L6711 datasheet(36/50 Pages) STMICROELECTRONICS

36 / 50 page

Output voltage monitor and protections

L6711

36/50

14.5.2

Constant current operation

This happens when the on-time limitation is reached after the valley current in each phase

reaches IOCPx (IINFOx > IOCTH).

The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until

the current read becomes lower than IOCPx (IINFOx < IOCTH) skipping clock cycles. The high

side mosfet can be then turned ON with a TON imposed by the control loop after the LS turn-

off and the device works in the usual way until another OCP event is detected.

This means that the average current delivered can slightly increase in Quasi-Constant-

Current operation since the current ripple increases. In fact, the ON time increases due to

the OFF time rise because of the current has to reach the IOCPx bottom. The worst-case

condition is when the ON time reaches its maximum value.

When this happens, the device works in Constant Current and the output voltage decrease

as the load increase. Crossing the UVP threshold causes the device to latch driving high the

OSC pin (Figure 18 shows this working condition).

It can be observed that the peak current (Ipeak) is greater than IOCPx but it can be

determined as follow:

Where VoutMIN is the UVP threshold, (inductor saturation must be considered). When that

threshold is crossed, all mosfets are turned off, the FAULT pin is driven high and the device

stops working. Cycle the power supply or the OUTEN pin to restart operation.

The maximum average current during the Constant-Current behavior results:

In this particular situation, the switching frequency for each phase results reduced. The ON

time is the maximum allowed (TonMAX) while the OFF time depends on the application:

The transconductance resistor Rg can be designed considering that the device limits the

bottom of the inductor current ripple and also considering the additional current delivered

during the quasi-constant-current behavior as previously described in the worst case

conditions.

Moreover, when designing D-VID compatible systems, the additional current due to the

output filter charge during dynamic VID transitions must be considered.

where

PEAK

OC Px

Vout

MI N

–

-------------------------------------- TonMAX

⋅

OCPx

Vout

MIN

–

-------------------------------------- 0.40 TSW

⋅⋅

MAX,TOT

MAX

⋅

OCPx

Ipeak

OCPx

–

-------------------------------------

⎝⎠

⎛⎞

⋅

OFF

Ipeak

OCPx

–

ou t

-------------------------------------

⋅

on Ma x

OFF

---------------------------------------

OCPx max

() RSENSE max

()

⋅

OC TH min

()

---------------------------------------------------------------------

OCPx

OUT OCP

()

---------------------------

∆

------------

–

D-VID

--------------

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