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19-1381; Rev 0; 7/98
KIT ATION EVALU BLE AVAILA
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter
General Description Features
o 0.87V Guaranteed Start-Up o Up to 90% Efficiency o Built-In Synchronous Rectifier (no external diode) o Ultra-Small MAX Package, 1.1mm High o 37A Quiescent Current (85A from 1.5V battery) o 2A Logic-Controlled Shutdown o Power-Fail Detector o Dual ModeTM Output: Fixed 3.3V Adjustable 2V to 5.5V o 45mA Output Current at 3.3V for 1-Cell Input o 90mA Output Current at 3.3V for 2-Cell Input o Inductor-Damping Switch Suppresses EMI
MAX1678
The MAX1678 is a high-efficiency, low-voltage, synchronous-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 37A quiescent supply current. The device includes a 1, 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 MAX 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 2A shutdown mode.
Applications
Pagers Remote Controls Pointing Devices Personal Medical Monitors Single-Cell Battery-Powered Devices
PART MAX1678EUA
Ordering Information
TEMP. RANGE -40C to +85C PIN-PACKAGE 8 MAX
Note: To order these devices shipped in tape-and-reel, add a -T to the part number.
Typical Operating Circuit
Pin Configuration
INPUT 0.87V TO VOUT LX OUT
OUTPUT 3.3V
TOP VIEW
MAX1678
BATT ON OFF LOW-BATTERY DETECTOR INPUT SHDN PFI GND PFO FB LOW-BATTERY DETECTOR OUTPUT BATT PFI PFO SHDN 1 2 3 4 8 OUT LX GND FB
MAX1678
7 6 5
MAX
Dual Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468.
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
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 = +25C) (Note 1) ...............220mA Continuous Power Dissipation (TA = +70C) MAX (derate 4.1mW/C above +70C) .......................330mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +165C Lead Temperature (soldering, 10sec) .............................+300C
Note 1: The reverse battery current is measured from the Typical Operating Circuit's input terminal to GND when the battery is connected backward. A reverse current of 220mA will not exceed package dissipation limits but, if left for an extended time (more than 10 minutes), may degrade performance.
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.
ELECTRICAL CHARACTERISTICS
(VBATT = V SHDN = 1.3V, ILOAD = 0, FB = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Minimum Operating Input Voltage Maximum Operating Input Voltage Start-Up Voltage (Note 2) Start-Up Voltage Tempco Output Voltage (Fixed Mode) Output Voltage Range (Adjustable Mode) FB Set Voltage N-Channel On-Resistance P-Channel On-Resistance P-Channel Catch Diode Voltage Maximum Peak LX Current On-Time Constant Quiescent Current into OUT Quiescent Current into BATT Shutdown Current into OUT Shutdown Current into BATT Efficiency FB Input Current PFI Trip Voltage PFI Input Current PFO Low Output Voltage PFO Leakage Current SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Current VIL VIH SHDN = GND or BATT 0.8 x VBATT 0.1 10 VOL VIL,PFI ILX(MAX) K IQ,OUT IQ,BATT ISHDN,OUT ISHDN,BATT VOUT = 3.5V VBATT = 1V ILOAD = 20mA, VBATT = 2.5V (Figure 7) VFB = 1.3V Falling PFI hysteresis 2% VPFI = 650mV VPFI = 0, VOUT = 3.3V, ISINK = 1mA VPFI = 650mV, VPFO = 6V 590 0.9V < VBATT < 3.3V (tON = K / VBATT) VOUT = 3.5V 5.60 VFB VOUT VFB < 0.1V External feedback External feedback VOUT = 3.3V VOUT = 3.3V IDIODE = 100mA, P-channel switch off 3.16 2.0 1.19 1.23 1 1.5 0.8 550 8 37 4 0.1 2 90 0.1 614 0.1 0.04 0.01 10 632 10 0.4 1 11.2 65 8 1 3.5 SYMBOL VBATT(MIN) VBATT(MAX) RL = 3k, TA = +25C 0.87 -2 3.3 3.44 5.5 1.26 1.5 2.2 CONDITIONS MIN TYP 0.7 5.5 MAX UNITS V V V mV/C V V V V mA V-s A A A A % nA mV nA V A V V nA
0.2 x VBATT
2
