014

0.007

0.189

0.150

0.228

0.010

0.016

0

MAX

0.0690.010

0.019

0.010

0.197

0.157

0.244

0.020

0.050

8

MIN

1.350.10

0.35

0.19

4.80

3.80

5.80

0.25

0.40

0

MAX

1.750.25

0.49

0.25

5.00

4.00

6.20

0.50

1.27

8

INCHES MILLIMETERS

8-PIN PLASTIC

SMALL-OUTLINEPACKAGE

HE

D

e

A

A1 C

h x 45

0.127mm0.004in.

B

1.27 BSC0.050 BSC

21-325A

PC Layout and Grounding The MAX756/MAX757 high peak currents and high-fre-quency operation make PC layout important for mini-mizing ground bounce and noise. The distancebetween the MAX756/MAX757s GND pin and theground leads of C1 and C2 in Figure 1 must be kept toless than 0.2" (5mm). All connections to the FB and LXpins should also be kept as short as possible. Toobtain maximum output power and efficiency and mini-mum output ripple voltage, use a ground plane andsolder the MAX756/MAX757 GND (pin 7) directly to theground plane.

TRANSISTOR COUNT: 758SUBSTRATE CONNECTED TO OUT

8 _______________________________________________________________________________________

Max756CSD

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