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19-0204; Rev 1; 5/94
UAL IT MAN TION K T VALUA A SHEE E T WS DA FOLLO
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
____________________________Features
o Regulates from Inputs Above & Below the Output o 1V to 6.2V Supply-Voltage Range o Internal 1A Active Rectifier with Input-to-Output Disconnect in Shutdown o Up to 210mA Load Currents, Guaranteed o 85% Efficiency o Only 3 External Components o Adjustable Current Limit o 195A Quiescent Supply Current o 20A Shutdown Supply Current o 3V/3.3V/5V and Adjustable Output Voltage Versions o Available in 8-Pin DIP and SO Packages
_______________General Description
The MAX877/MAX878/MAX879 are pulse-skipping, stepup/step-down DC-DC converters that provide a regulated output from inputs both above and below the output. They require only three external components--an inductor (typically 22H) and two filter capacitors. The MAX877 delivers a regulated 5V output from 2.5V to 6.2V inputs. The MAX878 generates pin-selectable voltages of 3.0V or 3.3V from 1.5V to 6.2V inputs. The MAX879 output can be adjusted from 2.5V to 6V via an external resistor divider from 2.5V to 6.2V inputs. A unique high-power, internal, synchronous rectifier design (Active RectifierTM) enables the devices to regulate in a switched linear mode if the input voltage is higher than the desired output voltage. When the input voltage falls below the output voltage, the MAX877/MAX878/MAX879 will smoothly switch into a pulse-skipping boost mode and step up from input voltages as low as 1V. In shutdown, the active rectifier disconnects the output from the source. This stops the current drain from input to output associated with conventional step-up converters. High-frequency operation (up to 300kHz) allows the use of small surface-mount inductors. Supply current is 195A under no load, and only 20A in shutdown mode. For 1-cell (1V) step-up converters with similar performance and the same pinout, refer to the MAX777/MAX778/MAX779 data sheet.
MAX877/MAX878/MAX879
______________Ordering Information
PART MAX877CPA MAX877CSA MAX877C/D MAX877EPA MAX877ESA MAX877MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP
________________________Applications
Two or Three NiCd Cells to 3V/3.3V Conversion Three or Four Alkaline Cells to 5V Conversion One Lithium Cell to 3V/3.3V Conversion Pagers Palmtop and Notebook Computers Battery-Powered and Hand-Held Instruments
Ordering Information continued on last page. * Contact factory for dice specifications.
__________Typical Operating Circuit
__________________Pin Configuration
TOP VIEW
INPUT 1.8V TO 6.2V
2
22F
IN ILIM MAX878 SHDN SEL 8 PGND 4 LX OUT AGND 3 5 6
22H
ILIM 1 IN 2 AGND 3 PGND 4
8
N.C. (MAX877)* SHDN ON/OFF OUT LX
1 7
MAX877 MAX878 MAX879
7 6 5
OUTPUT 3.3V 210mA
100F
DIP/SO * SEL (MAX878), FB (MAX879)
TM Active Rectifier is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (IN to PGND) .......................................0V to +7V Output Short-Circuit Duration to PGND, AGND (Note 1)....30sec Voltage Applied to: LX (switch off).......................................................-0.3V to +7V (switch on) ...................................30sec short to IN or OUT --- ---- OUT, SHDN...........................................................-0.3V to +7V FB ..........................................................-0.3V to (OUT + 0.3V) AGND to PGND ........................................................-0.3V, +0.3V Reverse Battery Current....................................................900mA Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW CERDIP (derate 8.00mW/C above +70C) .................640mW Operating Temperature Ranges: MAX87_C_A ........................................................0C to +70C MAX87_E_A .....................................................-40C to +85C MAX87_MJA ..................................................-55C to +125C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C Note 1: The output may be shorted to ground continuously if the package power dissipation is not exceeded.
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.
