LT1950
13
1950fa
Synchronizing
The SYNC pin is used to synchronize the LT1950 main
oscillator to an external clock. The SYNC pin can be driven
directly from a logic level output, requiring less
than 0.8V for a logic level low and greater than 2.2V for a
logic level high. Duty cycle must be between 10% and
90%. When synchronizing the part, slope compensation
will be reduced by approximately SYNC f/f
OSC
. If the
reduction of slope compensation affects performance,
R
SLOPE
can be reduced to increase slope compensation
and reestablish correct operation. If unused, the pin is left
open or shorted to ground. If left open, be aware that the
internal pin resistance is 20k and board layout should be
checked to avoid noise coupling to the pin.
SLOPE COMPENSATION
Programmability
The LT1950 allows its default level of slope compensation
to be easily increased by use of a single resistor connected
between the SLOPE pin and the V
REF
pin. The ability to
adjust slope compensation allows the designer to tailor his
application for a wider inductor value range as well as to
optimize the loop bandwidth. A resistor, R
SLOPE
, con-
nected between the SLOPE pin and V
REF
increases the
LT1950 slope compensation from its default level to as
high as 3X of default. The curves in Figure 7 show the
typical I
SENSE
maximum threshold vs duty cycle for vari-
ous values of R
SLOPE
. It can be seen that slope compensa-
tion subtracts from the maximum I
SENSE
threshold as duty
cycle increases from 0%. For example, with R
SLOPE
open,
I
SENSE
max threshold is 100mV at low duty cycle, but falls
to approximately 86mV at 80% duty cycle. This must be
accounted for when designing a converter to operate up to
a maximum load current and over a given duty cycle range.
The application and inductor value will define the
minimum amount of slope compensation. Refer to the
APPLICATIO S I FOR ATIO
WUU
U
Electrical Characteristics for 1X, 2X and 3X default slope
compensation vs R
SLOPE
.
Requirement in Current Mode Converters/Advantage
of Adjustability
The LT1950 uses a current mode architecture to provide
fast response to load transients and to ease frequency
compensation requirements. Current mode switching regu-
lators which operate with duty cycles above 50% and have
continuous inductor current, must add slope compensa-
tion to their current sensing loop to prevent subharmonic
oscillations. (For more information on slope compensa-
tion see Application Note 19). Typical current mode switch-
ing regulators have a fixed internal slope compensation.
This can place constraints on the value of the inductor. If
too large an inductor is used, the fixed internal slope
compensation will be greater than needed, causing opera-
tion to approach voltage mode. If too small an inductor is
used, the fixed internal slope compensation will be too
small, resulting in subharmonic oscillations. The LT1950
increases the range of usable inductor values by allowing
slope compensation to be adjusted externally.
Figure 7. I
SENSE
Maximum Threshold vs Duty Cycle
DUTY CYCLE (%)
0
I
SENSE
MAX THRESHOLD (mV)
1950 F07
20
60 80
100
90
80
70
60
50
40
30
20
40
100
R
SLOPE
= OPEN
R
SLOPE
= 8k
R
SLOPE
= 3.3k
