L5989D Datasheet, PDF (17/51 Pages) STMicroelectronics – 4 A continuous (more than 5 A pulsed) step-down switching regulator with synchronous rectification

L5989D
Functional description
5.3
External voltage reference
An external 1.8 V regulated voltage is provided. This reference is useful to set the voltage at
the multifunction pin (see Chapter 5.1) or to source current to ILIM-ADJ and FSW pins (see
and Chapter 5.5.2). The typical current capability is 4 mA.
5.4
Soft-start
When VCC is above the selected UVLO threshold the start-up phase takes place. At start-
up, a voltage ramp is generated charging the external capacitor C_SS with an internal current
generator. The device is in inhibit mode as long as SS/INH pin is below the INH threshold.
The L5989D implements the soft-start phase by clamping the output of the error amplifier
and, being based on a voltage mode control, the duty cycle. In fact the comparison between
the output of the error amplifier and the internal saw tooth waveform generates the duty
cycle needed to keep the output voltage in regulation.
Two different current sources charge the external capacitor depending on the pin voltage in
order to reduce the power consumption in INH mode.
I_SS
=
⎧^ISS1 ^{= 5μA}
⎩⎨^ISS2 ^{= 22μA}
0 < V_SS/INH < 1
1 < V_SS/INH < 2.9
The equation for the soft-start time is:
T_SS
=
ΔT₁ + ΔT₂
=
-^C----^S----^S-
I_SS1
×
(1
–
0)
+
-^C----^S----^S-
I_SS2
×
( 2.9
–
1)
Considering I_SS2/I_SS1 = 22/5 = 4.4, the proper soft-start capacitor is simply calculated as
follows:
C(nF) = Tss(mS) × 3.5
During the soft-start phase (V_SS < 2.9 V):
● the sink capability is always disabled (independently from the multifunction pin settings)
to be compatible with pre-biased output voltage
● in case the overcurrent limit is detected, a constant current protection is provided in
order to deliver extra current for charging the output capacitor (see Chapter 5.5.2 for
description of current protection management).
Doc ID 15778 Rev 3
17/51