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS
(VBATT = V SHDN = 1.3V, ILOAD = 0, FB = GND, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER Maximum Operating Input Voltage Output Voltage (Fixed Mode) Output Voltage Range (Adjustable Mode) FB Set Voltage N-Channel On-Resistance P-Channel On-Resistance On-Time Constant Quiescent Current into OUT Quiescent Current into BATT Shutdown Current into OUT Shutdown Current into BATT FB Input Current PFI Trip Voltage PFI Input Current PFO Low Output Voltage PFO Leakage Current SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Current VIL VIH SHDN = GND or BATT 0.8 x VBATT 10 VOL VIL,PFI K IQ,OUT IQ,BATT ISHDN,OUT ISHDN,BATT VOUT = 3.5V VBATT = 1V VFB = 1.3V Falling PFI hysteresis 2% VPFI = 650mV VPFI = 0, VOUT = 3.3V, ISINK = 1mA VPFI = 650mV, VPFO = 6V 580 VFB SYMBOL VBATT(MAX) VOUT VFB < 0.1V External feedback External feedback VOUT = 3.3V VOUT = 3.3V 0.9V < VBATT < 3.3V (tON = K / VBATT) VOUT = 3.5V 5.60 3.12 2.0 1.17 CONDITIONS MIN MAX 5.5 3.48 5.5 1.28 1.5 2.2 11.2 65 8 1 3.5 10 642 10 0.4 1 0.2 x VBATT UNITS V V V V V-s A A A A nA mV nA V A V V nA
MAX1678
3
Note 2: Start-up is guaranteed by correlation to measurements of device parameters (i.e., switch on-resistance, on-time, off-time, and output voltage trip point). Note 3: Specifications to -40C are guaranteed by design and not production tested.
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
Typical Operating Characteristics
(Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = 22H)
MAX1678-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = SUMIDA 47H)
MAX1678-02
EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = TDK 47H)
90 80 VIN = 2.0V VIN = 1.5V
MAX1678-03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 10 L1 = 22H SUMIDA CD43-220 R1 = 200k, R2 = 200k VIN = 0.85V VIN = 1.2V VIN = 1.5V VIN = 2.0V
100 90 80 EFFICIENCY (%) VIN = 1.2V VIN = 0.85V VIN = 1.5V VIN = 2.0V
100
60 50 40 30 20 10 0 0.01 0.1
EFFICIENCY (%)
70
70 60 50 40 30 VIN = 0.85V VIN = 1.2V
L1 = 47H SUMIDA CD43-470 R1 = 200k, R2 = 200k 1 10 100 200
20 10 0 0.01 0.1
L1 = 47H TDK NLC453232T-470K R1 = 200k, R2 = 200k 1 10 100 200
100 200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = 22H)
MAX1678-04
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = SUMIDA 47H)
MAX1678-05
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = TDK 47H)
90 80 VIN = 2.5V VIN = 1.5V VIN = 2.0V
MAX1678-06 MAX1678-09
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 10 L1 = 22H SUMIDA CD43-220 FB = GND VIN = 0.85V VIN = 1.2V VIN = 2.5V VIN = 2.0V VIN = 1.5V
100 90 80 EFFICIENCY (%) VIN = 2.5V
VIN = 2.0V
100
60 50 40 30 20 10 0 0.01 0.1
VIN = 0.85V V = 1.5V IN
EFFICIENCY (%)
70
VIN = 1.2V
70 60 50 40 30 VIN = 0.85V VIN = 1.2V
L1 = 47H SUMIDA CD43-470 FB = GND 1 10 100 200
20 10 0 0.01 0.1 1
L1 = 47H TDK NLC453232T-470K FB = GND 10 100 200
100 200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = 22H)
MAX1678-07
EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = SUMIDA 47H)
90 80 EFFICIENCY (%) VIN = 4.5V VIN = 3.0V VIN = 2.0V VIN = 1.2V VIN = 0.85V EFFICIENCY (%)
MAX1678-08
EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = TDK 47H)
100 90 80 70 60 50 40 30 VIN = 1.2V VIN = 0.85V VIN = 2.0V VIN = 4.5V VIN = 3.0V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 10 L1 = 22H SUMIDA CD43-220 R1 = 619k, R2 = 200k VIN = 0.85V VIN = 1.2V VIN = 4.5V VIN = 3.0V VIN = 2.0V
100
70 60 50 40 30 20 10 0 0.01 0.1
L1 = 47H SUMIDA CD43-470 R1 = 619k, R2 = 200k 1 10 100 200
20 10 0 0.01 0.1
L1 = 47H TDK NLC453232-470K R1 = 619k, R2 = 200k 1 10 100 200
100 200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
4
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.)
NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE
MAX1678-11
MAX1678
EFFICIENCY WITH DIFFERENT INDUCTORS
85 80 EFFICIENCY (%) 75
DS1608C-473 CD43-470 22H NLC453232T-220K 47H NLC453232T-470K
BATT AND OUT QUIESCENT CURRENT vs. TEMPERATURE
40 QUIESCENT CURRENT (A) 35 30 25 20 15 10 5 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) IBATT VBATT = 1.3V VOUT = 3.6V FB = GND
MAX1678-12
NO-LOAD BATTERY CURRENT (A)
VBATT = 1.2V VOUT = 3.3V ILOAD = 20mA
MAX1678-10
90
1000 VOUT = 5.0V R1 = 3M, R2 = 1M VOUT = 3.0V FB = GND 100
45 IOUT
70 65 60
DT1608C-223
LQH4N470K
CD43-220
LQH3C470K
47H
22H
47H
22H
47H
47H
55 50
10
VOUT = 2.4V R1 = 1M, R2 = 1M L1 = 47H SUMIDA CD43-470 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
COILCRAFT SUMIDA
MURATA
TDK
SHUTDOWN BATTERY CURRENT vs. INPUT VOLTAGE
MAX1678-13
ON-TIME CONSTANT (K) vs. TEMPERATURE
MAX1678-14
MINIMUM START-UP INPUT VOLTAGE vs. LOAD CURRENT
L1 = 47H SUMIDA CD43-470 3.3V FIXED MODE WITHOUT DIODE
MAX1678-15
12 SHUTDOWN BATTERY CURRENT (A) 10 8 6 4 2 0 0 1 2 3 4 5 6 INPUT VOLTAGE (V) 3.3V FIXED MODE L1 = 47H SUMIDA CD43-470
9.0 VBATT = 1.3V 8.8 ON-TIME CONSTANT (V-s) 8.6 8.4 8.2 8.0 7.8 7.6 -40 -20 0 20 40 60 80
1.3 1.2 START-UP INPUT VOLTAGE (V) 1.1 1.0 0.9 0.8 0.7 0.6
WITH EXTERNAL SCHOTTKY DIODE (FIGURE 3)
100
0
5
10
15
20
25
30
35
TEMPERATURE (C)
LOAD CURRENT (mA)
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = 22H)
MAX1678-16
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = SUMIDA 47H)
MAX1678-17
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = TDK 47H)
L1 = 47H TDK NLC453232T-470K
MAX1678-18
140 MAXIMUM LOAD CURRENT (mA) 120 100 80 60 40 20 0 VOUT = 3.3V VOUT = 5.0V L1 = 22H SUMIDA CD43-220 VOUT = 2.4V
140 MAXIMUM LOAD CURRENT (mA) 120 100 80 60 40 20 0 VOUT = 2.4V VOUT = 5.0V L1 = 47H SUMIDA CD43-470
140 MAXIMUM LOAD CURRENT (mA) 120 100 80 60 VOUT = 2.4V 40 20 0 VOUT = 5.0V VOUT = 3.3V
VOUT = 3.3V
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
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5
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
Typical Operating Characteristics (continued)
(Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.)
