12
LT3431
sn3431 3431fs
APPLICATIO S I FOR ATIO
WUUU
To calculate actual peak switch current with a given set of
conditions, use:
II
I
VVVVV
LfV
SW PEAK
OUT
P
OUT
OUT F IN OUT F
IN
()
=+
=+
+−
()
()()( )
I
2
L-P
() –
2
Reduced Inductor Value and Discontinuous Mode
If the smallest inductor value is of most importance to a
converter design, in order to reduce inductor size/cost,
discontinuous mode may yield the smallest inductor solu-
tion. The maximum output load current in discontinuous
mode, however, must be calculated and is defined later in
this section.
Discontinuous mode is entered when the output load
current is less than one-half of the inductor ripple
current (I
LP-P
). In this mode, inductor current falls to
zero before the next switch turn on (see Figure 8). Buck
converters will be in discontinuous mode for output
load current given by:
I
OUT
Discontinuous Mode
The inductor value in a buck converter is usually chosen
large enough to keep inductor ripple current (I
LP-P
) low;
this is done to minimize output ripple voltage and maxi-
mize output load current. In the case of large inductor
values, as seen in the equation above, discontinuous
mode will be associated with “light loads.”
When choosing small inductor values, however, discon-
tinuous mode will occur at much higher output load
currents. The limit to the smallest inductor value that can
be chosen is set by the LT3431 peak switch current (I
P
)
and the maximum output load current required, given by:
Example: For V
IN
= 12V, V
OUT
= 5V, V
F
= 0.52V, f = 500kHz
and L = 2.2µH.
I
OUT(MAX)
Discontinuous
Mode
I
OUT(MAX)
= 1.66A
Discontinuous Mode
What has been shown here is that if high inductor ripple
current and discontinuous mode operation can be toler-
ated, small inductor values can be used. If a higher output
load current is required, the inductor value must be
increased. If I
OUT(MAX)
no longer meets the discontinuous
mode criteria, use the I
OUT(MAX)
equation for continuous
mode; the LT3431 is designed to operate well in both
modes of operation, allowing a large range of inductor
values to be used.
Short-Circuit Considerations
For a ground short-circuit fault on the regulated output,
the maximum input voltage for the LT3431 is typically
limited to 21V. If a greater input voltage is required,
increasing the resistance in series with the inductor may
suffice (see short-circuit calculations at the end of this
section). Alternatively, the LT3430 can be used since it is
identical to the LT3431 but runs at a lower frequency of
200kHz, allowing higher sustained input voltage capability
during output short-circuit.
The LT3431 is a current mode controller. It uses the V
C
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
peak current is reached. The internal clamp on the V
C
node, nominally 2V, then acts as an output switch peak
current limit. This action becomes the switch current limit
specification. The maximum available output power is
then determined by the switch current limit.
A potential controllability problem could occur under
short-circuit conditions. If the power supply output is
short circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, V
C
, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V
C
. However, there is finite response
<
+()(––)
()( )()()
VVVVV
VfL
OUT F IN OUT F
IN
2
=
=
()
+
I
IfLV
VVVVV
P
PIN
OUT F IN OUT F
2
2
2
2
()( )
••
()(––)
I
LP-P
=
+
3 500 10 4 7 10 12
25052125052
236
•(•)(.• )()
(.)(––.)
–
I
OUT(MAX)
Discontinuous Mode
