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LT3758

3758f

APPLICATIONS INFORMATION

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 RT resistor

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 deﬁning 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 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 VOUT is set by a resistor divider, as

shown in Figure 1. The positive and negative VOUT are set

by the following equations:

OUT POSITIVE

OUT NEGATIV

.•

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

–.

•

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).

Soft-Start

The LT3758 contains several features to limit peak switch

currents and output voltage (VOUT) 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 VOUT 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 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 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 IL slewing rate is limited by

the soft-start function.

Besides start-up (with SHDN/UVLO), soft-start can also

be triggered by the following faults:

1. INTVCC > 17.5V

2. INTVCC < 4.5V

3. Thermal lockout

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 IS2 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:

µA

•

125

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