--- ---- (V IN = 2.7V, I LOAD = 0mA, LX = 22H, C OUT = 100F, S H D N and ILIM connected to IN, AGND connected to PGND, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER CONDITIONS ILOAD = 0mA, TA = +25C Minimum Start-Up Voltage (Notes 2, 6) MAX877/MAX879 (VOUT = 5V), 0mA < ILOAD < 180mA, TA = +25C MAX878/MAX879 (VOUT = 3.3V), 0mA < ILOAD < 120mA, TA = +25C (Notes 2, 3) MAX877C/MAX879C: 0mA ILOAD 240mA, 2.7V VIN 6.2V; Output Voltage (MAX879 set to 5V) (Note 3) MAX877E/MAX879E: 0mA ILOAD 220mA, 2.7V VIN 6.2V; MAX877M/MAX879M: 0mA ILOAD 180mA, 2.7V VIN 6.2V MAX878C/MAX879C: 0mA ILOAD 210mA, 1.8V VIN 6.2V; Output Voltage (MAX879 set to 3.3V) (Note 3) SEL = 0V MAX878E/MAX879E: 0mA ILOAD 200mA, 1.8V VIN 6.2V; MAX878M/MAX879M: 0mA ILOAD 180mA, 1.8V VIN 6.2V MAX878C: 0mA ILOAD 210mA, 1.8V VIN 6.2V; SEL = Open MAX878E: 0mA ILOAD 200mA, 1.8V VIN 6.2V; MAX878M: 0mA ILOAD 180mA, 1.8V VIN 6.2V Output Voltage Range Efficiency No-Load Supply Current Shutdown Supply Current --- - ---- SHDN Bias Current MAX879, ILOAD = 0mA (Note 4) MAX877/MAX879 (VOUT = 5V), ILOAD = 100mA, VIN = 4V MAX878/MAX879 (VOUT = 3.3V), ILOAD = 100mA, VIN = 2.5V ILOAD = 0mA (switch off) --- - ---- SHDN = 0V --- - ---- 0V < SHDN < VIN --- - ---- VIN < SHDN < 5V MAX87_C, MAX87_E MAX87_M 2.88 2.5 85 82 195 20 20 15 12 310 30 35 100 40 3.00 3.12 6.0 V % A A nA A 3.17 3.30 3.43 V 4.80 5.00 5.20 V 6.2 MIN TYP 1 2.5 1.5 V V MAX UNITS
ELECTRICAL CHARACTERISTICS
Maximum Operating Voltage
2
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5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
MAX877/MAX878/MAX879
- ----- -- (V IN = 2.7V, I LOAD = 0mA, LX = 22H, C OUT = 100F, S H D N and ILIM connected to IN, AGND connected to PGND, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER --- - ---- SHDN Threshold --- - ---- SHDN Enable Delay Current Limit Current-Limit Temperature Coefficient ISW = 400mA Switch Saturation Voltage ISW = 600mA ISW = 1000mA VIN = 2.5V Maximum Switch On Time VIN = 1.8V VIN = 1V Minimum Switch Off Time MAX877/MAX879 MAX878 ISW = 400mA Rectifier Forward Voltage Drop Error-Comparator Trip Point (VREF) FB Pin Bias Current Switch Off Leakage Current Rectifier Off Leakage Current ISW = 600mA ISW = 1000mA MAX879, VIN = 1.8V to 5V (Note 5) MAX879 197.5 VIN = 1V to 6.2V VIN = 2.7V 1.3 150 1.0 -0.3 0.275 0.33 0.50 4.0 5.9 12.6 1.3 2.3 0.21 0.31 0.50 202.5 10 0.1 0.1 207.5 40 mV nA A A V s s V CONDITIONS MIN TYP VIN/2 +0.25 1.7 MAX UNITS V s A %/C
Note 2: Output in regulation, VOUT = VOUT (nominal) 4%. Note 3: At high VIN to VOUT differentials, the maximum load current is limited by the maximum allowable power dissipation in the package (see Absolute Maximum Ratings and Maximum Output Current graphs in the Typical Operating Characteristics). Note 4: Minimum value is production tested. Maximum value is guaranteed by design and is not production tested. Note 5: VOUT is set to a target value of 5V by 0.1% external feedback resistors. VOUT is measured to be within 5V 2.5% to guarantee error-comparator trip point. Note 6: Startup guaranteed under these load conditions.
