Datasheet

LT3958
12
3958fa
For more information www.linear.com/LT3958
applicaTions inForMaTion
Operating Frequency and Synchronization
The choice of operating frequency may be determined by
on-chip power dissipation (a low switching frequency may
be required to ensure IC junction temperature does not
exceed 125°C), otherwise it is a trade-off between efficiency
and component size. Low frequency operation improves
efficiency by reducing gate drive current and MOSFET
and diode switching losses. However, lower frequency
operation requires a physically larger inductor. Switching
frequency also has implications for loop compensation.
The LT3958 uses a constant-frequency architecture that
can be programmed over a 100kHz to 1000kHz range
with a single external resistor from the RT pin to ground,
as shown in Figure 1. The RT pin must have an external
resistor to SGND for proper operation of the LT3958.
A table for selecting the value of R
T
for a given operating
frequency is shown in Table 1.
Table 1. Timing Resistor (R
T
) Value
SWITCHING FREQUENCY (kHz) R
T
(kΩ)
100 140
200 63.4
300 41.2
400 30.9
500 24.3
600 19.6
700 16.5
800 14
900 12.1
1000 10.5
The operating frequency of the LT3958 can be synchro-
nized to an external clock source. By providing a digital
clock signal into the SYNC pin, the LT3958 will operate
at the SYNC clock frequency
. The LT3958 detects the ris
-
ing edge of each Sync clock cycle. If this feature is used,
an R
T
resistor should be chosen to program a switching
frequency 20% slower than SYNC pulse frequency. Tie
the SYNC pin to SGND if this feature is not used. It is
recommended that the Sync input clock has a minimum
pulse width of 200ns.
Duty Cycle Consideration
Switching duty cycle is a key variable defining converter
operation. As such, its limits must be considered. Minimum
on-time is the smallest time duration that the LT3958 is
capable of turning on the power MOSFET. This time is
generally about 250ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LT3958 keeps the power switch off for at least
200ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time, minimum off-time and the switching
frequency define the minimum and maximum switching
duty cycles a converter is able to generate:
Minimum duty cycle = minimum on-time • frequency
Maximum duty cycle = 1 – (minimum off-time • frequency)
Programming the Output Voltage
The output voltage V
OUT
is set by a resistor divider, as
shown in Figure 1. The positive and negative V
OUT
are set
by the following equations:
V
OUT,POSITIVE
= 1.6V • 1+
R2
R1






V
OUT,NEGATIVE
= –0.8V • 1+
R2
R1






The resistors R1 and R2 are typically chosen so that
the error caused by the current flowing into the FBX pin
during normal operation is less than 1% (this translates
to a maximum value of R1 at about 158k).
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.