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MPS - The MPQ4470 - The new 5A synchronous step-down converter

2013-05-06  Power Supply Modules   

MPS is proud to present the MPQ4470, a new monolithic 5A synchronous step-down converter for industrial and automotive applicati

The MPQ4470 offers all developers of applications with somewhat higher current requirements an ultra compact and highly efficient solution. The MPQ4470 features internal 40m? (High Side) and 20m? (Low Side) MOSFETs for an input voltage range from 4.5V to 36V (40V absolute max.) in a 3x4mm housing. With regard to circuit technology, this makes the MPQ4470 a noticeably simplified alternative to Buck-Controller modules with external MOSFETs. The MPQ4470 can be provided for a temperature range from -40°C to +125°C, and the automotive version with AECQ100-G1 is currently at the qualification stage.

In order to provide the best possible load step behaviour for demanding FPGAs or GSM modules, the MPQ4470 uses an adaptive Constant On Time (COT) control scheme.  Normally, COT controllers suffer from the disadvantage that the switching frequency depends on the input voltage and the ESR of the output capacitor, which in many cases is undesirable. In the MPQ4470, the turn-on time is changed as a function of the input voltage, with the result that an almost constant switching frequency is provided for different input voltages. In Figure 1, a ripple signal is fed into the feedback network (R1 and R2) via R4 and C4 in order to render the switching frequency independent of the ESR of the output capacitor. Thanks to these circuit details, MPS has created a COT controller with an almost constant switching frequency, which also functions in an entirely stable manner with the low-ESR ceramic output capacitors.

In order to be able to optimize the size of the external components in relation to the switching losses for the particular application, the MPQ4470 offers the user an adjustable switching frequency. In a 24V on 3.3V and 3A load application, the efficiency drops from 92.3% at 350kHz to 91.8% (500kHz) or 90.6% (700kHz) respectively – a doubling of the switching frequency results at this working point at 170mW (1.7%) higher losses. If the circuit is provided with 12V at the input, an efficiency of 94.1% at 3A with 350kHz is achieved, and 93.7% with 700kHz.  At the lower input voltage, the efficiency is only reduced by 0.4%, being 40mW. If 12V and 24V supplies at the same switching frequency are compared, the difference is 1.8% (350kHz) or 3.1% (700kHz) respectively.

The switching losses increase as a linear function of the frequency and as a quadratic function of the supply voltage

The COT process was chosen in order to provide a perceptibly better load step behavior in comparison with more traditional controllers. In the COT, a comparator reacts immediately when the output voltage falls, with a turn-on pulse. With the Buck controller, with a fixed switching frequency (f_sw), the limit frequency of the control loop in practice is located mostly in the range from one fifth to one tenth of this frequency, which means that at f_sw=500kHz the controlling time constant is 10µs to 20µs. Accordingly, for good smoothing of a load step, and depending on the design of the controller, approximately 30µs to 100µs is needed.

The COT circuit makes do with less output capacitance for the same control deviation

Figure 4 shows a substantial load step from 0.5A to 2.6A, with a rise time of 2A/µs. Although only 2x 22µF ceramic capacitors are used at the output, the output voltage only falls by 80mV. By comparison, a GSM system requires a current rise time of “only” 0.15A/µs.

Figure 5 shows a simulated GSM load step with 0.25A/µs from 0.5A to 2.5A in a 12V on 3.3V application with 2x 22µF output capacitance. Thanks to its design as a monolithic chip in a compact housing with low parasitical inductivity, the MPQ4470 provides clear EMC advantages in comparison with the controller + FET solutions. This also applies in comparison with monolithic Buck ICs from other manufacturers, which by contrast with the MPQ4470 are based on a number of individual chips, connected by bond wires. With the MPQ4470, the antenna surface of the input circuit with high di/dt, which is determinant with regard to interference, can be kept very small between input capacitor and the two FET’s (see Figure 5).

The compact monolithic solution provides EMC advantages

With regard to the PCB structure, the intention is that in the layer beneath the MPQ4470, with a small interval, there should be a ground plane. Values of 50µm are ideal, smaller than 250µm should be striven for.

For a reduction of the rise time, and to reduce overshoots at the switching node, a small resistor may be placed between the bootstrap pin (BST) and C3 from Figure 1.  This resistor reduces the charging current of the gate-source capacitance when the upper FET is switched on. The recommended standard value is 5?. This value is used with all the measurements referred to in the text. Depending on the value and application, this arrangement can reduce the noise radiation by about 3-6dB, although this does increase the switching losses. At 3.3V and 3A load at the output, and a 500kHz switching frequency, these increase at a change from 5? to 22? by 1% (100mW) for a 24V feed. At an input voltage of 12V, these losses increase by only 0.4% (40mW).

And the result:

The MPQ4470 is a monolithic synchronous step-down converter, which, thanks to its excellent efficiency and little external wiring allows for applications with high power density to be achieved. The wide input voltage range from 4.5V to 36V makes it of great interest for a large number of industrial and automotive applications. The adaptive Constant On Time architecture used for the converter also combines the advantages of the very good load step response of a traditional COT controller and the almost constant switching frequency which up to now has only been achieved by PWM controllers.

For personal consultation, please contact us!

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Figure 1: MPQ4470 circuit – 5A with 2x 22µF MLCC at the output. f_sw=500kHz
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Figure 2: MPQ4470 Evaluation Board - 94.6% efficiency in the 3x4mm housing
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Figure 3: MPQ4470 Efficiency comparison for 12V & 24V supply and f_sw=350kHz and f_sw=700kHz
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Figure 4: Load step from 0.5A to 2.6A with 2A/µs – 80mV deviation with only 2x 22µF output capacitance. f_sw=500kHz CH1 = Vout (50mV/div AC) CH3 = I_load (500mA/div)
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Figure 5: Simulated GSM load step with 0.25A/µs from 0.5A to 2.5A in a 12V on 3.3V application 2x 22µF at the output
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Figure 6: Very small input circuit of the MPQ4470 with high di/dt

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