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3
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
__________________________________________Typical Operating Characteristics
(Circuit of Figure 4, TA = +25C, unless otherwise noted.)
MAX877/MAX879 EFFICIENCY vs. LOAD CURRENT
MAX877-1
MAX878 EFFICIENCY vs. LOAD CURRENT
MAX877-2
MAX877/MAX879 MAXIMUM OUTPUT CURRENT AT LOW INPUT VOLTAGE
300 OUTPUT CURRENT (mA) 250 200 150 100 50 VOUT = 5V 0
MAX877-3
90 VOUT = 5V 80 EFFICIENCY (%) 70 60 50 40 30 0.1
VIN = 4V
90 VOUT = 3.3V OR 3.0V 80 EFFICIENCY (%) 70 60 50 40 30 0.1 VIN = 2.5V VIN = 3.3V VIN = 1.8V
350
VIN = 5V
VIN = 2.5V VIN = 6.2V VIN = 1.8V 1 10 100 1000
VIN = 5V VIN = 6.2V 1 10 100 1000
0
1
2
3
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
MAX878 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX877-4
NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE AND TEMPERATURE
MAX877-5
SHUTDOWN SUPPLY CURRENT vs. INPUT VOLTAGE AND TEMPERATURE
70 60 50 40 30 20 10 0 1 2 3 TA = 0C TA = -40C 4 5 6 7 TA = +85C
MAX877-6
350 300 OUTPUT CURRENT (mA) 250 200 150 100 50 0 0 1 2 3 4 5 6 7 INPUT VOLTAGE (V) VOUT = 3.0V TA = +70C DIP PACKAGE DISSIPATION LIMIT SO PACKAGE DISSIPATION LIMIT
300 280 NO-LOAD SUPPLY CURRENT (A) 260 240 220 200 180 160 140 120 100 1
(SWITCH = OFF) TA = +85C TA = +25C
80 SHUTDOWN SUPPLY CURRENT (A)
TA = 0C TA = -40C 2 3 4 5 6 7
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
SHDN THRESHOLD VOLTAGE vs. INPUT VOLTAGE AND TEMPERATURE
MAX877-7
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
3.5 SHDN THRESHOLD VOLTAGE (V) 3.0 2.5 2.0 1.5 1.0 0.5 1 2 3 4 5 6 7 INPUT VOLTAGE (V) TA = +25C TA = -55C TA = +125C
A A
GND
B B
2ms/div A: IOUT, 200mA/div, 0mA to 200mA B: VOUT, 50mV/div, AC COUPLED MAX878, VOUT = 3.3V, VIN = 2.5V
2ms/div A: VIN, 2V/div, 2V to 4V B: VOUT, 50mV/div, AC COUPLED IOUT = 240mA MAX878, VOUT = 3.3V, CINBYPASS = 47F
4
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5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 4, TA = +25C, unless otherwise noted.)
