Datasheet

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General Description

The MAX1678 is a high-efficiency, low-voltage, syn-

chronous-rectified, step-up DC-DC converter intended

for use in devices powered by 1 to 3-cell alkaline,

NiMH, or NiCd batteries or a 1-cell lithium battery. It

guarantees a 0.87V start-up voltage and features a low

37µA quiescent supply current.

The device includes a 1Ω, N-channel MOSFET power

switch, a synchronous rectifier that acts as the catch

diode, a reference, pulse-frequency-modulation (PFM)

control circuitry, and circuitry to reduce inductor ring-

ing—all in an ultra-small, 1.1mm-high µMAX package.

The output voltage is preset to 3.3V or can be adjusted

from +2V to +5.5V using only two resistors. Efficiencies

up to 90% are achieved for loads up to 50mA. The

device also features an independent undervoltage

comparator (PFI/PFO) and a logic-controlled 2µA shut-

down mode.

Applications

Pagers

Remote Controls

Pointing Devices

Personal Medical Monitors

Single-Cell Battery-Powered Devices

Features

♦ 0.87V Guaranteed Start-Up

♦ Up to 90% Efficiency

♦ Built-In Synchronous Rectifier (no external diode)

♦ Ultra-Small µMAX Package, 1.1mm High

♦ 37µA Quiescent Current (85µA from 1.5V battery)

♦ 2µA Logic-Controlled Shutdown

♦ Power-Fail Detector

♦ Dual Mode™ Output: Fixed 3.3V

Adjustable 2V to 5.5V

♦ 45mA Output Current at 3.3V for 1-Cell Input

♦ 90mA Output Current at 3.3V for 2-Cell Input

♦ Inductor-Damping Switch Suppresses EMI

MAX1678

1-Cell to 2-Cell, Low-Noise,

High-Efficiency, Step-Up DC-DC Converter

________________________________________________________________

Maxim Integrated Products

OUT

GND

FBSHDN

PFO

PFI

BATT

MAX1678

µMAX

TOP VIEW

OUT

OUTPUT

3.3V

INPUT

0.87V TO V

OUT

LOW-BATTERY

DETECTOR INPUT

LOW-BATTERY

DETECTOR OUTPUT

OFF

PFO

BATT

MAX1678

SHDN

PFI

GND FB

Typical Operating Circuit

19-1381; Rev 0; 7/98

PART

MAX1678EUA -40°C to +85°C

TEMP. RANGE PIN-PACKAGE

8 µMAX

EVALUATION KIT

AVAILABLE

Note: To order these devices shipped in tape-and-reel, add a -T

to the part number.

Pin Configuration

Ordering Information

Dual Mode is a trademark of Maxim Integrated Products.

Summary of content (12 pages)

