Datasheet

LT3958
11
3958f
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 effi ciency
and component size. Low frequency operation improves
effi ciency 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 defi ning 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 defi ne 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 fl owing into the FBX pin
during normal operation is less than 1% (this translates
to a maximum value of R1 at about 158k).