SWITCHING WAVEFORMS-- CONTINUOUS CONDUCTION SWITCHING WAVEFORMS-- DISCONTINUOUS CONDUCTION
MAX877/MAX878/MAX879
STEP-DOWN CONVERSION
A
A
A
B 0mA C 5s/div A: SWITCH VOLTAGE (LX PIN), 2V/div B: INDUCTOR CURRENT, 0.5A/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div MAX877, VIN = 1.5V, IOUT = 100mA B 0mA C
B 0mA C 5s/div A: SWITCH VOLTAGE (LX PIN), 2V/div B: INDUCTOR CURRENT, 0.5A/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div MAX878, VIN = 6.0V, VOUT = 5.0V, IOUT = 210mA
2s/div A: SWITCH VOLTAGE (LX PIN), 2V/div B: INDUCTOR CURRENT, 0.5A/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div MAX877, VIN = 3V, IOUT = 70mA
STEP-UP/STEP-DOWN OPERATION
MAX878 START-UP TIME
A A GND GND
B
B GND
20ms/div A: INPUT VOLTAGE, 2V/div, 2V to 6V B: OUTPUT VOLTAGE, 50mV/div MAX878, IOUT = 100mA, VOUT = 3.3V A: SHDN, 2V/div B: VOUT, 1V/div VOUT = 3V
10ms/div
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5
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
______________________________________________________________Pin Description
PIN NAME FUNCTION Sets switch current-limit input. Connect to IN for 1A current limit. A resistor from ILIM to IN sets lower peak inductor currents. Input supply. Analog ground. Not internally connected to PGND. Power ground must be low impedance; solder directly to ground plane or star ground. Connect to AGND, close to the device. 1A NPN power switch collector and active-rectifier PNP emitter. Voltage output. Connect filter capacitor close to pin. Shutdown input disables power supply when low. Also disconnects load from input. Threshold is set at VIN/2. Connect to IN for normal operation. No connect, not internally connected. Selects the main output voltage: 3.3V when connected to AGND, 3.0V when left open. Feedback input for adjustable-output operation. Connect to an external voltage divider between VOUT and AGND.
1
2 3 4 5 6 7
ILIM
IN AGND PGND LX OUT --- - ---- SHDN N.C. (MAX877)
8
SEL (MAX878) FB (MAX879)
_______________Detailed Description
Operating Principle
The MAX877/MAX878/MAX879 combine a switch-mode regulator with an NPN bipolar power switch and current limit, a precision voltage reference, and a synchronous rectifier--all in a single monolithic device. In shutdown mode, the internal rectifier is completely turned off and disconnects the load from the source. Only two external components are required in addition to the input bypass capacitor--a 22H inductor, and a 100F filter capacitor. A minimum-off-time, current-limited, pulse-frequencymodulation (PFM) control scheme combines the high output power and efficiency of pulse-width modulation (PWM) with the low quiescent currents of traditional PFM pulse skippers. External conditions (inductor value, load, and input voltage) determine the way the converter operates, as follows: At light loads, the current through the inductor starts at zero, rises to a peak value, and drops down to zero in each cycle (discontinuous-conduction mode). In this case, the switching frequency is governed by a pair of one-shots, which set a maximum on-time inversely pro6
portional to VIN [tON = 8.8/(VIN - 0.25)] and a minimum off-time (1.3s for MAX877/MAX879, or 2.3s for MAX878). With a 22H inductor, LX's peak current is about 400mA and is independent of input voltage. Efficiency at light loads is improved because of lower peak currents. At very light loads, more energy is stored in the coil than is required by the load in each cycle. The converter regulates by skipping entire cycles. Efficiency is typically 65% to 75% in the pulse-skipping mode. Pulse-skipping waveforms can be irregular, and the output waveform contains a low-frequency component. Larger, low equivalent-series-resistance (ESR) filter capacitors can help reduce the ripple voltage if needed. At heavy loads above approximately 100mA, the converter enters continuous-conduction mode, where current always flows in the inductor. The switch ON state is controlled on a cycle-by-cycle basis, either by the tON(max) time or the preset current limit in the switch. This prevents exceeding the switch current rating or saturating the inductor. At very heavy loads, the inductor current self-oscillates between this peak current limit and some lower value governed by the minimum off-time, the inductance value, and the input/output differential.
