Datasheet

I
OUT(MAX)
=
V
IN
x D
2 x F x L
I
CLMIN
-
x (1 - D)
I
PEAK
= I
L
+
'I
L
2
IC Device Ratings www.ti.com
7 IC Device Ratings
The DC/DC polarity-inverting converter needs to be rated for the peak switch current, I
PEAK
, and maximum
input voltage, V
INMAX
, as described in Equation 6. Peak switch current is:
(6)
Since the ground pin, GND, of the LM22670 is connected to the output voltage, the maximum input
voltage rating has to be able to withstand the application input voltage, V
IN
, plus the absolute value of
output voltage, V
OUT
. Maximum input voltage rating of the IC is as shown in Equation 7:
V
INMAX
= V
IN
+ |V
OUT
| (7)
Maximum load current, I
OUT(MAX)
, is dependent upon the duty-cycle, D, and the inductor value, L. This is
important because the LM22670 3A step-down switching regulator cannot typically deliver a 3A load
current in a polarity-inverting topology as shown in Figure 3.
Figure 3. LM22670 Input Voltage vs Maximum Load Current (V
OUT
= -5 V, L = 10 µH)
The formula for maximum load current in a given circuit is shown in Equation 8:
(8)
where, F is the switching frequency and I
CLMIN
is the minimum current limit threshold as specified in the
Electrical Characteristics section of the LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER®, Step-Down
Voltage Regulator with Features Data Sheet (SNVS584).
8 Diode Ratings
Diode, D1, has to be able to meet the following parameters:
I
DMAX
= I
PEAK
(9)
V
DMAX
= V
IN
+ |V
OUT
|, (10)
where, I
DMAX
is the maximum current rating and V
DMAX
is the maximum voltage rating of the diode, D1.
A Shottky diode with a low forward voltage rating is recommended to achieve high converter efficiency
and low EMI.
4
AN-1888 LM22670 Evaluation Board Inverting Topology SNVA363E–September 2008–Revised April 2013
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