12
LT1956/LT1956-5
1956f
APPLICATIO S I FOR ATIO
WUUU
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 LT1956 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 LT1956 is typically
limited to 25V. 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 1.5A LT1766 can be used since
it is identical to the LT1956 but runs at a lower frequency
of 200kHz, allowing higher sustained input voltage capa-
bility during output short circuit.
The LT1956 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 switch 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
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
t
ON(MIN)
. When combined with the large ratio of V
IN
to
(V
F
+ I • R), the diode forward voltage plus inductor I • R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
ft
VIR
V
ON
F
IN
•
•
≤
+
solu
tion. The maximum output load current in discontinu-
ous 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 Discontinous Mode
VVVVV
VfL
OUT
OUT F IN OUT F
IN
<
+()(––)
()( )()()2
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 LT1956 peak switch current (I
P
)
and the maximum output load current required given by:
I
OUT(MAX)
LP-P
DiscontinuousMode
I
I
IfLV
VVVVV
PPIN
OUT F IN OUT F
==
+−
22
22()
()()( )
()(– )
Example: For V
IN
= 15V, V
OUT
= 5V, V
F
= 0.63V, f = 500kHz
and L = 4µH
I Discontinuous Mode
OUT MAX()
. ( • )( • )( )
(.)(––.)
=
+
−
1 5 500 10 4 10 15
25063155063
236
I
OUT(MAX)
Discontinuous Mode = 0.639A
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
