Datasheet LTC3400-1 (Analog Devices) - 5

LTC3400-1
U OPERATIO
The LTC3400-1 is a1.2MHz, synchronous boost converter to provide a peak current control command for the PWM. housed in a 6-lead ThinSOT package. Able to operate from Peak switch current is limited to approximately 850mA an input voltage below 1V, the device features fixed independent of input or output voltage. The current signal frequency, current mode PWM control for exceptional line is blanked for 40ns to enhance noise rejection. and load regulation. With its low RDS(ON) and gate charge Zero Current Comparator: The zero current comparator internal MOSFET switches, the device maintains high monitors the inductor current to the output and shuts off efficiency over a wide range of load current. Detailed the synchronous rectifier once this current reduces to ap- descriptions of the three distinct operating modes follow. proximately 20mA. This prevents the inductor current from Operation can be best understood by referring to the Block reversing in polarity improving efficiency at light loads. Diagram. Antiringing Control: The antiringing control circuitry pre-
Low Voltage Start-Up
vents high frequency ringing of the SW pin as the inductor current goes to zero by damping the resonant circuit The LTC3400-1 will start up at a typical VIN voltage of formed by L and C 0.85V or higher. The low voltage start-up circuitry controls SW (capacitance on SW pin). the internal NMOS switch up to a maximum peak inductor
Burst Mode Operation
current of 850mA (typ), with an approximate 1.5µs off- time during start-up, allowing the device to start up into an Portable devices frequently spend extended time in low output load. Once V power or standby mode, only switching to high power OUT exceeds 2.3V, the start-up cir- cuitry is disabled and normal fixed frequency PWM opera- drain when specific functions are enabled. In order to tion is initiated. In this mode, the LTC3400-1 operates improve battery life in these types of products, high power independent of V converter efficiency needs to be maintained over a wide IN, allowing extended operating time as the battery can droop to several tenths of a volt without output power range. In addition to its high efficiency at affecting output voltage regulation. The limiting factor for moderate and heavy loads, the LTC3400-1 includes auto- the application becomes the ability of the battery to supply matic Burst Mode operation that improves efficiency of sufficient energy to the output. the power converter at light loads. Burst mode operation is initiated if the output load current falls below an
Low Noise Fixed Frequency Operation
internally programmed threshold (see Typical Perfor- mance graph, Output Load Burst Mode Threshold vs V Oscillator: The frequency of operation is internally set to IN). Once initiated, the Burst Mode operation circuitry shuts 1.2MHz. down most of the device, only keeping alive the circuitry Error Amp: The error amplifier is an internally compensated required to monitor the output voltage. This is referred to transconductance type (current output) with a transconduc- as the sleep state. In sleep, the LTC3400-1 draws only tance (gm) = 33 microsiemens. The internal 1.23V reference 19µA from the output capacitor, greatly enhancing effi- voltage is compared to the voltage at the FB pin to generate ciency. When the output voltage has drooped approxi- an error signal at the output of the error amplifier. A volt- mately 1% from nominal, the LTC3400-1 wakes up and age divider from VOUT to ground programs the output commences normal PWM operation. The output capacitor voltage via FB from 2.5V to 5V using the equation: recharges and causes the LTC3400-1 to reenter sleep if V the output load remains less than the sleep threshold. The OUT = 1.23V • [1 + (R1/R2)] frequency of this intermittent PWM or burst operation is Current Sensing: A signal representing NMOS switch proportional to load current; that is, as the load current current is summed with the slope compensator. The drops further below the burst threshold, the LTC3400-1 summed signal is compared to the error amplifier output turns on less frequently. When the load current increases 34001f 5