Datasheet

LT3957
11
3957f
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 LT3957 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 SGND,
as shown in Figure 1. 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 LT3957 can be synchronized
to an external clock source. By providing a digital clock
signal into the SYNC pin, the LT3957 will operate at the
SYNC clock frequency. The LT3957 detects the rising edge
of each 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. It is recommended that
the SYNC pin has a minimum pulse width of 200ns. Tie
the SYNC pin to SGND if this feature is not used.
Duty Cycle Consideration
Switching duty cycle is a key variable defi ning con-
verter operation. As such, its limits must be considered.
Minimum on-time is the smallest time duration that the
LT3957 is capable of turning on the power MOSFET. This
time is typically about 240ns (see Minimum On-Time in
the Electrical Characteristics table). In each switching
cycle, the LT3957 keeps the power switch off for at least
220ns (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).