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MAX878CPA-MAX878CSA-MAX879CSA-MAX879ESA
5V/3.3V/3V/Adjustable-Output / Step-Up/Step-Down DC-DC Converters
_______________General DescriptionThe MAX877/MAX878/MAX879 are pulse-skipping, step-
up/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 22µH) and two filter capacitors.
The MAX877 delivers a regulated 5V output from 2.5V to
6.2V inputs. The MAX878 generates pin-selectable volt-
ages 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 Rectifier™) 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 195µA
under no load, and only 20µA 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.
________________________ApplicationsTwo 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
____________________________FeaturesRegulates from Inputs Above & Below the Output1V to 6.2V Supply-Voltage RangeInternal 1A Active Rectifier with Input-to-Output
Disconnect in ShutdownUp to 210mA Load Currents, Guaranteed85% EfficiencyOnly 3 External ComponentsAdjustable Current Limit195µA Quiescent Supply Current20µA Shutdown Supply Current3V/3.3V/5V and Adjustable Output Voltage VersionsAvailable in 8-Pin DIP and SO Packages
______________Ordering Information
Ordering Information continued on last page.* Contact factory for dice specifications.
MAX877/MAX878/MAX879
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
________________________________________________________________Maxim Integrated Products1
__________________Pin Configuration
__________Typical Operating Circuit
Call toll free 1-800-998-8800 for free samples or literature.™ Active Rectifier is a trademark of Maxim Integrated Products.
MAX877/MAX878/MAX879
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
ABSOLUTE MAXIMUM RATINGSSupply 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, –S—H—D—N...........................................................-0.3V to +7V..........................................................-0.3V to (OUT + 0.3V)
AGND to PGND........................................................-0.3V, +0.3V
Reverse Battery Current....................................................900mA
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C)............727mW
SO (derate 5.88mW/°C above +70°C).........................471mW
CERDIP (derate 8.00mW/°C above +70°C).................640mW
Operating Temperature Ranges:
MAX87_C_A........................................................0°C to +70°C
MAX87_E_A.....................................................-40°C to +85°C
MAX87_MJA..................................................-55°C to +125°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10sec).............................+300°C
ELECTRICAL CHARACTERISTICS(VIN= 2.7V, ILOAD= 0mA, LX = 22µH, COUT= 100µF, –S—H—D—Nand ILIM connected to IN, AGND connected to PGND,
TA = TMINto TMAX, typical values are at TA = +25°C, 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.
Note 1:The output may be shorted to ground continuously if the package power dissipation is not exceeded.
MAX877/MAX878/MAX879
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
_______________________________________________________________________________________3
ELECTRICAL CHARACTERISTICS (continued)(VIN= 2.7V, ILOAD= 0mA, LX = 22µH, COUT= 100µF, –S—H—D—N–and ILIM connected to IN, AGND connected to PGND,
TA = TMINto TMAX, typical values are at TA = +25°C, unless otherwise noted.)
Note 2:Output in regulation, VOUT= VOUT(nominal) ±4%.
Note 3:At high VINto VOUTdifferentials, the maximum load current is limited by the maximum allowable power dissipation in the
package (see Absolute Maximum Ratingsand 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:VOUTis set to a target value of 5V by 0.1% external feedback resistors. VOUTis measured to be within 5V ±2.5%
to guarantee error-comparator trip point.
Note 6:Startup guaranteed under these load conditions.
MAX877/MAX878/MAX879
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics(Circuit of Figure 4, TA = +25°C, unless otherwise noted.)
MAX877/MAX879
EFFICIENCY vs. LOAD CURRENT
MAX877-1
LOAD CURRENT (mA)
EFFICIENCY (%)10090
MAX878
EFFICIENCY vs. LOAD CURRENT
MAX877-2
LOAD CURRENT (mA)
EFFICIENCY (%)100
MAX877/MAX879 MAXIMUM OUTPUT
CURRENT AT LOW INPUT VOLTAGE
MAX877-3
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
MAX878 MAXIMUM OUTPUT
CURRENT vs. INPUT VOLTAGE
MAX877-4
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
SHDN THRESHOLD VOLTAGE
vs. INPUT VOLTAGE AND TEMPERATURE
MAX877-7
INPUT VOLTAGE (V)
SHDN THRESHOLD VOLTAGE (V)
NO-LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE AND TEMPERATURE
MAX877-5
SUPPLY VOLTAGE (V)
NO-LOAD SUPPLY CURRENT (µA)
SHUTDOWN SUPPLY CURRENT
vs. INPUT VOLTAGE AND TEMPERATURE
MAX877-6
INPUT VOLTAGE (V)
SHUTDOWN SUPPLY CURRENT (706
MAX877/MAX878/MAX879
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
_______________________________________________________________________________________5
____________________________Typical Operating Characteristics (continued)(Circuit of Figure 4, TA = +25°C, unless otherwise noted.)
MAX877/MAX878/MAX879
_______________Detailed Description
Operating PrincipleThe 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 22µH inductor, and a 100µF filter capacitor.
A minimum-off-time, current-limited, pulse-frequency-
modulation (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 volt-
age) determine the way the converter operates, as fol-
lows:
At light loads, the current through the inductor starts atzero, 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 pro-
portional to VIN[tON= 8.8/(VIN- 0.25)] and a minimum
off-time (1.3µs for MAX877/MAX879, or 2.3µs for
MAX878). With a 22µH 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 equiv-
alent-series-resistance (ESR) filter capacitors can help
reduce the ripple voltage if needed.
At heavy loadsabove approximately 100mA, the con-
verter enters continuous-conduction mode, where cur-
rent 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 sat-
urating 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.
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters_______________________________________________________________________________________
______________________________________________________________Pin Description
