L6732TR datasheet(17/36 Pages) STMICROELECTRONICS

17 / 36 page

L6732

Device description

17/36

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 function):

●

Peak Current Limit

●

Valley Current Limit

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

masking time of about 100ns. The valley-current-protection is enabled when the low-side

MOSFET(s) is turned on after a 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: 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

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 output

(comp), in order to avoid its saturation and thus recover faster when the output returns in

regulation. Figure 12. shows the behaviour of the device during an over current condition that

persists also in the soft-start phase.

Figure 11. OVP: the low-side MOSFET is turned-on in advance.

VOUT

VFB

LGate

109%

MIN

VALLEY

MAX

Vout

Vin

⋅

−

(4)

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