2014-04-29  Semiconductor ICs   MPS

With the MPQ4569, MONOLITHIC POWER SYSTEMS offers a new synchronous step-down converter for input voltages from 4.5V to 75V and

The MPQ4569 combines high efficiency over a wide load range and a broad input voltage range as well. There’s an adjustable Soft Start and a Power OK signal, and users can also take advantage in their applications of the reference from 1.0V (± 1%) up to 100µA. The EN pin allows the converter to be put in circuit with a defined source voltage using a voltage divider, with the switch-on threshold having a hysteresis of 22%. The adjustable Soft Start is particularly useful in applications with a high internal resistance of the source.

Thanks to the low working quiescent current of only 20µA, this IC is ideal for applications in 24V automobile on-board networks, which still have to provide certain functions even when the ignition is switched off. The MPQ4569 is available as the AECQ100 G1 version. Another range of application for the MPQ4569 is in industrial systems with functional safety in accordance with IEC61508/SIL. The IC is also a very good solution for the supply of µC and cell-monitoring circuits in multi cell Li-XX battery systems. For these systems, the combination of high input voltage and a quiescent current in the range of a quarter of the self-discharge current of typically 18650 cells is an important factor.

The MPQ4569 can deliver up to 300mA to small non-isolated PoE systems

With an output voltage of 5V, the MPQ4569 achieves efficiency values of 84% to 90% for loads from 10mA to 300mA and input voltages from 12V to 60V. To achieve high efficiency over a broad load current and input voltage range the MPQ4569 uses a hysteresis “peak current mode” procedure. If the voltage at the Fb pin falls below the reference voltage, the upper MOSFET is switched on until the current in the switch and the coil has reached the peak value of 700mA. The upper MOFSET then switches off, and the lower MOFSET switches on until the current in the coil drops to zero. Depending on the output load the time interval of these switching cycles varies; in other words: the effective switching frequency is determined by the load current, the input/output voltage ratio, and by the value of the inductivity.

             L*Ipk                    L*Ipk
TON = –––––––– ; TOFF = –––––  with IPK - 0.7A
            Vin-Vout                  Vout

Der empfohlene minimale Wert der Induktivität beträgt:
Vin, max*0.15µs
Lmin = ––––––––––––––––0.7A

For 24V systems with a maximum input voltage of 30V the value is 6.5µH, and for the maximum permissible input voltage of 75V it is 16µH. If highest possible efficiency is needed at very low mA load current, a higher inductivity value - in the 100µH range - should be used. This circuitry arrangement for the MPQ4569 will reduce switching losses in relation to the power output. And with high input voltages the efficiency at small load currents can be improved still further by feeding the bias pin of the MPQ4569 from the output with a voltage of between 2.9V and 5.5V. With output voltages of more than 5.5V, for example, a Zener diode can be connected between the output and the bias pin. As an alternative, the voltage can also be drawn from another part of the circuit.

Efficiency data for 5V Arrangement
Efficiency data for 10V output with Zener diode at the bias pin
Circuit arrangement for the MPQ4569
Block circuit diagram of the MPQ4569
Blockschaltbild des MPQ4569
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