Datasheet

9
LT1676
which tantalum capacitors are generally unavailable. Rela-
tively bulky “high frequency” aluminum electrolytic types,
specifically constructed and rated for switching supply
applications, may be the only choice.
Minimum Load Considerations
As discussed previously, a lightly loaded LT1676 with V
C
pin control voltage below the boost threshold will operate
in low dV/dt mode. This affords greater controllability at
light loads, as minimum t
ON
requirements are relaxed. In
many applications, it is possible to operate the LT1676
down to zero external load without “pulse skipping”!
In these cases, the LT1676’s modest V
CC
current
requirement of several milliamperes provides enough of a
load to avoid pulse skipping.
However, some users may be indifferent to pulse skipping
behavior, but instead may be concerned with maintaining
maximum possible efficiency at light loads. This require-
ment can be satisfied by forcing the part into Burst Mode
TM
operation. The use of an external comparator whose
output controls the shutdown pin allows high efficiency at
light loads through Burst Mode operation behavior (see
Typical Applications and Figure 8).
Maximum Load/Short-Circuit Considerations
The LT1676 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
this 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
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
APPLICATIONS INFORMATION
WUU
U
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
•
•
≤
+
where:
f = switching frequency
t
ON
= switch ON time
V
F
= diode forward voltage
V
IN
= Input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at I
PK
, but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1676 clock frequency
of 100KHz, a V
IN
of 48V and a (V
F
+ I • R) of say 0.7V, the
maximum t
ON
to maintain control would be approximately
140ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
when the FB pin voltage is abnormally low thereby indicat-
ing some sort of short-circuit condition. Figure 2 shows
the typical response of Oscillator Frequency vs FB divider
Thevenin voltage and impedance. Oscillator frequency is
unaffected until FB voltage drops to about 2/3 of its normal
value. Below this point the oscillator frequency decreases
roughly linearly down to a limit of about 25kHz. This lower
Burst Mode is a trademark of Linear Technology Corporation.
FB DIVIDER THEVENIN VOLTAGE (V)
0
0
f
OSC
(kHz)
20
40
60
80
100
120
0.25 0.50 0.75 1.00
1676 F02
1.25
R
TH
LT1676
FB
R
TH
= 22k
R
TH
= 4.7kR
TH
= 10k
Figure 2. Oscillator Frequency vs FB Divider
Thevenin Voltage and Impedance