Datasheet

LT3757/LT3757A
12
3757afd
applicaTions inForMaTion
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 LT3757 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 LT3757 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 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).
In the applications where V
OUT
is pulled up by an external
positive power supply, the FBX pin is also pulled up through
the R2 and R1 network. Make sure the FBX does not exceed
its absolute maximum rating (6V). The R5, D2, and D3 in
Figure 1 provide a resistive clamp in the positive direction.
To ensure FBX is lower than 6V, choose sufficiently large
R1 and R2 to meet the following condition:
6V • 1+
R2
R1






+ 3.5V •
R2
8kΩ
> V
OUT(MAX)
where V
OUT(MAX)
is the maximum V
OUT
that is pulled up
by an external power supply.
Soft-Start
The LT3757 contains several features to limit peak switch
currents and output voltage (V
OUT
) overshoot during
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
OUT
is far from its final value,
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 LT3757 addresses this mechanism with the SS pin. As
shown in Figure 1, the SS pin reduces the power MOSFET
current by pulling down the V
C
pin through Q2. In this
way the SS allows the output capacitor to charge gradu-
ally toward its final 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
L
slewing rate is limited by the soft-start function.
Besides start-up, soft-start can also be triggered by the
following faults:
1. INTV
CC
> 17.5V
2. INTV
CC
< 2.6V
3. Thermal lockout
Any of these three faults will cause the LT3757 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
S2
starts
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:
T
SS
= C
SS
•
1.25V
10µA