Datasheet
Table Of Contents
- Figure 1. Application circuit
- 1 Pin settings
- 2 Maximum ratings
- 3 Electrical characteristics
- 4 Functional description
- 5 Application information
- 5.1 Input capacitor selection
- 5.2 Inductor selection
- 5.3 Output capacitor selection
- 5.4 Compensation network
- 5.5 Thermal considerations
- 5.6 Layout considerations
- 5.7 Application circuit
- Figure 18. Demonstration board application circuit
- Table 9. Component list
- Figure 19. PCB layout (component side)
- Figure 20. PCB layout (bottom side)
- Figure 21. PCB layout (front side)
- Figure 22. Junction temperature vs output current
- Figure 23. Junction temperature vs output current
- Figure 24. Junction temperature vs output current
- Figure 25. Efficiency vs output current
- Figure 26. Efficiency vs output current
- Figure 27. Efficiency vs output current
- Figure 28. Load regulation
- Figure 29. Line regulation
- Figure 30. Short circuit behavior
- Figure 31. Load transient: from 0.1 A to 0.7 A
- Figure 32. Soft-start
- 6 Application ideas
- 7 Package mechanical data
- 8 Order codes
- 9 Revision history
L5980 Functional description
Doc ID 13003 Rev 6 11/42
4.2 Soft-start
The soft-start is essential to assure correct and safe start up of the step-down converter. It
avoids inrush current surge and makes the output voltage increases monothonically.
The soft-start is performed by a staircase ramp on the non-inverting input (V
REF
) of the error
amplifier. So the output voltage slew rate is:
Equation 1
where SR
VREF
is the slew rate of the non-inverting input while R1 and R2 the resistor divider
to regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of
9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles. So the
soft-start time and then the output voltage slew rate depend on the switching frequency.
Figure 7. Soft-start scheme
Soft-start time results:
Equation 2
For example with a switching frequency of 250 kHz the SS
TIME
is 8 ms.
SR
OUT
SR
VREF
1
R1
R2
------- -+
⎝⎠
⎛⎞
⋅=
SS
TIME
32 64⋅
Fsw
-----------------=