Datasheet

L6730 - L6730B Device description
Doc ID 11938 Rev 3 25/52
enters in HICCUP mode (L6730): the high-side and low-side MOSFET(s) are turned off, the
soft-start capacitor is discharged with a constant current of 10 µA and when the voltage at
the SS pin reaches 0.5 V the soft-start phase restarts. During the soft-start phase the OCP
provides a constant-current-protection. If during the T
ON
the OCH comparator triggers an
over current the high-side MOSFET(s) is immediately turned-off (after the masking time and
the internal delay) and returned-on at the next pwm cycle. The limit of this protection is that
the Ton can’t be less than masking time plus propagation delay (see Chapter 5.9: Adjustable
masking time on page 27) because during the masking time the peak-current-protection is
disabled. In case of very hard short circuit, even with this short T
ON
, the current could
escalate. The valley-current-protection is very helpful in this case to limit the current. If
during the off-time the OCL comparator triggers an over current, the high-side MOSFET(s)
is not turned-on until the current is over the valley-current-limit. This implies that, if it is
necessary, some pulses of the high-side MOSFET(s) will be skipped, guaranteeing a
maximum current due to the following formula:
In constant current protection a current control loop limits the value of the error amplifier’s
output (comp), in order to avoid its saturation and thus recover faster when the output
returns in regulation. Figure 19 shows the behaviour of the device during an over current
condition that persists also in the soft-start phase.
Using the L6730B there is the possibility to set the constant-current-protection also after the
soft-start. The following figures show the behaviour of the L6730B during an overcurrent
event.
Figure 20 shows the intervention of the peak OCP: the high-side MOSFET(s) is turned-off
when the current exceeds the OCP threshold. In this way the duty-cycle is reduced, the
V
OUT
is reduced and so the maximum current can be fixed even if the output current is
escalating. Figure 21 shows the limit of this protection: the on-time can be reduced only to
the masking time and, if the output current continues to increase, the maximum current can
increase too. Notice how the Vout remains constant even if the output current increases
because the on-time cannot be reduced anymore.
Figure 19. Constant current and hiccup mode during an OCP (L6730)
MINONVALLEYMAX
T
L
VoutVin
II
,
⋅
−
+=
(4)
VSS
VCOMP
I
L