L6730B datasheet(25/52 Pages) STMICROELECTRONICS

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L6730 - L6730B

Device description

Doc ID 11938 Rev 3

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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 TON 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 TON, 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

VOUT 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)

MIN

VALLEY

MAX

Vout

Vin

⋅

−

(4)

VSS

VCOMP

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