SWITCHING WAVEFORM
A
MAX1678-19
LOAD-TRANSIENT RESPONSE
MAX1678-20
A
B
B
C C 5s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 A: LX, 2V/div B: VOUT, 50mV/div AC COUPLED C: INDUCTOR CURRENT, 100mA/div 100s/div VOUT = 3.3V, VBATT = 1.2V, COUT = 10F, L1 = SUMIDA CD43-470, A: VOUT, 50mV/div, AC COUPLED B: INDUCTOR CURRENT, C: LOAD, 2mA to 12mA 100mA/div
LINE-TRANSIENT RESPONSE
MAX1678-21
POWER-UP RESPONSE
MAX1678-22
A
A
B B
C
200s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 A: VOUT, 50mV/div, AC COUPLED B: VIN, 1V/div, 1.2V to 2.2V
100s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 B: INDUCTOR CURRENT, 100mA/div A: VOUT, 1V/div C: SHDN, 5V/div
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME BATT PFI PFO SHDN FB GND LX OUT Battery-Power Input Power-Fail Input. When the voltage at PFI is below 614mV, PFO sinks current. Open-Drain Power-Fail Output. PFO sinks current when PFI is below 614mV. Active-Low Shutdown. Connect SHDN to BATT for normal operation. Dual-Mode Feedback Input. Connect FB to GND for fixed-output operation (3.3V). Connect FB to a feedback-resistor network for adjustable output voltage operation (2V to 5.5V). FB regulates to 1.23V. Ground N-Channel MOSFET Switch Drain and P-Channel Synchronous-Rectifier Drain Power Output and IC Power Input (bootstrapped). OUT is the feedback input for 3.3V operation. Connect the filter capacitor close to OUT. FUNCTION
6
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
BACKUP tOFF TIMER ZERO-CROSSING DETECTION
BATT DAMPING SWITCH tON = K/VBATT DAMP EN PFI TON TOFF PDRV NDRV P
OUT
0.5REF
CONTROL LOGIC
LX PFO
MAX1678
FB
REF
N
RFRDY 1.23V REF
REF 0.5REF
START-UP OSCILLATOR
GND OUT SHDN START-UP COMPARATOR 1.7V
Figure 1. Functional Diagram
Detailed Description
The MAX1678 consists of an internal 1, N-channel MOSFET power switch, a built-in synchronous rectifier that acts as the catch diode, a reference, PFM control circuitry, and an inductor damping switch (Figure 1). The device is optimized for applications that are powered by 1 to 3-cell alkaline, NiMH, or NiCd batteries, or a 1-cell lithium battery such as pagers, remote controls, and battery-powered instruments. They are designed to meet the specific demands of the operating states characteristic of such systems: 1) Primary battery is good and load is active: In this state the load draws tens of milliamperes and the MAX1678 typically offers 80% to 90% efficiency.
2) Primary battery is good and load is sleeping: In this state the load draws hundreds of microamperes and the DC-DC converter IC draws very low quiescent current. Many applications maintain the load in this state most of the time. 3) Primary battery is dead and DC-DC converter is shut down: In this state the load is sleeping or supplied by the backup battery, and the MAX1678 draws 0.1A current from the OUT pin. 4) Primary and backup battery dead: The DC-DC converter can restart from this condition.
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7
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
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). During the on-time, current ramps up in the inductor, storing energy in the magnetic field. When the MOSFET turns off during the second half of each cycle, the magnetic field collapses, causing the inductor voltage to force current through the synchronous rectifier, transferring the stored energy to the output filter capacitor and the load. The output filter capacitor stores charge while the current from the inductor is high, then holds up the output voltage until the second half of the next switching cycle, smoothing power flow to the load. The ideal on-time of the N-channel MOSFET changes as a function of input voltage. The on-time is determined as follows: t ON = K VBATT
OUT VIN L1 LX N VLX VOUT VBATT VBATT (DEAD TIME) (ON TIME) t
IL
K VBATT
K VOUT - VBATT IPEAK = K L
IPEAK
(ON TIME) (DEAD TIME) tON tOFF tON OR DEAD TIME
t
Figure 2. Switching Waveforms
VOUT COUT
where K is typically 8V-s. The peak inductor current (assuming a lossless circuit) can be calculated from the following equation: K IPEAK = L The P-channel MOSFET (synchronous rectifier) turns on when the N-channel MOSFET turns off. The circuit operates at the edge of discontinuous conduction; therefore, the P-channel synchronous rectifier turns off immediately after the inductor current ramps to zero. During the dead time after the P-switch has been turned off, the damping switch connects LX and BATT. This suppresses EMI noise due to LC ringing of the inductor and parasitic capacitance at the LX node (see Damping Switch section). The error comparator starts another cycle when VOUT falls below the regulation threshold. With this control scheme, the MAX1678 maintains high efficiency over a wide range of loads and input/output voltages while minimizing switching noise.
