L6701TR datasheet(29/44 Pages) STMICROELECTRONICS

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L6701

13 Output voltage Monitor and Protections

29/44

Output voltage Monitor and Protections

L6701 monitors through pin VSEN the regulated voltage in order to manage the OVP, UVP and

PGOOD (when applicable) conditions. Protections are active also during soft-start (See

Section 12 for details) while are masked during D-VID transitions with an additional 32 clock

cycle delay after the transition has finished to avoid false triggering.

In addition, preliminary over-voltage protection is also provided to protect the load from high-

side MOSFET failures before the system turn-ON.

13.1

Under Voltage

If the output voltage monitored by VSEN drops more than -400mV below the programmed

reference for more than one clock period, the device turns off all MOSFETs driving high the

FAULT pin and latches the condition: to recover it is required to cycle Vcc or the EN pin. This is

independent by the selected operative mode.

13.2

Preliminary Over Voltage

To provide a protection while VCC is below the UVLOVCC threshold is fundamental to avoid

damage to the load in case of failed HS MOSFETs. In fact, since the device is supplied from the

12V bus, it is basically “blind” for any voltage below the turn-on threshold (UVLOVCC). In order

to give full protection to the load, a preliminary-OVP protection is provided while VCC is within

UVLOVCC and UVLOOVP.

According to the DAC_SEL pin status, this protection turns-on the low side MOSFETs as long

as the FBR pin voltage is greater than 1.9V for VR10 and 2.1V for VR9 and K8 with a 300mV

hysteresis (See Table 10). When set, the protection drives the LS MOSFET with a gate-to-

source voltage depending on the voltage applied to VCC. This protection depends also on the

EN pin status as detailed in Figure 13. Preliminary OVP is always active before UVLOVCC for

all operative modes with intervention thresholds dependent on the DAC_SEL pin status.

A simple way to provide protection to the output in all conditions when the system is OFF (then

avoiding the unprotected red region in Figure 13-Left) consists in supplying the controller

through the 5VSB bus with an OR-ing diode solution as shown in Figure 13-Right: 5VSB is

always present before +12V and, in case of HS short, the LS MOSFET is driven with 5V

assuring a reliable protection of the load.

When using the OR-ing diode solution, OR-ing diodes need to be sized according to the device

current consumption: the two diodes will then results to be different since the diode connected

to the 12V bus needs to carry the current for normal operations (IRMS) and the diode connected

to the 5VSB (IRMS-PREOVP) need to carry only the current in case of Pre-OVP protection is

active. Device current consumption (IRMS) in normal operations depends on the external

MOSFET configuration as follow:

Device current consumption when Pre-OVP is active depends on the output filter configuration

since LS MOSFETs switching frequency depends on the leakage that is charging the output

filter. Test on the bench is required but, for an over-sized solution, the same diode identified for

the +12V bus can be used.

RMS

GHS

GLS

()

CCDRx

⋅

BOO Tx

⋅

()

⋅⋅

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