Datasheet

DocID022446 Rev 8 11/44
L7985 Functional description
44
5.2 Soft-start
The soft-start is essential to assure correct and safe startup of the step-down converter. It
avoids inrush current surge and makes the output voltage increase 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 R1and R2 is the resistor
divider to regulate the output voltage (see Figure 7). The soft-start staircase consists of 64
steps of 9.5 mV each, 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.
5.3 Error amplifier and compensation
The error amplifier (E/A) provides the error signal to be compared with the sawtooth to
perform the pulse width modulation. Its non-inverting input is internally connected to a 0.6 V
voltage reference, while its inverting input (FB) and output (COMP) are externally available
for feedback and frequency compensation. In this device the error amplifier is a voltage
mode operational amplifier, therefore, with high DC gain and low output impedance.
SR
OUT
SR
VREF
1
R1
R2
------- -+


=
SS
TIME
32 64
Fsw
-----------------=