MAX1678
PDRV TIMING CIRCUIT NDRV START-UP OSCILLATOR GND P
Figure 3. External Schottky Diode to Improve Start-Up with Heavy Load
Start-Up Operation
The MAX1678 contains a low-voltage start-up oscillator (Figure 1). This oscillator pumps up the output voltage to approximately 1.7V, the level at which the main DCDC converter can operate. The 150kHz fixed-frequency oscillator is powered from the BATT input and drives an NPN switch. During start-up, the P-channel synchronous
8
rectifier remains off and its body diode (or an external diode, if desired) is used as an output rectifier. The minimum start-up voltage is a function of load current (see Typical Operating Characteristics). In normal operation, when the voltage at the OUT pin exceeds 1.7V, the DCDC converter is powered from the OUT pin (bootstrapped) and the main control circuitry is enabled. Once started, the output can maintain the load as the battery voltage decreases below the start-up voltage. To improve start-up capability with heavy loads, add a Schottky diode in parallel with the P-channel synchronous rectifier (from LX to OUT) as shown in Figure 3 (see Typical Operating Characteristics).
_______________________________________________________________________________________
1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter
Shutdown Mode
Pulling the SHDN pin low places the MAX1678 in shutdown mode (ISHDN = 2A typical). In shutdown, the internal switching MOSFET turns off, PFO goes high impedance, and the synchronous rectifier turns off to prevent the flow of reverse current from the output back to the input. However, there is still a forward current path through the synchronous-rectifier body diode from the input to the output. Thus, in shutdown, the output remains one diode drop below the battery voltage (VBATT). To disable the shutdown feature, connect SHDN (a logic input) to BATT or OUT.
VOUT VIN OUT
MAX1678
MAX1678
PDRV P BATT
TIMING CIRCUIT DAMP
DAMPING SWITCH P LX
Reverse-Battery Protection
The MAX1678 can sustain/survive battery reversal up to the package power-dissipation limit. An internal 5 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.
NDRV N GND
Figure 4. Simplified Diagram of Damping Switch
Power-Fail Comparator
The MAX1678 has an on-chip comparator for power-fail detection. This comparator can detect a loss of power at the input or output (Figures 7 and 8). If the voltage at the power-fail input (PFI) falls below 614mV, the PFO output sinks current to GND. Hysteresis at PFI is 2%. The power-fail monitor threshold is set by two resistors, R3 and R4, using the following equation: V R3 = R4 x TH - 1 VPFI where VTH is the desired threshold of the power-fail detector, and VPFI is the 614mV threshold of the powerfail comparator. Since PFI leakage is 10nA max, select feedback resistor R4 in the 100k to 1M range.
2s/div
1V/div
VBATT = 2.5V VOUT = 3.3V L1 = 47H
Figure 5. LX Ringing Without Damping Switch (example only)
Damping Switch
The MAX1678 is designed with an internal damping switch to minimize ringing at the LX node. The damping switch (Figure 4) connects the LX node to BATT, effectively depleting the inductor's remaining energy. When the energy in the inductor is insufficient to supply current to the output, the capacitance and inductance at LX form a resonant circuit that causes ringing. The damping switch supplies a path to quickly dissipate this energy, suppressing the ringing at LX. This does not reduce the output ripple, but does reduce EMI. Figures 5 and 6 show the LX node voltage waveform without and with the damping switch.
1V/div
VBATT = 1.8V VOUT = 3.3V L1 = 47H 2s/div
Figure 6. LX Ringing With Damping Switch
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
Applications Information
Output Voltage Selection
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. Since FB leakage is 10nA max, select feedback resistor R2 in the 100k to 1M range. R1 is given by: V R1 = R2 x OUT - 1 VREF where VREF = 1.23V. IOUT(MAX) = M x V 1 K x x BATT 2 L VOUT
where M is an empirical factor that takes into account losses in the MAX1678 internal switches and in the inductor resistance. K is the V-s factor that governs the inductor charge time. Nominally, M = 0.9 and K = 8V-s. M should be further reduced by 0.1 for each ohm of inductor resistance. The inductor's saturation-current rating must exceed the worst-case peak current limit set by the MAX1678's timing algorithm: KMAX IPEAK = L where K MAX = 11.2V-s. It is usually acceptable to exceed most coil saturation-current ratings by 20% with no ill effects; however, the maximum recommended IPEAK for the MAX1678 internal switches is 550mA, so inductor values below 22H are not recommended. For optimum efficiency, inductor series resistance should be less than 150mV/IPEAK. Table 1 lists suggested inductors and suppliers.
