Datasheet LTC3400, LTC3400B (Analog Devices) - 5
| Manufacturer | Analog Devices |
| Description | 600mA, 1.2MHz Micropower Synchronous Boost Converters in ThinSOT |
| Pages / Page | 12 / 5 — OPERATIO. Low Voltage Start-Up. Burst Mode Operation. Low Noise Fixed … |
| File Format / Size | PDF / 223 Kb |
| Document Language | English |
OPERATIO. Low Voltage Start-Up. Burst Mode Operation. Low Noise Fixed Frequency Operation
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LTC3400/LTC3400B
U OPERATIO
The LTC3400/LTC3400B are 1.2MHz, synchronous boost to provide a peak current control command for the PWM. converters housed in a 6-lead ThinSOT package. Able to Peak switch current is limited to approximately 850mA operate from an input voltage below 1V, the devices independent of input or output voltage. The current signal feature fixed frequency, current mode PWM control for is blanked for 40ns to enhance noise rejection. exceptional line and load regulation. With its low RDS(ON) Zero Current Comparator: The zero current comparator and gate charge internal MOSFET switches, the devices monitors the inductor current to the output and shuts off maintain high efficiency over a wide range of load current. the synchronous rectifier once this current reduces to ap- Detailed descriptions of the three distinct operating modes proximately 20mA. This prevents the inductor current from follow. Operation can be best understood by referring to reversing in polarity improving efficiency at light loads. the Block 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/LTC3400B will start up at a typical VIN volt- formed by L and C age of 0.85V or higher. The low voltage start-up circuitry SW (capacitance on SW pin). controls the internal NMOS switch up to a maximum peak
Burst Mode Operation
inductor current of 850mA (typ), with an approximate 1.5µs off-time during start-up, allowing the devices to Portable devices frequently spend extended time in low start up into an output load. Once V power or standby mode, only switching to high power OUT exceeds 2.3V, the start-up circuitry is disabled and normal fixed frequency drain when specific functions are enabled. In order to PWM operation is initiated. In this mode, the LTC3400/ improve battery life in these types of products, high power LTC3400B operate 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 output power range. In addition to its high efficiency at of a volt without affecting output voltage regulation. The moderate and heavy loads, the LTC3400 includes auto- limiting factor for the application becomes the ability of the matic Burst Mode operation that improves efficiency of battery to supply 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 draws only 19µA tance (gm) = 33 microsiemens. The internal 1.23V reference from the output capacitor, greatly enhancing efficiency. voltage is compared to the voltage at the FB pin to generate When the output voltage has drooped approximately 1% an error signal at the output of the error amplifier. A volt- from nominal, the LTC3400 wakes up and commences age divider from VOUT to ground programs the output normal PWM operation. The output capacitor recharges voltage via FB from 2.5V to 5V using the equation: and causes the LTC3400 to reenter sleep if the output load V remains less than the sleep threshold. The frequency of OUT = 1.23V • [1 + (R1/R2)] this intermittent PWM or burst operation is proportional to Current Sensing: A signal representing NMOS switch load current; that is, as the load current drops further current is summed with the slope compensator. The below the burst threshold, the LTC3400 turns on less summed signal is compared to the error amplifier output frequently. When the load current increases above the 3400fa 5
U OPERATIO
The LTC3400/LTC3400B are 1.2MHz, synchronous boost to provide a peak current control command for the PWM. converters housed in a 6-lead ThinSOT package. Able to Peak switch current is limited to approximately 850mA operate from an input voltage below 1V, the devices independent of input or output voltage. The current signal feature fixed frequency, current mode PWM control for is blanked for 40ns to enhance noise rejection. exceptional line and load regulation. With its low RDS(ON) Zero Current Comparator: The zero current comparator and gate charge internal MOSFET switches, the devices monitors the inductor current to the output and shuts off maintain high efficiency over a wide range of load current. the synchronous rectifier once this current reduces to ap- Detailed descriptions of the three distinct operating modes proximately 20mA. This prevents the inductor current from follow. Operation can be best understood by referring to reversing in polarity improving efficiency at light loads. the Block 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/LTC3400B will start up at a typical VIN volt- formed by L and C age of 0.85V or higher. The low voltage start-up circuitry SW (capacitance on SW pin). controls the internal NMOS switch up to a maximum peak
Burst Mode Operation
inductor current of 850mA (typ), with an approximate 1.5µs off-time during start-up, allowing the devices to Portable devices frequently spend extended time in low start up into an output load. Once V power or standby mode, only switching to high power OUT exceeds 2.3V, the start-up circuitry is disabled and normal fixed frequency drain when specific functions are enabled. In order to PWM operation is initiated. In this mode, the LTC3400/ improve battery life in these types of products, high power LTC3400B operate 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 output power range. In addition to its high efficiency at of a volt without affecting output voltage regulation. The moderate and heavy loads, the LTC3400 includes auto- limiting factor for the application becomes the ability of the matic Burst Mode operation that improves efficiency of battery to supply 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 draws only 19µA tance (gm) = 33 microsiemens. The internal 1.23V reference from the output capacitor, greatly enhancing efficiency. voltage is compared to the voltage at the FB pin to generate When the output voltage has drooped approximately 1% an error signal at the output of the error amplifier. A volt- from nominal, the LTC3400 wakes up and commences age divider from VOUT to ground programs the output normal PWM operation. The output capacitor recharges voltage via FB from 2.5V to 5V using the equation: and causes the LTC3400 to reenter sleep if the output load V remains less than the sleep threshold. The frequency of OUT = 1.23V • [1 + (R1/R2)] this intermittent PWM or burst operation is proportional to Current Sensing: A signal representing NMOS switch load current; that is, as the load current drops further current is summed with the slope compensator. The below the burst threshold, the LTC3400 turns on less summed signal is compared to the error amplifier output frequently. When the load current increases above the 3400fa 5
