l6995 STMicroelectronics, l6995 Datasheet - Page 9

• Manufacturer Part Number: l6995
• Description: Step Down Controller For High Differential Input-output Conversion
• Manufacturer: STMicroelectronics

ductor value is, the smaller the threshold is. On the other hand, the bigger the inductor value is, the slower the
transient response is. In PFM mode the frequency changes, with the output current changing, more than in
PWM mode; in fact if the output current increase, the output voltage decreases more quickly; so the successive
turn-on arrives before, increasing the switching frequency. The PFM waveforms may appear more noisy and
asynchronous than normal operation, but this is normal behaviour mainly due to the very low load. If the PFM
is not compatible with the application it can be disabled connecting to V
1.4 Softstart
If the supply voltages are already applied, the SHDN pin gives the start-up. The system starts with the high side
MOSFET off and the low side MOSFET on. After the SHDN pin is turned on the SS pin voltage begins to in-
crease and the system starts to switch. The softstart is realized by gradually increasing the current limit thresh-
old to avoid output overvoltage. The active soft start range for the V
increase linearly) starts from 0.6V to 1.5V. In this range an internal current source (5 A Typ) charges the ca-
pacitor on the SS pin; the reference current (for the current limit comparator) forced through ILIM pin is propor-
tional to SS pin voltage and it saturates at 5 A (Typ.) when SS voltage is close to 1.5V and the maximum current
limit is active. Undervoltage protection is disabled until SS pin voltage reaches 1.5V; instead the overvoltage is
always present (see figure 5).
Once the SS pin voltage reaches the 1.5V value, the voltage on SS pin doesn't impact the system operation
anymore. If the SHDN pin is turned on before the supplies, the correct start-up sequence is the following: first
turn-on the power section and after the logic section (V
Figure 5. Soft -Start Diagram
Because the system implements the soft start controlling the inductor current, the soft start capacitor selection
is function of the output capacitance, the current limit and the soft start active range ( V
In order to select the softstart capacitor it must be imposed that the output voltage reaches the final value before
the soft start voltage reaches the under voltage value (1.5V). In other words the output voltage charging time
has to be lower than the uvp time.
The UVP time is given by:
Eq 7
In order to calculate the output volatge chargin time it should be calculated, before, the output volatrge function
versus time. This function can be calculated from the inductor current function; the inductor current function can
T
uv p
C
SS
=
V
------------
Iss
u vp
Ilim current
4.1V
5 A
1.5V
0.6V
C
Vss
SS
CC
pin).
Maximum current limit
Soft-start active range
SS
CC
voltage (where the output current limit
the NOSKIP pin.
Time
Time
SS
).
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