Datasheet

Functional description A7986A
12/42 DocID022801 Rev 5
5.3 Error amplifier and compensation
The error amplifier (EA) 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. 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 zeros to obtain a safe
phase margin.
5.4 Overcurrent protection
The A7986A implements the overcurrent protection 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 the pulse-
by-pulse overcurrent protection. Under 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/μs
Output voltage swing 0 to 3.3 V
Maximum source/sink current 17 mA/25 mA