5.3

Error amplifier and light-load management

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 0.8 V voltage reference and 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 summarized in Table 6. Error amplifier characteristics:

Table 6. Error amplifier characteristics

Characteristics

Value

Low frequency gain (A0)

100 dB

GBWP

23 MHz

Output voltage swing

0 to 3.5 V

Source/sink current capability

3.1 mA / 5 mA

In continuous conduction working 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 Compensation network for details on the compensation network design).

In case of light load (i.e. if the output current is lower than the half of the inductor current ripple) the A7987 enters

pulse-skipping working mode. The HS MOS is kept off if the COMP level is below 200 mV (typ.); when this bottom

level is reached the integrated switch is turned on until the inductor current reaches ISKIP value. So, in

discontinuous conduction working mode (DCM), the HS MOS on-time is only related to the time necessary to

charge the inductor up to ISKIP level. Due to current sensing comparator delay, the actual inductor charge current

could be slightly impacted by VIN and inductance level.

In order to let the bootstrap capacitor recharge, in case of extremely light load, the A7987 is able to pull down the

LX net through an integrated small LS MOS. In this way the bootstrap recharge current can flow from VIN through

CBOOT, LX and the LS MOS.

This mechanism is activated if the HS MOS has been kept turned off for more than 3 ms (typ.).

5.4

Low VIN operation

In normal operation (i.e. VOUT programmed lower than input voltage), when the HS MOS is turned off, a

minimum off-time (TOFFMIN) interval is performed. In case the input voltage falls close or below the programmed

output voltage (low-dropout, LDO), the A7987 control loop is able to increase the duty cycle up to 100%.

However, in order to keep the boot capacitor properly recharged, a maximum HS MOS on-time is limited

(TONMAX). When this limit is reached the HS MOS is turned off and a pull-down resistor between LX and GND is

turned on until one of the following conditions is met:

•

A negative current limit (300 mA typ.) is reached

•

A time-out (1 µs typ.) is reached

So the A7987 is able to work in low-dropout operation and recover the programmed output voltage as soon as the

proper input voltage level is restored.

5.5

Overcurrent protection

The A7987 implements an overcurrent protection by sensing the current flowing through the power MOSFET. Due

to the noise created by the switching activity of the power MOSFET, the current sensing circuitry 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 120 ns (typ.).

A7987

Error amplifier and light-load management

DS12928 - Rev 3

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