LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 14

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3758
The operating frequency of the LT3758 can be synchronized
to an external clock source. By providing a digital clock
signal into the SYNC pin, the LT3758 will operate at the
SYNC clock frequency. If this feature is used, an R
should be chosen to program a switching frequency 20%
slower than SYNC pulse frequency. The SYNC pulse should
have a minimum pulse width of 200ns. Tie the SYNC pin
to GND if this feature is not used.
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 LT3758 is
capable of turning on the power MOSFET. This time is
generally about 220ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LT3758 keeps the power switch off for at least
220ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time and 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
shown in Figure 1. The positive and negative V
by the following equations:
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).
14
V
V
OUT POSITIVE
OUT NEGATIV
,
,
E E
=
=
1 6
– .
.
0 8
OUT
V
•
V
⎛
⎝ ⎜
is set by a resistor divider, as
•
1
⎛
⎝ ⎜
+
1
R
R
+
2
1
R
R
⎞
⎠ ⎟
2
1
⎞
⎠ ⎟
OUT
T
resistor
are set
Soft-Start
The LT3758 contains several features to limit peak switch
currents and output voltage (V
start-up or recovery from a fault condition. The primary
purpose of these features is to prevent damage to external
components or the load.
High peak switch currents during start-up may occur in
switching regulators. Since V
the feedback loop is saturated and the regulator tries to
charge the output capacitor as quickly as possible, resulting
in large peak currents. A large surge current may cause
inductor saturation or power switch failure.
The LT3758 addresses this mechanism with the SS pin.
As shown in Figure 1, the SS pin reduces the power
MOSFET current by pulling down the VC pin through
Q2. In this way the SS allows the output capacitor to
charge gradually toward its fi nal value while limiting the
start-up peak currents. The typical start-up waveforms
are shown in the Typical Performance Characteristics
section. The inductor current I
the soft-start function.
Besides start-up (with SHDN/UVLO), soft-start can also
be triggered by the following faults:
Any of these three faults will cause the LT3758 to stop
switching immediately. The SS pin will be discharged by
Q3. When all faults are cleared and the SS pin has been
discharged below 0.2V, a 10μA current source I
charging the SS pin, initiating a soft-start operation.
The soft-start interval is set by the soft-start capacitor
selection according to the equation:
1. INTV
2. INTV
3. Thermal lockout
T
SS
=
C
CC
CC
SS
> 17.5V
< 4.5V
•
. 1 25
10
µA
V
OUT
L
slewing rate is limited by
is far from its fi nal value,
OUT
) overshoot during
S2
starts
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