L7985 datasheet(12/42 Pages) STMICROELECTRONICS

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Functional description

L7985

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DocID022446 Rev 7

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/

s

Output voltage swing

0 to 3.3 V

Maximum source/sink current

17 mA/25 mA

VIN V

–

OUT

RDSON IOUT



DCR IOUT



–

LFSW



------------------------------------------------------------------------------------------------------------ D



VOUT VF RDSON IOUT



DCR IOUT



LFSW



-----------------------------------------------------------------------------------------------------------

–





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