L6739 datasheet(21/30 Pages) STMICROELECTRONICS

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L6739

Application details

To place the poles and zeros of the compensation network, the following suggestions may

be followed:

Set the gain RF / RFB in order to obtain the desired closed loop regulator

bandwidth according to the approximated formula (suggested values for RFB are

in the range of some k

):

Equation 12

Place FZ1 below FLC (typically 0.5 * FLC):

Place FP1 at FESR:

Place FZ2 at FLC and FP2 at half of the switching frequency:

Check that compensation network gain is lower than open loop EA gain before

F0dB.

Check phase margin obtained (it should be greater than 45°) and repeat if

necessary.

9.2

Layout guidelines

The L6739 device provides control functions and high current integrated drivers to

implement high current step-down DC-DC converters. In this kind of application, a good

layout is very important.

The first priority when placing components for these applications has to be reserved to the

power section, minimizing the length of each connection and loop as much as possible. To

minimize noise and voltage spikes (EMI and losses) power connections (highlighted in

Figure 11) must be a part of a power plane and realized by wide and thick copper traces:

loop must be minimized. The critical components, i.e. the power MOSFETs, must be close

to one another. The use of a multi-layer printed circuit board is recommended.

The input capacitance (CIN), or at least a portion of the total capacitance needed, has to be

placed close to the power section in order to eliminate the stray inductance generated by

the copper traces. Low ESR and ESL capacitors are preferred, MLCCs are recommended

to be connected near the HS drain.

Use a proper number of VIAs when power traces have to move between different planes on

the PCB in order to reduce both parasitic resistance and inductance. Moreover, reproducing

RFB

----------

F0dB

FLC

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

V

OSC

VIN

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



 R

F FLC



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

 R

F CF FESR

–



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

RFB

FSW



------------------ 1

–

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

 R

S FSW



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

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