L6730C datasheet(22/50 Pages) STMICROELECTRONICS

22 / 50 page

Device description

L6730C - L6730D

22/50

The L6730D can always sink current and so the OVP will operate always in advance. The

device realizes the over-current-protection (OCP) sensing the current both on the high-side

MOSFET(s) and the low-side MOSFET(s) and so 2 current limit thresholds can be set (see

OCH pin and OCL pin in Table 3: Pins connection):

●

Peak Current Limit

●

Valley Current Limit

The Peak Current Protection is active when the high-side MOSFET(s) is turned on, after an

adjustable masking time (see Chapter 5.10 on page 25). The valley-current-protection is

enabled when the low-side MOSFET(s) is turned on after a fix masking time of about 400ns. If,

when the soft-start phase is completed, an over current event occurs during the on time (peak-

current-protection) or during the off time (valley-current-protection) the device enters in

HICCUP mode (L6730C): the high-side and low-side MOSFET(s) are turned off, the soft-start

capacitor is discharged with a constant current of 10µA and when the voltage at the SS pin

reaches 0.5V 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 25) 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 16. shows the behaviour of the device during an over current condition that

persists also in the soft-start phase.

Figure 15. OVP with sink disabled: delay on the OVP operation.

126%

VOUT

VFB

LGate

MIN

VALLEY

MAX

Vout

Vin

⋅

−

(4)

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