Quad Output, Integrated MOSFET Power Supply : MC34700
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The document provides an overview of the MC34700 quad-output integrated MOSFET power supply. It has three switching regulators and one linear regulator that can provide up to 1.5A, 1.25A, 1.25A, and 400mA respectively. It features voltage feedforward, current limiting, overvoltage/undervoltage protection and enables various operational modes like normal, standby, and fault. The document describes the internal block diagram, compensation scheme, and typical applications like set-top boxes and printers.
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Quad Output, Integrated MOSFET Power Supply : MC34700
- 13. Bode Plot of the Buck Converter Bode Plot of the Buck Converter
Editor's Notes
- Welcome to the training module on Quad Output, Integrated MOSFET Power Supply . This training module gives overview operation of the device and its application areas.
- The MC34700 is a compact, high-efficiency power supply with on-chip power MOSFETs that features three step-down switching regulators and one low dropout linear regulator. The switching regulators utilize voltage-mode control with external compensation, allowing flexibility in optimizing the performance of the MC34700 device for a given application.
- The device is ideal for space constrained applications where multiple power rails are required and simplicity of design and implementation of the power supply is necessary. The MC34700 is well suited for power supply designs in wide variety of applications, including set top boxes, cable modems, laser printers, fax machines, point-of-sale terminals, small appliances, telecom line cards and DVD players.
- When power is first applied to the MC34700, the internal regulators and bias circuits need to be up and stable before the power on reset or POR signal is released. The POR waits until the gate drive regulator’s voltage has reached about 4.0 V before it allows the rest of the internal blocks to be enabled. Each regulator has an independent enable pin. This allows the user to program the power up sequence to suit the application. As each regulator is turned on, it will execute a soft start ramp of the output voltage. This is done to prevent the output voltage from overshooting the regulation point. Without a soft start ramp, the output voltage will ramp up faster than the control loop can typically respond to, resulting in an overshoot
- This is the functional block diagram of MC34700. It consists of analog circuitry which includes a VDDI regulator, Band gap and Power on reset. Others are Logic and control blocks which enable inputs, supervisory logic, current monitor, thermal monitor and a power supply block which has a LDO and Dc to DC converter.
- There are 3 different modes of operation. Normal Mode, Stand-By Mode, Fault Mode.
- The MC34700 monitors the regulators for several fault conditions to protect both the system load and the IC from overstress An over-voltage (OV) condition occurs when the output voltage exceeds the over-voltage threshold. To avoid false trips of the OV monitor, the power good circuit has a 10 μs glitch filter. Once the output voltage falls below the OV threshold and back into regulation, the fault is cleared and the power good signal goes high.
- A current limit condition for the switching regulator occurs when the sense FET acts as a currne t detection device by sampling a fraction of the current in the power mosfet. This sampled current is compared to internal reference which determines whether the current is exceeding the limit or not. The temperature of the power MOSFETs in the switching regulators and the LDO are monitored using a thermal sensing transistor located near the power devices.
- The devices has 4 outputs: namely DC/DC1, DC/DC2, DC/DC3 and one LDO. DC/DC1 is capable of generating a 2.0 to 5.25 V output at 1.5 A. DC/DC2 is capable of generating a 0.7 to 3.6 V output at 1.25 A. LDO is capable of generating a 0.7 to 3.6 V output at 400 mA.
- The MC34700 has independent inputs for each regulator. This allows a high degree of flexibility as far as how the IC can be configured. Only Buck Converter 1 has a 9.0 to 18 V input voltage range. All the other regulators have a 1.5 to 6.0 V input voltage range. Some sample configurations are show and all the regulators require an input voltage higher than the output voltage. The can be connected in either single or dual input supply configurations.
- The compensation network consists of the error amplifier (internal to the 34700), and the external resistors and capacitors. If designed properly, the compensation network will yield a closed loop transfer function with a high cross-over (0 dB) frequency, and adequate phase margin to be stable. Choose the closest standard value for the compensation components. it is recommended to use a mathematical model to plot the closed loop response
- A more intuitive representation of the mathematical model, is an asymptotic bode plot of the buck converter’s gain versus frequency is shown The open loop error amplifier gain bounds the compensation gain
- When the 34700 is configured for cascaded operation, where the output of one regulator powers the input of another regulator, the start-up sequence also needs to be cascaded. The output voltage of the first regulator needs to be up and stable before enabling the downstream regulator, otherwise start-up overshoot can occur. If the application requires a simpler implementation of the cascaded sequence start-up, a single enable signal can be used to start the first regulator in the sequence. When sequencing the regulators on, one parameter that must be considered is the leakage specification
- Here is an typical application circuit shown
- Thank you for taking the time to view this presentation on “ MC34700” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the Freescale Semiconductor site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit Element 14 e-community to post your questions.