MAX1745EUB+

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MAX1745EUB+ Application Field and Working Principle

MAX1745EUB+ is an advanced low-power, low-noise DC-DC step-down switching controller designed to work in power management applications with the need for high efficiency. This device belongs to PMIC - Voltage Regulators - DC DC Switching Controllers product family.

MAX1745EUB+ is capable of constant frequency current mode control for power supply sequencing and power path control, and also features digital soft-start and line compensation for voltage ripple reduction and fast transient response.

Application Field

The MAX1745EUB+ device is suitable for use in a variety of systems such as notebook PCs, servers, digital cameras and other devices operating at 3.3V, 5V and other higher voltages . With the use of external MOSFETs, it is possible to convert a 24V input to 15V outputs. The external P-channel MOSFETs can be left in Diode mode and the chip controls the N-channel MOSFETs to make a synchronous rectification.

Features

• Output voltage up to 10V with a 3.0V~24V input
• Integrated low-side current sensing
• High-side current sensing
• Soft-start, controlled by an internal 1ms timer
• Max average current-mode control with adjustable switching frequency from 1kHz to 2MHz
• Open loop compensation
• External sync to leading-edge or trailing-edge clock
• Latched short-circuit protection, adjustable current limit and thermal limit
• Fault Flag output (FF) for over-current, short-circuit and thermal protection events
• No power-on reset circuit needed
• vRT voltage input to control the output voltage

Working Principle

The Maxim1745EUB+ device is a difference amplifier based on a constant current mode implementation with synchronous rectification. It uses two identical MOSFETs, one of which connects to the output voltage via an inductor and the other connected to the ground. It also uses an external MOSFET, such as a p-channel MOSFET, to control the current flow direction into the inductor. This dc-dc conversion technique is known as synchronous rectification.

The MAX1745EUB+ converts the input voltage (Vin) to an output voltage (Vout) by controlling the switching frequency of the external MOSFETs. The switching frequency is controlled by a constant frequency current mode PWM controller. The comparison reference voltage of the PWM controller is obtained from a voltage divider connected to the output voltage. When the PWM’s reference voltage (Vref) is lower than the output voltage, the PWM controller switches ON the external MOSFETs (Q1 and Q2). This causes the current in the inductor to increase, which increases the output voltage.

The output voltage clamp is another feature of the MAX1745EUB+. The output voltage clamp limits the output voltage to a certain value, depending on the voltage applied on the vRT pin. The vRT voltage is compared to the PWM’s reference voltage (Vref). If the vRT voltage is higher than the PWM’s reference voltage, the PWM controller switches off the external MOSFETs, thus limiting the output voltage to the vRT voltage.

In addition, the MAX1745EUB+ also provides a fast over-current detection and protection. When an over-current condition is detected, the PWM controller will shuts off the external MOSFETs and sets the FF output to a high voltage state.

Conclusion

The MAX1745EUB+ is a low-power, low-noise step-down switching controller for power management applications, capable of constant frequency current mode control for power supply sequencing and power path control. The device can be used to convert 3.3V, 5V or higher input voltages to the output voltages with the help of external MOSFETs and can also provide a robust current sensing design, soft-start and line compensation for fast transient response. In addition, along with its fast over-current detection and protection mechanism, it is an ideal choice for power management applications.

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