Datasheet

Functional description L7985
12/44 DocID022446 Rev 8
The uncompensated error amplifier characteristics are the following:
In continuous conduction mode (CCM), the transfer function of the power section has two
poles due to the LC filter and one zero due to the ESR of the output capacitor. Different
kinds of compensation networks can be used depending on the ESR value of the output
capacitor. If the zero introduced by the output capacitor helps to compensate the double
pole of the LC filter, a type II compensation network can be used. Otherwise, a type III
compensation network must be used (see Section 6.4 on page 18 for details of the
compensation network selection).
Anyway, the methodology to compensate the loop is to introduce zeroes to obtain a safe
phase margin.
5.4 Overcurrent protection
The L7985 implements overcurrent protection by sensing current flowing through the power
MOSFET. Due to the noise created by the switching activity of the power MOSFET, the
current sensing is disabled during the initial phase of the conduction time. This avoids an
erroneous detection of a fault condition. This interval is generally known as “masking time”
or “blanking time”. The masking time is about 200 ns.
If the overcurrent limit is reached, the power MOSFET is turned off, implementing pulse-by-
pulse overcurrent protection. In the overcurrent condition, the device can skip turn-on pulses
in order to keep the output current constant and equal to the current limit. If, at the end of the
“masking time”, the current is higher than the overcurrent threshold, the power MOSFET is
turned off and one pulse is skipped. If, at the following switching on, when the “masking
time” ends, the current is still higher than the overcurrent threshold, the device skips two
pulses. This mechanism is repeated and the device can skip up to seven pulses. While, if at
the end of the “masking time”, the current is lower than the overcurrent threshold, the
number of skipped cycles is decreased by one unit (see Figure 8).
So, the overcurrent/short-circuit protection acts by switching off the power MOSFET and
reducing the switching frequency down to one eighth of the default switching frequency, in
order to keep constant the output current around the current limit.
This kind of overcurrent protection is effective if the output current is limited. To prevent the
current from diverging, the current ripple in the inductor during the on-time must not be
higher than the current ripple during the off-time. That is:
Equation 3
Table 5. Uncompensated error amplifier characteristics
Parameter Value
Low frequency gain 100 dB
GBWP 4.5 MHz
Slew rate 7 V/µs
Output voltage swing 0 to 3.3 V
Maximum source/sink current 17 mA/25 mA
V
IN
V–
OUT
R
DSON
I
OUT
 DCR I
OUT
––
LF
SW

------------------------------------------------------------------------------------------------------------ D
V
OUT
V
F
R
DSON
I
OUT
 DCR I
OUT
++ +
LF
SW

----------------------------------------------------------------------------------------------------------- 1D–=