LTM4641
APPLICATIONS INFORMATION—EMI PERFORMANCE
OVPGM to SGND. Furthermore, the OV
trip voltage
whereV
isthemaximuminputvoltagethattheinput
PGM
INH(MAX)
can be made more accurate than its default setting by
paralleling the existing (internal) OV resistor-divider
to the power stage (V ) will see in the application, and
INH
f
is the DC/DC converter’s full load switching frequency
PGM
SW
with an external resistor divider comprised of low T.C.R.
0.1%-tolerance resistors, for example. See Appendix F
for details on how to adjust or tighten the fast OOV
comparator trip threshold.
of operation. C should be NPO, C0G or X7R-type (or
SW
better) material.
The snubber resistor (R ) value is then given by:
SW
5nH
CSW
RSW
=
The Switching Node: SW Pin
(31)
The SW pin provides access to the midpoint of the power
MOSFETs in LTM4641’s power stage.
The snubber resistor should be low ESL and capable of
withstanding the pulsed currents present in snubber cir-
cuits. A value between 0.7Ω and 4.2Ω is normal.
Connecting an optional series RC network from SW to
GND can dampen high frequency (~30MHz+) switch node
ringing caused by parasitic inductances and capacitances
in the switched-current paths. The RC network is called
a snubber circuit because it dampens (or “snubs”) the
resonance of the parasitics, at the expense of higher
power loss.
EMIperformanceofLTM4641(onDC1543)withandwith-
out a snubber is compared and contrasted in Figures 13 to
16. In the examples shown, the snubber networks reduce
EMI signal amplitude by as much as ~5dB.
Access to SW is also provided to make it possible to
deliberately induce a short circuit between the input of
To use a snubber, choose first how much power to allocate
to the task and how much PCB real estate is available to
implement the snubber. For example, if PCB space al-
lows a low inductance 1W resistor to be used—derated
LTM4641’s power stage (V ) and its switch node—to
INH
evaluate, in hardware, the performance of the LTM4641
when a high side MOSFET fault condition is simulated.
conservatively to 600mW (P
)—then the capacitor in
SNUB
the snubber network (C ) is computed by:
SW
70
60
50
PSNUB
CSW
=
(30)
2
V
•fSW
INH(MAX)
EN55022
40
CLASS B
70
LIMIT
30
60
50
40
30
20
10
0
20
10
EN55022
CLASS B
LIMIT
0
–10
30
814.8
1010
226.2
422.4
618.6
FREQUENCY (MHz)
4641 F13
Figure 14. Radiated Eꢃissions Scan of LTꢁ4641 Producing
5VOUT at 10A, froꢃ 12VIN. DC1543 Hardware with Ad Hoc
Snubber Network Installed Directly Between SW Probe Point
and GND, CSW = 10nF, RSW = 1Ω (1W-Rated). fSW = 550kHz.
CIN(BULK) = 2 × 100μF, CIN(ꢁLCC) = 4 × 10μF X7R + 2 × 4.7μF
X7R. ꢁeasured in a 10 ꢁeter Chaꢃber. Quasi-Peak Detect
ꢁethod
–10
30
618.6
FREQUENCY (MHz)
814.8
1010
226.2
422.4
4641 F13
Figure 13. Radiated Eꢃissions Scan of LTꢁ4641 Producing
5VOUT at 10A, froꢃ 12VIN. DC1543 Hardware with No Snubber
Network Installed. fSW = 550kHz. CIN(BULK) = 2 × 100μF,
CIN(ꢁLCC) = 4 × 10μF X7R + 2 × 4.7μF X7R. ꢁeasured in a
10 ꢁeter Chaꢃber. Quasi-Peak Detect ꢁethod
4641f
35
Linear [ Linear ]
