6

LTC3801/LTC3801B

sn3801 3801fs

5

µs/DIV

3801 F01a

OPERATIO

(Refer to the Functional Diagram)

Undervoltage Lockout Protection

To prevent operation of the external P-channel MOSFET

with insufficient gate drive, an undervoltage lockout cir-

cuit is incorporated into the LTC3801/LTC3801B. When

the input supply voltage drops below approximately 1.7V,

the P-channel MOSFET and all internal circuitry other than

the undervoltage block itself are turned off. Input supply

current in undervoltage is approximately 1

µA.

Short-Circuit Protection

If the output is shorted to ground, the frequency of the

oscillator is folded back from 550kHz to approximately

210kHz while maintaining the same minimum on time.

(with Burst Mode Operation)

(Burst Mode Operation Disabled)

5

µs/DIV

3801 F01b

Figure 1. Output Ripple Waveforms for the Front Page Circuit

LOAD CURRENT (mA)

0.1

10

1

70

75

80

85

90

100

1000

10000

3801 F02

65

60

55

50

95

100

Figure 2. LTC3801B Efficiency vs Load Current

This lower frequency allows the inductor current to safely

discharge, thereby preventing current runaway. After the

short is removed, the oscillator frequency will gradually

way back to 0.8V.

Overvoltage Protection

10% for any reason, such as an output short circuit to a

higher voltage, the overvoltage comparator will hold the

external P-channel MOSFET off. This comparator has a

typical hysteresis of 40mV.

Slope Compensation and Inductor’s Peak Current

The switch on duty cycle in normal operation is given by:

Duty

V

VV

D

IN

D

Cycle =

+

at the average inductor current. For duty cycles less than

40%, the inductor’s peak current is determined by:

I

VV

R

MAX

ITH RUN

SENSE

=

/

–.

07

10

However, for duty cycles greater than 40%, slope com-

pensation begins and effectively reduces the peak