The LTC1628 is a third generation switching regulator controller from Linear Technology for CPU and system power. It is a monolithic current mode controller that provides dual outputs that operate 180° out of phase. This article briefly decribes its use in a power supply application that is highly efficient.
The schematic of the step down converter is shown in Figure 1. The current pulses coming from the switches are interleaved, eliminating overlap. The result is a large reduction in total RMS input current (Figure 2) which allows significant reductions in the input capacitance required and in conducted and radiated electromagnetic interference (EMI). The lower RMS current pulses have the additional benefit of allowing longer battery life.
The LTC1628 drives all N-channel synchronous power MOSFET stages. The gate drivers are three to four times the strength of those of previous controllers for faster transitions and less loss in the MOSFETs. The LTC1628's constant frequency current mode architecture allows up to 300 kHz operation. It delivers high current with output voltages as low as 0,8 V and works efficiently with inputs from 3,5 to 36 V. Its 0,2% load and line regulation provide tight control of critical voltages. The IC includes 3,3 and 5 V standby regulators.
The unique OPTI-LOOP compensation scheme used in the LTC1628 reduces the output capacitance requirement while optimising transient response and minimising output voltage ripple. OPTI-LOOP gives the user more freedom of capacitor choice and generally allows fewer output capacitors (Table 1).
Table 1: LTC1628 advantages
* Reduced Cin and Cout
* Reduced EMI
* Improved transient response
* Reduced output voltage ripple
* Ultrahigh efficiency
* Extended battery life
* Drives larger MOSFETs with less loss
* Eliminates diodes on the output
* Will not latch off except for true faults
* New burst disable mode
* Lower output voltage operation
* Smaller system size
The LTC1628 can operate in three modes. Forced PWM mode is low noise but can be less efficient in low load current conditions.
Burst Mode is very efficient especially at low output current levels but can generate some noise. A new 'burst disable' mode is both efficient and low noise due to constant frequency operation down to 1% of the maximum load current.
Soft-latch overvoltage protection
Competing controllers use 'hard-latch' crowbar schemes for overvoltage protection that result in nuisance trips caused by noise or transients. Additionally, large Schottky diodes must be placed across the outputs to prevent excessive negative voltages when the crowbar is enabled. Latching OVP circuits are also incompatible with on-the-fly CPU core voltage changes.
The LTC1628 uses a new 'soft-latch' OVP circuit to avoid these constraints. Only if a true fault exists, such as a shorted top MOSFET, will the synchronous MOSFET remain on. When the overvoltage condition is removed, the LTC1628 automatically resumes normal operation. Soft-latch also provides easy fault diagnosis and protection from other overvoltage faults, such as a lower supply rail shorted to a higher voltage.
Applications such as notebook computers, PDAs, handheld terminals and automotive electronics will benefit from the lower input filtering requirements, reduced EMI and increased efficiency associated with 2-phase operation. A single controller version of the LTC1628, the LTC1735, is also available.
For further information contact Linear Technology distributor in South Africa, Insight-Memec, (021) 674 4124.
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