PAGE 1
9-1381; Rev 0; 7/98 KIT ATION EVALU E L B AVAILA 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Features ♦ 0.87V Guaranteed Start-Up The device includes a 1Ω, N-channel MOSFET power switch, a synchronous rectifier that acts as the catch diode, a reference, pulse-frequency-modulation (PFM) control circuitry, and circuitry to reduce inductor ringing—all in an ultra-small, 1.1mm-high µMAX package. The output voltage is preset to 3.3V or can be adjusted from +2V to +5.
PAGE 2
MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS BATT, OUT,LX, SHDN to GND ..............................-0.3V to +6.0V OUT, LX Current.......................................................................1A FB, PFI, PFO to GND ................................-0.3V to (VOUT + 0.3V) Reverse Battery Current (TA = +25°C) (Note 1) ...............220mA Continuous Power Dissipation (TA = +70°C) µMAX (derate 4.1mW/°C above +70°C) .......................
PAGE 3
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678 ELECTRICAL CHARACTERISTICS (VBATT = V SHDN = 1.3V, ILOAD = 0, FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) PARAMETER Maximum Operating Input Voltage Output Voltage (Fixed Mode) SYMBOL MIN VBATT(MAX) VOUT Output Voltage Range (Adjustable Mode) FB Set Voltage CONDITIONS VFB MAX UNITS 5.5 V VFB < 0.1V 3.12 3.48 V External feedback 2.0 5.5 V External feedback 1.17 1.
PAGE 4
Typical Operating Characteristics (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = SUMIDA 47µH) 50 VIN = 1.2V VIN = 0.85V 10 70 VIN = 1.2V 60 VIN = 0.85V 50 40 L1 = 47µH SUMIDA CD43-470 R1 = 200kΩ, R2 = 200kΩ 0.1 1 10 10 0.1 1 10 100 200 0.01 10 100 200 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = TDK 47µH) 100 VIN = 2.0V VIN = 2.5V 90 100 MAX1678-05 MAX1678-04 VIN = 1.
PAGE 5
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter 100 VOUT = 2.4V R1 = 1MΩ, R2 = 1MΩ L1 = 47µH SUMIDA CD43-470 10 4 4 MAX1678-12 20 40 60 80 8.2 8.0 L1 = 47µH SUMIDA CD43-470 3.3V FIXED MODE 1.2 1.1 WITHOUT DIODE 1.0 0.9 WITH EXTERNAL SCHOTTKY DIODE (FIGURE 3) 0.8 0.7 0.6 -20 0 20 40 60 80 0 100 5 10 15 20 25 30 MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = 22µH) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = SUMIDA 47µH) MAXIMUM LOAD CURRENT vs.
PAGE 6
Typical Operating Characteristics (continued) (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25°C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE MAX1678-19 MAX1678-20 SWITCHING WAVEFORM A A B B C C 100µs/div VOUT = 3.3V, VBATT = 1.2V, COUT = 10µF, L1 = SUMIDA CD43-470, A: VOUT, 50mV/div, AC COUPLED B: INDUCTOR CURRENT, C: LOAD, 2mA to 12mA 100mA/div 5µs/div VOUT = 3.3V, VBATT = 1.
PAGE 7
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678 BACKUP tOFF TIMER ZERO-CROSSING DETECTION BATT OUT DAMPING SWITCH 0.5REF tON = K/VBATT DAMP TON TOFF PDRV EN CONTROL LOGIC NDRV P PFI LX PFO FB MAX1678 REF RFRDY N START-UP OSCILLATOR REF 1.23V REF 0.5REF GND OUT 1.7V SHDN START-UP COMPARATOR Figure 1.
PAGE 8
MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Operating Principle The MAX1678 employs a proprietary constant-peakcurrent control scheme that combines the ultra-low quiescent current of traditional pulse-skipping PFM converters with high-load efficiency. When the error comparator detects that the output voltage is too low, it turns on the internal N-channel MOSFET switch for an internally calculated on-time (Figure 2).
PAGE 9
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter VOUT VIN OUT MAX1678 PDRV BATT P DAMPING SWITCH TIMING CIRCUIT P DAMP LX Reverse-Battery Protection The MAX1678 can sustain/survive battery reversal up to the package power-dissipation limit. An internal 5Ω resistor in series with a diode limits reverse current to less than 220mA, preventing damage. Prolonged operation above 220mA reverse-battery current can degrade the device’s performance.
PAGE 10
MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Applications Information IOUT(MAX) = M x Output Voltage Selection V 1 K x x BATT 2 L VOUT The MAX1678 operates with a fixed 3.3V or adjustable output. To select fixed-voltage operation, connect FB to GND (Figure 7). For an adjustable output between 2V and 5.5V, connect FB to a resistor voltage-divider between OUT and GND (Figure 8). FB regulates to 1.23V.
PAGE 11
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Minimizing Noise and Voltage Ripple EMI and output voltage ripple can be minimized by following these simple design rules: 1) Place the DC-DC converter and digital circuitry on the opposite corner of the PC board from sensitive RF and analog input stages. 2) Use a closed-core inductor, such as toroid or shielded bobbin, to minimize fringe magnetic fields.
PAGE 12
___________________Chip Information TRANSISTOR COUNT: 840 Package Information 8LUMAXD.