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5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
22H VIN 22F RLIM 2 IN DELAY TIMER tOFF SWITCH DRIVER RECTIFIER CONTROL PGND 4 1:N ILIM 1 5 LX
ACTIVE RECTIFIER
OUT 6
VOUT 100F
DELAY TIMER tON
MAX878
SEL 8 VREF 7 SHDN SHUTDOWN CONTROL AGND 3 0.2025V
Figure 1. MAX878 Block Diagram
With ILIM shorted to IN, the peak switch current of the internal NPN power switch is set to 1A. It can be set to a lower value by connecting a resistor between ILIM and IN (see Current Limit section). This enables the use of physically smaller inductors with lower saturationcurrent ratings. At 1A, the switch voltage drop (VSW) is about 500mV. VSW decreases to about 250mV at 0.1A. Conventional PWM converters generate constantfrequency switching noise, while this architecture produces variable-frequency switching noise. The output ripple is the product of the peak inductor current and the output capacitor's ESR. Unlike conventional pulse-skippers, the MAX877/MAX878/MAX879 peak currents are scaled down at light loads, resulting in lower output ripple.
Step-Down Mode and Power Dissipation
In battery-powered applications, for example, where the input voltage exceeds the output voltage, the MAX877/MAX878/MAX879 behave as "switched" linear regulators. If the output voltage starts to drop, the switch turns on and energy is stored in the coil, as in normal step-up mode. After the switch turns off, the voltage at LX flies high. The active rectifier turns on when LX rises above VIN. As in a linear regulator, the voltage difference between V IN and V OUT appears across the rectifier (actually a PNP transistor) until the current goes to zero and the rectifier turns off. At high VIN to VOUT differentials, the maximum load current is limited by the maximum allowable power dissipation in the package (see Typical Operating Characteristics).
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7
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
Active Rectifier
The internal active rectifier of the MAX877/MAX878/ MAX879 replaces the external Schottky catch diode in normal boost operation. The rectifier consists of a PNP pass transistor and a unique control circuit which, in shutdown mode, entirely disconnects the load from the source. This is a distinct advantage over standard boost topologies, since it prevents battery drain in shutdown. The MAX877/MAX878/MAX879 can withstand a momentary short at the output in normal operation. The active rectifier also acts as a zero-dropout regulator if the input exceeds the regulated output.The device still switches to deliver power to the output, and the difference between the input and output voltage appears across the rectifier. Efficiency is similar to that of a linear regulator if the MAX877/MAX878/MAX879 are used as step-down converters. The maximum output current (IOUT (MAX)) with larger input/output differentials is determined by package power dissipation. IOUT (MAX) can be approximated by: IOUT (MAX)
CURRENT-LIMIT RESISTOR vs. PEAK INDUCTOR CURRENT
MAX877/878/879-FG02
1200 PEAK INDUCTOR CURRENT (mA) 1000 800 600 VIN = 2.5V 400 200 0 0 2 4 6 8 10 12
14
RESISTOR VALUE (k)
(
PDISS ------------ (VIN - VOUT)
Figure 2. Current-Limit Resistor vs. Peak Inductor Current
)
x 0.9
Shutdown (S H D N ) is a high-impedance, active-low input.-Connect it to IN for normal operation. Keeping ------ S H D N at ground holds the converters in shutdown mode. Since the active rectifier is turned off in this mode, the path from input to load is cut, and the output effectively drops to 0V. The supply current in shutdown mode ranges from 4A at VIN = 1V to 50A at VIN = 5V. The shutdown-circuit----- threshold is set nominally to VIN/2 -- + 250mV. When S H D N is below this--threshold, the ----- device is shut down; it is enabled with SHDN ------- the above threshold. When driven from external logic, SHDN can be driven to a higher voltage than VIN, (6.2V max).
-------
Shutdown
The MAX879's output voltage is set by two resistors, R1 and R2 (Figure 3), which form a voltage divider between the output and the FB pin. The output voltage can be set from 2.5V to 6.0V by the equation: VOUT = VREF (R1 + R2) R2 where VREF = 0.2025V. To simplify the resistor selection: VOUT R1 = R2 -1 VREF
(
)
Current Limit
Connecting ILIM to IN sets an LX current limit of 1A. For smaller output power levels that do not require the maximum peak current, reduce the peak inductor current by connecting a resistor between ILIM and IN. This optimizes overall efficiency and allows very small, low-cost coils with lower current ratings. See Figure 2 to select the resistor (see also Inductor Selection section).
