SLUU130A – September 2002 – Revised February 2003
4 Design Procedure
4.1TPS4000X Family Device Selection
The TPS4000X family of devices offers four selections to encompass the frequency and output current mode choices. The TPS40003 is selected for the following reasons. First, the internal oscillator components set a fixed switching frequency of 600 kHz. This allows minimally sized filter components in this compact design. The other choice related to the TPS4000X family involves the selection of Discontinuous Current Mode (DCM) operation or Continuous Current Mode (CCM) operation at lighter loads. In this design the TPS40003 is selected to keep the current continuous all the way to zero load, to provide robust control characteristics.
4.2Inductance Value
The output inductor value is selected to set the ripple current to a value most suited to overall circuit functionality. The inductor value is calculated in equation (1).
| VOUT |
|
| VOUT | 1.2 V | 1 * | 1.2 V | + 1.07 mH | (1) | |||||
L + |
|
|
| 1 | * |
|
| + |
|
|
|
| ||
f I |
|
| V |
| 600 kHz 1.25 A | 3.6 V |
| |||||||
|
|
|
|
|
| |||||||||
|
| RIPPLE |
|
| IN(max) |
|
|
|
|
|
| |||
where IRIPPLE is chosen to be 25% of IOUT, or 1.25 A. A common value of 1 ∝ H is selected.
4.3Input Capacitor Selection
Bulk input capacitor selection is based on allowable input voltage ripple and required RMS current carrying capability. In typical buck converter applications, the converter is fed from an upstream power converter with its own output capacitance. In this converter, onboard capacitance is provided to supply the current required during the top MOSFET
C + | I | Dt | + | 5 A 606 ns | + 20 mF | (2) |
| DV | 0.15 V |
|
To meet this requirement with the lowest size and cost, a single 22 ∝ F, X5R ceramic capacitor might be considered. Although these capacitors have an extremely small resistance a typical datasheet indicates that the part undergoes a 30° C temperature rise with 2 ARMS current at 500 kHz. With VIN = 3.0 V our circuit requires nearly 2 ARMS of current, so for a conservative design two capacitors are selected to allow for conservative current derating. These capacitors function as power bypass components and should be located near the MOSFET package, to keep the high frequency current flow in a tight loop. The low impedance characteristics of the dual ceramic capacitors help to reduce noise on the VDD supply of the device. Specifically, the high side MOSFET current sense is referenced to this point, so noise at the device must be kept to a low level.
6
