11
LT1680
clamped to 2.5V through a 20k series input resistance and
will therefore draw 0.5mA when tied directly to 12V. This
additional current can be minimized by making the con-
nection through an external resistor (100k is typically used).
Oscillator Synchronization
The LT1680 oscillator generates a modified sawtooth
waveform at the C
T
pin between low and high thresholds
of 0.8V (vl) and 2.5V (vh) respectively. The oscillator can
be synchronized by driving a TTL level pulse into the SYNC
pin. This pin connects to a one shot circuit that reduces the
oscillator high threshold to 2V for about 200ns. The SYNC
input signal should have minimum on/off times of ≥1µs.
The inductor core type is determined by peak current and
efficiency requirements. The inductor core must with-
stand this peak current without saturating, and the series
winding resistance and core losses should be kept as
small as is practical to maximize conversion efficiency.
The LT1680 peak current threshold is 40% greater than
the average limit threshold. Slope compensation effects
reduce this margin as duty cycle increases. This margin
must be maintained to prevent peak current limit from
corrupting the programmed value for average current
limit. Programming the peak ripple current to less than
15% of the desired average current limit value will assure
proper operation of the average current limit feature
through 90% duty cycle (see Slope Compensation).
Slope Compensation
Current mode switching regulators that operate with a
duty cycle greater than 50% and have continuous inductor
current can exhibit duty cycle instability. While a regulator
will not be damaged and may even continue to function
acceptably during this type of subharmonic oscillation, an
irritating high-pitched squeal is usually produced.
The criterion for current mode duty cycle instability is
met when the increasing slope of the inductor ripple
current is less than the decreasing slope, which is the
case at duty cycles greater than 50%. This condition is
illustrated in Figure 5a. The inductor ripple current starts
at I
1
, the beginning of each oscillator switch cycle.
Current increases at a rate S1 until the current reaches
the control trip level I
2
. The controller servo loop then
disables the switch and inductor current begins to de-
crease at a rate S2. If the current switch point (I
2
) is
perturbed slightly and increased by ∆I, the cycle time
ends such that the minimum current point is increased by
a factor of 1 + (S2/S1) to start the next cycle. On each
successive cycle, this error is multiplied by a factor of S2/
S1. Therefore, if S2/S1 is ≥1, the system is unstable.
Subharmonic oscillations can be eliminated by augment-
ing the increasing ripple current slope (S1) in the control
loop. This is accomplished by adding an artificial ramp on
the inductor current waveform internal to the IC (with a
slope S
X
) as shown in Figure 5b. If the sum of the slopes
0.8V
1680 F04
2V
2.5V
(vl)
SYNC
V
CT
(vh)
FREE RUN SYNCHRONIZED
Figure 4. Free Run and Synchronized Oscillator
Waveforms (at C
T
Pin)
Inductor Selection
The inductor for an LT1680 converter is selected based on
output power, operating frequency and efficiency require-
ments. Generally, the selection of inductor value can be
reduced to desired maximum ripple current in the inductor
(∆I). For a boost converter, the minimum inductor value
for a given operating ripple current can be determined
using the following relation:
L
VV V
If V
MIN
IN OUT IN
O OUT
=
()
()()( )
–
∆
Given an inductor value (L), the peak inductor current is
the sum of the average inductor current (I
AVG
) and half the
inductor ripple current (∆I), or:
II
VV V
Lf V
PK AVG
IN OUT IN
O OUT
=+
()
()()( )( )
–
2
APPLICATIO S I FOR ATIO
WUUU
