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LT3748

3748fa

APPLICATIONS INFORMATION

selected MOSFET switch at the expected VIN and INTVCC

voltages and multiply that charge required with each

turn-on event by the maximum operating frequency. The

maximum operating frequency in a given application can

be approximated from the primary transformer inductance,

the windings ratio (NPS), the nominal output voltage and

the maximum input voltage. Unless the part is limited by

minimum on- or off-times, this maximum frequency will

occur when the part is regulating in boundary mode at the

minimum peak switch current, and can be derived from:

fSW(MAX) ≈

VIN(MAX) •VOUT + VF(DIODE)

() •NPS

LPRI •ILIM(MIN) •VOUT + VF(DIODE) •NPS + VIN(MAX)

()

With the maximum INTVCCcurrentcalculated,theexpected

dropout when VIN drops below 7V can be extracted from

the curves in the Typical Performance Characteristics sec-

tion. The LT3748 is tested as low as VIN = 5V but the hard

limit on minimum VIN operation is the INTVCC regulator

dropout and the 3.6V under voltage lockout. Figure 12

illustrates an example where operation with VIN = 5V

and IINTVCC = 20mA might be fully functional at room

temperature, but when the dropout for the same current

exceeds 1.4V and trips the UVLO at higher temperatures

the LT3748 will stop switching.

Overdriving INTVCC with a Third Winding

The LT3748 provides excellent output voltage regulation

without the need for an opto-coupler or third winding,

but for some applications with input voltages greater than

20V, an additional winding may improve overall system

efficiency. The third winding should be designed to out-

put a voltage between 7.2V and 20V. For a typical 48VIN,

10W application, overdriving the INTVCC pin may improve

efficiency by several percent at maximum load and as

much as 30% at light loads.

Loop Compensation

The LT3748 is compensated using an external resistor-

capacitor network on the VC pin. Typical values are in the

range of RC = 50k and CC = 1nF (see the numerous sche-

matics in the Typical Applications section for other possible

values). If too large of an RC value is used, the part will be

more susceptible to high frequency noise and jitter. If too

small of an RC value is used, the transient performance will

suffer. The value choice for CC is somewhat the inverse

of the RC choice: if too small a CC value is used, the loop

may be unstable and if too large a CC value is used, the

transient performance will also suffer. Transient response

plays an important role for any DC/DC converter.

LT3748

3.6V < BIAS < 20V,

VIN > BIAS

5V TO 100V

INTVCC

VIN

3748 F09

EXTERNAL SUPPLY

OR THIRD WINDING

LDO

LT3748

(VIN – DROPOUT) TO 7V

5V TO 100V

INTVCC

VIN

LDO

LT3748

5V TO 20V

INTVCC

VIN

OPTIONAL

LDO

Figure 11. INTVCC Pin Configurations

TEMPERATURE ( C)

–50

0.5

1.0

1.5

2.0

050

100

150

3748 F12

2.5

3.0

–25

125

VIN = 5V

INTVCC UVLO = 3.6V

IINTVCC = 20mA

Figure 12. INTVCC Current at Low VIN Can Cause the LT3748 to

Stop Switching Due to INTVCC Undervoltage Lockout

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