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MAX483MJA ,Low-Power / Slew-Rate-Limited RS-485/RS-422 TransceiversGeneral Description ________
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MAX1896EUT-T
1.4MHz SOT23 Current-Mode Step-Up DC-DC Converter
General DescriptionThe MAX1896 step-up DC-DC converter incorporates
high-performance current-mode, fixed-frequency,
pulse-width modulation (PWM) circuitry and an internal
0.7ΩN-channel MOSFET to provide a highly efficient
regulator with fast response.
High switching frequency (1.4MHz) allows fast loop
response and easy filtering with small components. The
MAX1896 can produce an output voltage as high as
13V from an input as low as 2.6V. Soft-start is program-
mable with an external capacitor, which sets the input
current ramp rate. In shutdown mode, current con-
sumption is reduced to 0.01µA.
The MAX1896 is available in a space-saving 6-pin
SOT23 package. The ultra-small package and high
switching frequency allow cost and space-efficient
implementations.
ApplicationsNotebook Computers
LCD Displays
PCMCIA Cards
Portable Applications
Hand-Held Devices
Features>90% EfficiencyAdjustable Output Up to 13VGuaranteed 12V/120mA Output from 5V Input2.6V to 5.5V Input RangeLT1613 Pin Compatible0.01µA Shutdown CurrentProgrammable Soft-StartSpace-Saving 6-Pin SOT23 Package
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Pin Configuration
Typical Operating Circuit19-2221; Rev 1; 3/04
Ordering Information
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGSStresses 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.
LX to GND..............................................................-0.3V to +14V
IN, SHDN, FB to GND...............................................-0.3V to +6V
SS to GND...................................................-0.3V to (VIN+ 0.3V)
RMS LX Pin Current..............................................................0.6A
Continuous Power Dissipation (TA= +70°C) (Note 1)
6-Pin SOT23 (derate 9.1mW/°C above +70°C)...........727mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
Note 1:Thermal properties are specified with product mounted on PC board with one square-inch of copper area and still air.
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Note 2:Current limit varies with duty cycle due to slope compensation. See the Output Current Capabilitysection.
Note 3:Specifications to -40°C are guaranteed by design and not production tested.
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Typical Operating Characteristics(Circuit of Figure 1, VIN= 3.3V, TA= +25°C, unless otherwise noted.)
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Pin Description
Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 3.3V, TA= +25°C, unless otherwise noted.)
MAX1896
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
Detailed DescriptionThe MAX1896 is a highly efficient power supply that
employs a current-mode, fixed-frequency pulse-width
modulation (PWM) architecture for fast-transient response
and low-noise operation. The functional diagram is shown
in Figure 2. As the load varies, the error amplifier sets the
inductor peak current necessary to supply the load and
regulate the output voltage. To maintain stability at high
duty cycle, a slope-compensation signal is internally
summed with the current-sense signal.
At light loads, this architecture allows the MAX1896 to
skip cycles to prevent overcharging the output voltage.
In this region of operation, the inductor ramps up to a
peak value of about 100mA, discharges to the output
and waits until another pulse is needed again.
Output-Current CapabilityThe output-current capability of the MAX1896 is a func-
tion of current limit, input voltage, and inductor value.
Because of the slope compensation used to stabilize
the feedback loop, the duty cycle affects the current
limit. The output-current capability is governed by the
following equation:
where:
ILIM= current limit specified at 50% (see Electrical
Characteristics)
VDIODE= catch diode forward drop at ILIM, (V)
fOSC= oscillator frequency, (Hz)
L = inductor value, (H)= conversion efficiency, 0.85 nominal
VIN= input voltage, (V)
VOUT= output voltage, (V)
Soft-StartThe MAX1896 can be programmed for soft-start upon
power-up with an external capacitor. When the
MAX1896 is turned on, the soft-start capacitor (CSS) is
charged at a constant current of 4µA, ramping up to
0.5V. During this time, the SS voltage directly controls
the peak-inductor current, allowing 0A at VSS= 0.5V to
the full current limit at VSS= 1.5V. The maximum load
current is available after the soft-start cycle is complet-
ed. When the MAX1896 is turned off, the soft-start
capacitor is internally discharged to ground.
ShutdownThe MAX1896 shuts down to reduce the supply current
to 0.01µA when SHDNis low. In this mode, the internal
reference, error amplifier, comparators, biasing circuit,
and N-channel MOSFET are turned off. The step-up
converter’s output is still connected to IN via the exter-
nal inductor and output rectifier.
Applications InformationThe MAX1896 operates well with a variety of external
components. The components in Figure 1 are suitable
for most applications. See the following sections to opti-
mize external components for a particular application.
Inductor SelectionInductor selection depends on input voltage, output volt-
age, maximum current, size, and availability of inductor
values. Other factors can include efficiency and ripple
voltage. Inductors are specified by their inductance (L),
peak current (IPK), and resistance (RL). The following
step-up circuit equations are useful in choosing the
inductor values based on the application. They allow the
trading of peak current and inductor value while consid-
ering component availability and cost.
The equation used here assumes a constant LIR, which
is the ratio of the inductor peak-to-peak AC current to
average DC inductor current. A good compromise
between the size of the inductor versus loss and output
ripple is to choose an LIR of 0.3 to 0.5. The peak induc-
tor current is then given by:
where:
IOUT(MAX)= maximum output current, (A)
VIN(MIN)= minimum input voltage, (V)
