LTC3801 LINER [Linear Technology], LTC3801 Datasheet - Page 6

• Manufacturer Part Number: LTC3801
• Description: Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC3801/LTC3801B
20mV
6
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 A.
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.
AC
/DIV
V
OUT
V
V
I
Figure 2. LTC3801B Efficiency vs Load Current
OUT
IN
OUT
Ripple for Front Page Circuit Using the LTC3801
= 4.2V
= 2.5V
= 100mA
100
95
90
85
80
75
70
65
60
55
50
0.1
V
OUT
(with Burst Mode Operation)
V
V
IN
U
IN
= 2.5V
= 6.6V
= 4.2V
1
(Refer to the Functional Diagram)
LOAD CURRENT (mA)
V
IN
10
5 s/DIV
= 3.3V
V
IN
= 9.8V
100
Figure 1. Output Ripple Waveforms for the Front Page Circuit
V
IN
1000
= 8.4V
3801 F02
10000
3801 F01a
This lower frequency allows the inductor current to safely
discharge, thereby preventing current runaway. After the
short is removed, the oscillator frequency will gradually
increase back to 550kHz as V
way back to 0.8V.
Overvoltage Protection
If V
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:
where V
at the average inductor current. For duty cycles less than
40%, the inductor’s peak current is determined by:
However, for duty cycles greater than 40%, slope com-
pensation begins and effectively reduces the peak
20mV
Duty
I
FB
MAX
AC
V
/DIV
OUT
exceeds its regulation point of 0.8V by more than
D
V
V
I
OUT
Cycle =
IN
OUT
Ripple for Front Page Circuit Using the LTC3801B
is the forward voltage drop of the external diode
= 4.2V
V
= 2.5V
= 100mA
ITH RUN
10
(Burst Mode Operation Disabled)
/
R
V
V
OUT
SENSE
IN
– . 0 7
V
V
D
V
D
5 s/DIV
FB
rises through 0.3V on its
3801 F01b
3801f