Micro Current Mode Boost DC/DC Converter

    Linear Company has launched four micro switching regulator ICs-LTC3401/LTC3402/LTC1872/LT1619 (see Table 1) for boost conversion of portable and distributed power systems.

Table 1 4 micro boost converters

    *Dependent on input and output voltage,**External switching transistor

    Each switching regulator in Table 1 operates in current mode with excellent load regulation and transient response. They feature cycle-by-cycle current limiting, undervoltage lockout, adaptive compensation, and anti-shock excitation circuitry. According to the load requirements, in order to maintain high efficiency when the output is light load, each switching regulator automatically enters Burst ModeTM operation (LTC3401 and LTC3402 are user selectable).

Switching frequency 3MHz efficiency 97% input less than 1V converter

    LTC3401 and LTC3402 can provide up to 500mA and 1A regulated output respectively from a single lithium-ion or alkaline battery. Figure 1 shows conversion from two batteries to 3.3V (1A) boost converter circuit.

    The main features of the LTC3401 and LTC3402 are: up to 3MHz switching frequency, 97% efficiency, and less than 1V input. They can provide the highest power density for a single-cell synchronous boost converter, providing a regulated output of up to 5W in an area of ​​only 0.05in2. Their miniature size, high efficiency and wide output current range are ideal for pagers, cordless phones, GPS receivers and handheld instruments.

    To achieve high efficiency each switching regulator contains a 0.16Ω N-channel MOSFET switching transistor and a 0.18Ω synchronous rectifier. The LTC3402 can provide a load current of up to 1A (2A switching current), an efficiency of 95%, and a power consumption of only 38μA (working in Burst Mode). The LTC3401 is a similar device with 500mA output capability.

    A timing resistor is used to program the switching frequency from 300KHz to 3MHz, allowing the circuit to achieve optimal RFI suppression and maximum efficiency. To improve switching harmonic rejection in sensitive audio and IF bands, the LTC3401 or LTC3402 oscillators can be synchronized to an external clock. During synchronization or when the MODE/SYNC pin is low, Burst Mode operation is disabled to prevent interference from low-frequency ripples.

    The LTC3401 and LTC3402 are designed to start up at an input voltage of 0.85V (typ). Once started, the IC operates on Vout rather than Vin. At this point, the internal circuitry is independent of the input voltage, so large input capacitors are not required. The input voltage can be reduced to less than 0.5V without affecting the operation, so that the power supply can provide sufficient energy at low voltage.     The LTC1872, a current mode boost DC/DC controller with an

output 4A switching frequency of 550KHz , has an operating frequency of 550KHz, an input voltage range of 2.5V~9.8V, and a load current of up to 2A.

Figure 2 shows a 3.3V to 5V boost converter circuit. Applications include board-level boost conversion for 1- and 2-cell lithium-ion battery-powered portable devices such as PDAs, GPS systems, and network systems.

    The device's high operating frequency allows the value and size of inductors and capacitors to be reduced, allowing designs to be packaged in an area less than 110mm2. Low operating current of 270μA, shutdown current of 8μA and efficiency up to 90% help extend battery life. The LTC1872 guarantees an output voltage accuracy of ±2.5%. The output voltage is limited only by the performance of external components. To prevent the N-channel MOSFET from operating below a safe input voltage level, the device has an undervoltage lockout feature. When the input supply voltage is below 2V, the N-channel MOSFET and controller are turned off, while the undervoltage circuit is still operating (it consumes only a few microamps).

The converter LT1619, which outputs 10A and has an efficiency of 90% and an input range of 1.9~18V

    , is a fixed frequency current mode controller that can be used to implement boost, SEPIC and flyback converters with the least external components. Figure 3 shows a high-efficiency 3.3V to 5V DC/DC converter circuit.

    Maintaining high conversion efficiency under heavy load conditions relies on the 50mV current sense voltage. This low voltage reduces energy loss in the sense resistor. Burst Mode operation maintains high efficiency under light load conditions. The operating frequency is generally 300KHz, but can also be synchronized to an external clock up to 500KHz. The no-load quiescent current of the device is 140μA, and the shutdown current is 15μA. In the boost converter circuit of Figure 3, if the minimum input voltage is insufficient to fully amplify the power MOSFET, the device pin (DRV) can be connected to the converter output. The DRV pin can also be input or powered from a discrete power supply.

    The LT1619 is not limited to low output voltage sources. It is also suitable for SLIC (Subscriber Line Interface Circuit) voltage generation, such as 5V to -48V conversion in telecommunications systems and SEPIC (Single-Ended Primary Inductance Converter) conversion in automotive systems (input range 4V~28V).

The 1.2MHz boost switching regulator

    LT1930 is a switching regulator in SOT-23 package. It uses constant frequency, internal compensation, current mode PWM structure. The 1.2MHz switching frequency and integrated 1A switching transistor enable high current output and the use of miniature, low-cost ceramic capacitors and small inductors. The input voltage range of LT1930 is 2.6V~16V.

    Figure 4 is a 3.3V to 5V boost converter using LT1930. This circuit can provide an output current of 480mA. In the load current range of 60mA~450mA, its efficiency remains above 83%. The maximum output voltage ripple of this circuit is 40mVp-p.

    Figure 5 is another typical application of LT1930, a 5V to 12V boost converter. This circuit provides 300mA output current with an efficiency of up to 87%. The maximum output voltage ripple of this circuit is 60mVp-p.

    The LT1930 is the industry's highest power SOT-23 switching regulator. In addition to being used in boost converters, it can also be used in SEPIC and flyback converter designs.