Output Voltage Selection
The MAX877's output voltage is fixed at 5V. The MAX878's output voltage can be set to 3V by leaving the SEL pin open, or to 3.3V by connecting SEL to AGND.
Since the input current at FB has a maximum of 40nA, large values (10k to 50k for R2) can be used without significant accuracy loss. For 1% error, the current through R2 should be at least 100 times FB's bias current. When large values are used for the feedback resistors (R1 > 50k), stray output impedance at FB can add a "lag" to the feedback response, destabilizing the regulator and creating a larger ripple at the output. Lead lengths and circuit board traces at the FB node should be kept short. Reduce ripple by adding a "lead" compensation capacitor (C3, 100pF to 50nF) in parallel with R1.
8
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5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
L1 22H
VIN
C1 22F
INPUT 1.0V TO 6.2V
5 6
2
IN
LX
OUT
VOUT
22F
2
IN ILIM MAX877 SHDN PGND LX OUT AGND 3 5 6
22H
7
MAX879
SHDN
R1 8
C3
C2 100F ON/OFF
1 7
OUTPUT 5V
100F
1
ILIM AGND 3
FB PGND 4
R2
4
Figure 3. MAX879 Adjustable Voltage
Figure 4. MAX877 Standard Application Circuit
__________Applications Information
Figure 4 shows a MAX877 step-up application circuit. This circuit starts up and operates with inputs ranging from 1.0V to 6.2V. Start-up time is a function of the load, typically less than 5ms. Output current capability is a function of the input voltage (see Typical Operating Characteristics). The converters will regulate down to the output voltage and seamlessly switch into boost mode as the input drops below the output voltage. This is especially useful in battery-powered applications, where the battery voltage may initially exceed the output voltage. To generate 5V from four alkaline cells in series, the input ranges from 6.2V to 3.6V. When the battery pack is fresh, the MAX877 will step down with the active rectifier acting as the switch. As the batteries approach 5V, or the desired output voltage, the converter's control circuitry will ensure a smooth transition into step-up mode. The converter operates until the batteries are less than 3V; efficiency is typically 80% with fresh batteries, and is close to 85% at VIN = 4V.
current limit, which is 1A without an external current limit (ILIM connected to IN). It is acceptable to operate the inductor at 120% of its saturation rating; however, this may slightly reduce efficiency. For highest efficiency, use an inductor with a low DC resistance, preferably under 0.2. Table 1 lists suggested inductor suppliers.
Capacitor Selection
The 100F, 10V surface-mount tantalum (SMT) output capacitor shown in the Typical Operating Circuit will provide a 25mV output ripple or less, stepping up from 3V to 5V at 200mA. Smaller capacitors, down to 10F, are acceptable for light loads or in applications that can tolerate higher output ripple. The input capacitor may be omitted if the supply has low output impedance and the input lead length is less than 2 inches (5cm) or the loads are small. The primary factor in selecting both the output and input filter capacitor is low ESR. The ESR of both bypass and filter capacitors affects efficiency. Optimize performance by increasing filter capacitors or using specialized lowESR capacitors. The smallest low-ESR SMT tantalum capacitors currently available are Sprague 595D or 695D series. Sanyo OS-CON organic-semiconductor throughhole capacitors also exhibit very low ESR, are rated for the wide temperature range, and are especially suitable for operation at cold temperatures (below 0C). Table 1 lists suggested capacitor suppliers.
Inductor Selection
The 22H inductor shown in the Typical Operating Circuit is sufficient for most MAX877/MAX878/MAX879 designs. Other inductor values ranging from 10H to 47H are also suitable. The inductor should have a saturation rating equal to or greater than the peak switch-
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9
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
Layout
The MAX877/MAX878/MAX879's high peak currents and high-frequency operation make PC layout important for minimum ground bounce and noise. Locate input bypass and output filter capacitors close to the device pins. All connections to the FB pin (MAX879) should also be kept as short as possible. A ground plane is recommended. Solder AGND (pin 3) and PGND (pin 4) directly to the ground plane. Refer to the MAX877/MAX878/MAX879 evaluation kit (EV kit) manual for a suggested surface-mount layout.
