LTC3429/LTC3429B
6
3429fa
OPERATIO
U
The LTC3429/LTC3429B are 500kHz, synchronous boost
converters housed in a 6-lead SOT-23 package. Able to
operate from an input voltage below 1V, the device fea-
tures fixed frequency, current mode PWM control for
exceptional line and load regulation. Low R
DS(ON)
internal
MOSFET switches enable the device to maintain high
efficiency over a wide range of load current. Detailed
descriptions of the different operating modes follow.
Operation can be best understood by referring to the Block
Diagram.
LOW VOLTAGE START-UP
The LTC3429/LTC3429B include an independent start-up
oscillator designed to start up at input voltages of 0.85V
typically. The frequency and duty cycle of the start-up
oscillator are internally set to 150kHz and 67% respec-
tively. In this mode, the IC operates completely open-loop
and the current limit is also set internally to 850mA. Once
the output voltage exceeds 2.3V, the start-up circuitry is
disabled and normal close-loop PWM operation is initi-
ated. In normal mode, the LTC3429/LTC3429B power
themselves from V
OUT
instead of V
IN
. This allows the
battery voltage to drop to as low as 0.5V without affecting
the circuit operation. The only limiting factor in the appli-
cation becomes the ability of the battery to supply suffi-
cient energy to the output. Soft-start and inrush current
limiting are provided during start-up as well as normal
mode operation.
Soft-Start
The LTC3429/LTC3429B provide soft-start by charging an
internal capacitor with a very weak current source. The
voltage on this capacitor, in turn, slowly ramps the peak
inductor current from zero to a maximum value of 850mA.
The soft-start time is typically 2.5ms, the time it takes to
charge the capacitor from zero to 1.35V. However, this
time varies greatly with load current, output voltage and
input voltage (see Typical Performance Characteristics,
Inrush Current Control and Soft-Start). The soft-start
capacitor is discharged completely in the event of a
commanded shutdown or a thermal shutdown. It is dis-
charged only partially in case of a short circuit at the
output.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is internally set to 500kHz.
Error Amp
The error amplifier is an internally compensated transcon-
ductance type (current output) with a transconductance
(g
m
) = 33 microsiemens. The internal 1.23V reference
voltage is compared to the voltage at the FB pin to generate
an error signal at the output of the error amplifier. A volt-
age divider from V
OUT
to ground programs the output
voltage via FB from 2.5V to 5V using the equation:
V
OUT
= 1.23V • [1 + (R1/R2)]
Current Sensing
Lossless current sensing converts the NMOS switch
current signal to a voltage to be summed with the internal
slope compensation. The summed signal is compared to
the error amplifier output to provide a peak current
control command for the PWM. Peak switch current is
limited to approximately 850mA independent of input or
output voltage. The switch current signal is blanked for
60ns to enhance noise rejection.
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
once this current reduces to approximately 27mA. This
prevents the inductor current from reversing in polarity
thereby improving efficiency at light loads.
Antiringing Control
The antiringing control circuitry prevents high frequency
ringing of the SW pin as the inductor current goes to zero
in discontinuous mode. The damping of the resonant
circuit formed by L and C
SW
(capacitance on SW pin) is
achieved by placing a 150Ω resistor across the inductor.
Synchronous Rectifier
To prevent the inductor current from running away, the
PMOS synchronous rectifier is only enabled when V
OUT
>
(V
IN
+ 0.1V) and the FB pin is >0.8V.