Maximum Output Current and Inductor Selection
The MAX1678 is designed to work well with a 47H inductor in most low-power applications. 47H is a sufficiently low value to allow the use of a small surfacemount coil, but large enough to maintain low ripple. The Typical Operating Characteristics section shows performance curves with several 47H and 22H coils. Low inductance values supply higher output current but also increase ripple and reduce efficiency. Note that values below 22H are not recommended due to MAX1678 switch limitations. Higher inductor values reduce peak inductor current (and consequent ripple and noise) and improve efficiency, but also limit output current. The relationship between current and inductor value is approximately:
Table 1. Suggested Inductors and Suppliers
PIN Coilcraft Murata Sumida TDK INDUCTOR DS1608C-223, DS1608C-473 LQH4N470K, LQH3C470K CD43-220, CD43-470 NLC453232T-220K, NLC453232T-470K
L1 47H
PHONE (847) 639-6400 (814) 237-1431 (847) 956-0666 (847) 390-4373
INPUT 0.87V TO VOUT C1 10F R3 OUT R4 R5
L1 47H, 200mA
INPUT 0.87V TO VOUT C1 10F 3.3VOUT C2 10F R3
BATT PFI OUT
LX OUT
BATT PFI
LX OUT
VOUT = 2V TO 5.5V
MAX1678
PFO SHDN GND FB
MAX1678
R4 R5 PFO SHDN GND FB
R1
C2
R2
Figure 7. 3.3V Standard Application Circuit
10
Figure 8. Adjustable Output Circuit
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter
Capacitor Selection
Choose input and output capacitors to service input and output peak currents with acceptable voltage ripple. Capacitor ESR is a major contributor to output ripple (usually more than 60%). A 10F, ceramic output filter capacitor typically provides 50mV output ripple when stepping up from 1.3V to 3.3V at 20mA. Low input to output voltage differences (i.e., 2 cells to 3.3V) require higher capacitor values (10F to 47F). The input filter capacitor (CIN) also reduces peak currents drawn from the battery and improves efficiency. Low-ESR capacitors are recommended. Ceramic capacitors have the lowest ESR, but low-ESR tantalums represent a good balance between cost and performance. Low-ESR aluminum electrolytic capacitors are tolerable, and standard aluminum electrolytic capacitors should be avoided. Capacitance and ESR variation over temperature need to be taken into consideration for best performance in applications with wide operating temperature ranges. Table 2 lists suggested capacitors and suppliers. 5) Follow sound circuit-board layout and grounding rules (see the PC Board Layout and Grounding section).
MAX1678
PC Board Layout and Grounding
High switching frequencies and large peak currents make PC board layout an important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane. Both of these factors can result in instability or regulation errors. The OUT pin must be bypassed directly to GND, as close to the IC as possible (within 0.2 inches or 5mm). Place power components--such as the MAX1678, inductor, input filter capacitor, and output filter capacitor--as close together as possible. Keep their traces short, direct, and wide (50 mil or 1.25mm), and place their ground pins close together in a star-ground configuration. Keep the extra copper on the board and integrate it into ground as a pseudo-ground plane. On multilayer boards, route the star ground using component-side copper fill, then connect it to the internal ground plane using vias. Place the external voltage-feedback network very close to the FB pin (within 0.2 inches or 5mm). Noisy traces, such as from the LX pin, should be kept away from the voltage-feedback network and separated from it using grounded copper. The MAX1678 evaluation kit manual shows an example PC board layout, which includes a pseudo-ground plane.
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. 3) Choose the largest inductor value that satisfies the load requirement, to minimize peak switching current and the resulting ripple and noise. 4) Use low-ESR input and output filter capacitors.
Table 2. Recommended Surface-Mount Capacitor Manufacturers
VALUE (F) 4.7 to 47 TAJ, TPS-series tantalum 4.7 to 10 4.7 to 22 X7R ceramic AVX X7R ceramic Taiyo Yuden 803-946-0690 408-573-4150 AVX TDK 803-946-0690 847-390-4373 DESCRIPTION 595D-series tantalum MANUFACTURER Sprague PHONE 603-224-1961
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1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678
___________________Chip Information
TRANSISTOR COUNT: 840
Package Information
8LUMAXD.EPS
12
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