LTC3421_15 Datasheet, PDF (8/16 Pages) Linear Technology – 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect

LTC3421
U
OPERATIO
LOW VOLTAGE START-UP
The LTC3421 includes an independent start-up oscillator
designed to start-up at input voltages of 0.85V typical. The
frequency and peak current limit during start-up are inter-
nally controlled. The device can start-up under some load
(see graph of Start-Up Current vs Input Voltage). Soft-
start and inrush current limiting are provided during start-
up as well as normal mode. The same soft-start capacitor
is used for each operating mode.
When either V_IN or V_OUT exceeds 2.25V, the IC enters
normal operating mode. Once the output voltage exceeds
the input by 0.3V, the IC powers itself from V_OUT instead
of V_IN. At this point the internal circuitry has no depen-
dency on the V_IN input voltage, eliminating the require-
ment for a large input capacitor. The input voltage can drop
as low as 0.5V without affecting circuit operation. The
limiting factor for the application becomes the availability
of the power source to supply sufficient energy to the
output at the low voltages and the maximum duty cycle,
which is clamped at 91% typical.
LOW NOISE FIXED FREQUENCY OPERATION
Shutdown
The part is shut down by pulling SHDN below 0.3V, and
activated by pulling the pin initially above 1V and maintain-
ing a high state down to 0.5V. Note that the SHDN pin can
be driven above V_IN or V_OUT as long as it is limited to less
than the absolute maximum rating.
Soft-Start
The soft-start time is programmed with an external capaci-
tor to ground on the SS pin. An internal current source
charges it with a nominal 2.5µA. The voltage on the SS pin
(in conjunction with the external resistor on the I_LIM pin)
is used to control the peak current limit until the voltage on
the capacitor exceeds 1.6V, at which point the external
resistor sets the peak current. In the event of a com-
manded shutdown or a thermal shutdown, the capacitor is
discharged automatically. Note that Burst Mode operation
is inhibited during the soft-start time.
t(ms) = C_SS(µF) • 320
8
Oscillator
The frequency of operation is set through a resistor from
the R_T pin to ground. An internally trimmed timing capaci-
tor resides inside the IC. The oscillator can be synchro-
nized with an external clock applied to the SYNC pin. When
synchronizing the oscillator, the free running frequency
must be set to an approximately 30% lower frequency
than the desired synchronized frequency.
Current Sensing
Lossless current sensing converts the peak current signal
to a voltage to sum in with the internal slope compensa-
tion. This summed signal is compared to the error ampli-
fier output to provide a peak current control command for
the PWM. The slope compensation in the IC is adaptive to
the input voltage and output voltage. Therefore, the con-
verter provides the proper amount of slope compensation
to ensure stability, but not an excess to cause a loss of
phase margin in the converter.
Error Amplifier
The error amplifier is a transconductance amplifier, with
its positive input internally connected to the 1.22V refer-
ence and its negative input connected to FB. A simple
compensation network is placed from COMP to ground.
Internal clamps limit the minimum and maximum error
amplifier output voltage for improved large-signal tran-
sient response. During sleep (in Burst Mode operation),
the compensation pin is high impedance; however, clamps
limit the voltage on the external compensation network,
preventing the compensation capacitor from discharging
to zero during the sleep time.
Current Limit
The programmable current limit circuit sets the maximum
peak current. This clamp level is programmed with a
resistor from I_LIM to ground. In Burst Mode operation, the
current limit is automatically set to a nominal value of 0.6A
peak for optimal efficiency.
I_LIM
=
150
R
where I is in amps and R is in kΩ.
3421f