LTC3125_15 Datasheet, PDF (10/18 Pages) Linear Technology – 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit

LTC3125
APPLICATIONS INFORMATION
Burst Mode operation is inhibited during start-up and
soft-start and until V_OUT is at least 0.38V greater than V_IN.
GSM and GPRS modems have become a popular wire-
less data transfer solution for use in notebook PCs and
other mobile systems. GSM transmission requires large
bursts of current that exceed the maximum peak current
specifications for CompactFlash and PCMCIA bus power.
The GSM standard specifies a 577μs, 2A (typical) trans-
mission burst within a 4.6ms period (12.5% duty cycle).
During the receive and standby periods the current con-
sumption drops to 70mA (typical), yielding an average
current requirement of 320mA.
Other standards (such as GPRS, Class 10) define a higher
data rate. One popular requirement transmits two 2A bursts
(3A worst case) within a 4.6ms frame period (70mA standby
current) demanding an 800mA average input current. The
LTC3125 external current limit programming resistor can
be easily adjusted for this requirement.
Further, the GSM module is typically specified to operate
over an input power range that is outside that allowed in
the PCMCIA or CompactFlash bus power specification.
The LTC3125 is a high efficiency boost converter with
programmable input average current limit that provides
the needed flexibility when designing a GSM/GPRS power
supply solution. The high efficiency of the converter maxi-
mizes the average output power without overloading the
bus. A bulk output capacitor is used to supply the energy
and maintain the output voltage during the high current
pulses.
V_IN > V_OUT OPERATION
The LTC3125 will maintain voltage regulation even when
the input voltage is above the desired output voltage.
Note that the efficiency and the maximum output current
capability are reduced. Refer to the Typical Performance
Characteristics for details.
SHORT-CIRCUIT PROTECTION
The LTC3125 output disconnect feature enables output
short circuit protection although input current limit func-
tionality is maintained. To reduce power dissipation under
10
short-circuit conditions; the peak switch current limit is
reduced to 800mA (typical).
SCHOTTKY DIODE
Although it is not necessary, adding a Schottky diode from
SW to V_OUT will improve efficiency by about 4%. Note that
this defeats the output disconnect, short-circuit protection
and average input limiting during start-up.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3125 demands care-
ful attention to board layout. A careless layout will result
in reduced performance. A large ground pin copper area
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3125 can utilize small surface mount chip induc-
tors due to its fast 1.6MHz switching frequency. Inductor
values between 2.2μH and 4.7μH are suitable for most
applications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst Mode
threshold) by reducing the inductor ripple current. Increas-
ing the inductance above 10μH will increase size while
providing little improvement in output current capability.
The minimum inductance value is given by:
( ) ^VIN(MIN) ^{• V}OUT(MAX) ^–VIN(MIN)
L>
Ripple•V_OUT(MAX) •f_SW
where:
Ripple = Allowable inductor current ripple
(amps peak-peak)
V_IN(MIN) = Minimum input voltage
V_OUT(MAX) = Maximum output voltage
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency dependent
3125fa