L7986 datasheet(12/43 Pages) STMICROELECTRONICS

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Functional description

L7986

12/43

DocID022450 Rev 6

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.

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 L7986 device 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.

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

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