Table 1. Component Suppliers
PRODUCTION METHOD INDUCTORS Sprague 595D Sprague 695D Matsuo 267 series AVX TPS series Sanyo OS-CON (low-ESR organic semiconductor) Nichicon PL series (low-ESR electrolytic) United Chemi-Con LXF series CAPACITORS
Sumida CD54-220 (22H) Surface Mount Murata-Erie LQHYN1501K04M00-D5 (15H) CoilCraft DO3316-223 (22H) Coiltronics CTX20-1 (22H)
Miniature Through-Hole
Sumida RCH654-220 (22H)
Low-Cost Through-Hole
Renco RL 1284-22 (22H) CoilCraft PCH-27-223 (22H)
AVX CoilCraft Coiltronics Matsuo Murata-Erie Nichicon Renco Sanyo Sprague Sumida United Chemi-Con
USA: USA: USA: USA: Japan: USA: USA: Japan: USA: USA: Japan: USA: USA: Japan: USA:
(207) 282-5111 (708) 639-6400 (407) 241-7876 (714) 969-2491 (06) 332-0871 (800) 831-9172 (708) 843-7500 (81) 7-5231-8461 (516) 586-5566 (619) 661-6835 (0720) 70-1005 (603) 224-1961 (708) 956-0666 (81) 3607-5111 (714) 255-9500
FAX (207) 283-1941 FAX (708) 639-1469 FAX (407) 241-9339 FAX (714) 960-6492 FAX (814) 238-0490 FAX (708) 843-2798 FAX (81) 7-5256-4158 FAX (516) 586-5562 FAX (619) 661-1055 FAX (0720) 70-1174 FAX (603) 224-1430 FAX (708) 956-0702 FAX (81) 2070-1174 FAX (714) 255-9400
10
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5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
__Ordering Information (continued)
PART MAX878CPA MAX878CSA MAX878C/D MAX878EPA MAX878ESA MAX878MJA MAX879CPA MAX879CSA MAX879C/D MAX879EPA MAX879ESA MAX879MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP
ILIM VREF N.C. (MAX877) SEL (MAX878) FB (MAX879)
___________________Chip Topography
MAX877/MAX878/MAX879
IN
SHDN
AGND
0.084" (2.134mm)
* Contact factory for dice specifications.
PGND
OUT
LX 0.068" (1.727mm)
TRANSISTOR COUNT: 170 SUBSTRATE CONNECTED TO AGND
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11
5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters MAX877/MAX878/MAX879
________________________________________________________Package Information
D1
DIM A A1 A2 A3 B B1 C D D1 E E1 e eA eB L INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.050 0.012 0.008 0.390 0.348 0.035 0.005 0.325 0.300 0.280 0.240 0.100 BSC 0.300 BSC 0.400 - 0.150 0.115 15 0 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.56 1.27 1.65 0.20 0.30 8.84 9.91 0.13 0.89 7.62 8.26 6.10 7.11 2.54 BSC 7.62 BSC - 10.16 2.92 3.81 0 15
21-324A
E D A3 A A2 E1
L
A1 e B
C B1 eA eB
8-PIN PLASTIC DUAL-IN-LINE PACKAGE
DIM A A1 B C D E e H h L
E
H
INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.197 0.189 0.157 0.150 0.050 BSC 0.244 0.228 0.020 0.010 0.050 0.016 8 0
MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 4.80 5.00 3.80 4.00 1.27 BSC 5.80 6.20 0.25 0.50 0.40 1.27 0 8
21-325A
D A e B
0.127mm 0.004in.
h x 45
A1
C
L
8-PIN PLASTIC SMALL-OUTLINE PACKAGE
12
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