L7987 datasheet(16/37 Pages) STMICROELECTRONICS

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Functional description

L7987

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DocID025589 Rev 3

(typ.). With the EN pin higher than 1.16 V, the device is enabled. If the EN pin is left floating,

an internal pull-down current ensures that the voltage at the pin reaches the inhibit threshold

and the device is disabled. The pin is also VCC compatible.

4.3

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 0.8V

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.

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 5.4 on page 23 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 L7987 device 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 is slightly impacted by VIN and inductance level.

In order to let the bootstrap capacitor recharge, in case of extremely light load the L7987 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.).

Table 6. Error amplifier characteristics

Parameters

Value

Low frequency gain (A0)

100 dB

GBWP

23 MHz

Output voltage swing

0 to 3.5 V

Source/sink current capability

2 mA / 5 mA

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