Datasheet LTC3429, LTC3429B (Analog Devices) - 6

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