LTC3801 Datasheet, PDF (6/12 Pages) Linear Technology – Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT

LTC3801/LTC3801B
U
OPERATIO (Refer to the Functional Diagram)
V_OUT Ripple for Front Page Circuit Using the LTC3801
(with Burst Mode Operation)
V_OUT Ripple for Front Page Circuit Using the LTC3801B
(Burst Mode Operation Disabled)
20mV_AC/DIV
20mV_AC/DIV
V_IN = 4.2V
V_OUT= 2.5V
I_OUT = 100mA
5µs/DIV
3801 F01a
V_IN = 4.2V
V_OUT= 2.5V
I_OUT = 100mA
5µs/DIV
Figure 1. Output Ripple Waveforms for the Front Page Circuit
3801 F01b
100
V_OUT = 2.5V
95
90
V_IN = 3.3V
85
80
75
V_IN = 4.2V
70
65 ^VIN ^{= 6.6V}
V_IN = 8.4V
60
V_IN = 9.8V
55
50
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
3801 F02
Figure 2. LTC3801B Efficiency vs Load Current
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 1µA.
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.
6
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_FB rises through 0.3V on its
way back to 0.8V.
Overvoltage Protection
If V_FB exceeds its regulation point of 0.8V by more than
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 Cycle = ^{VOUT + VD}
V_IN + V_D
where V_D is the forward voltage drop of the external diode
at the average inductor current. For duty cycles less than
40%, the inductor’s peak current is determined by:
I_MAX
=
V_ITH/RUN – 0.7V
10R_SENSE
However, for duty cycles greater than 40%, slope com-
pensation begins and effectively reduces the peak
3801f