L5988D datasheet(24/52 Pages) STMICROELECTRONICS

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Functional description

L5988D

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DocID15324 Rev 4

Figure 12. Minimum TON

5.7

Error amplifier

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 so 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. In case 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 has to be used (see Section 6.4 on page 28 for details about the

compensation network selection).

Anyway the methodology to compensate the loop is to introduce zeros to obtain a safe

phase margin.

Table 10. Uncompensated error amplifier

Parameter description

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

25 mA / 40 mA

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