LT3430 Linear Technology, LT3430 Datasheet - Page 12

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT3430
APPLICATIO S I FOR ATIO
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
the next switch turn on (see Figure 8). Buck converters will
be in discontinuous mode for output load current given by:
I
The inductor value in a buck converter is usually chosen
large enough to keep inductor ripple current (I
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 LT3430 peak switch current (I
and the maximum output load current required, given by:
Example: For V
and L = 4.7 H.
12
OUT
LP-P
Discontinuous Mode
I
Discontinuous Mode
I
Discontinuous
Mode
I
Discontinuous Mode
OUT(MAX)
OUT(MAX)
OUT(MAX)
). In this mode, inductor current falls to zero before
IN
= 15V, V
U
3
2
2 5 0 52 15 5 0 52
• (
= 1.21A
(
OUT
U
200 10
( )(
2
2
(
(
V
V
I
OUT
OUT
= 5V, V
I
P
LP-P
•
2
. )( – – . )
)
I
3
W
P
( )(
)( . •
V V
V V
F
2
2
F
F
4 7 10
= 0.52V, f = 200kHz
)(
)(
f L V
V
• •
IN
IN
IN
)( )( )
–
–
f L
IN
V
V
U
6
OUT
OUT
LP-P
)( )
15
) low;
–
–
V
V
F
F
P
)
)
)
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
mode criteria, use the I
mode; the LT3430 is designed to operate well in both
modes of operation, allowing a large range of inductor
values to be used.
Short-Circuit Considerations
The LT3430 is a current mode controller. It uses the V
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
this peak current is reached. The internal clamp on the V
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
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
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
t
(V
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
where:
f = switching frequency
t
V
V
I • R = inductor I • R voltage drop
ON(MIN)
ON
F
IN
F
f t
= diode forward voltage
+ I • R), the diode forward voltage plus inductor I • R
= switch minimum on time
= Input voltage
•
ON
. When combined with the large ratio of V
V
OUT(MAX)
F
V
IN
I R
•
no longer meets the discontinuous
C
OUT(MAX)
. However, there is finite response
equation for continuous
sn3430 3430is
C
, to its
IN
to
C
C