Datasheet

Functional description L7985
12/43 Doc ID 022446 Rev 3
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 for details of the compensation
network selection).
Anyway, the methodology to compensate the loop is to introduce zeroes to obtain a safe
phase margin.
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