Datasheet

MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
_______________________________________________________________________________________ 9
Input-Output (Dropout) Voltage
A step-down converter’s minimum input-to-output volt-
age differential (dropout voltage) determines the lowest
usable supply voltage. In battery-powered systems,
this limits the useful end-of-life battery voltage. To maxi-
mize battery life, the MAX1776 operates with duty
cycles up to 100%, which minimizes the dropout volt-
age and eliminates switching losses while in dropout.
When the supply voltage approaches the output volt-
age, the P-channel MOSFET remains on continuously to
supply the load.
Dropout voltage is defined as the difference between
the input and output voltages when the input is low
enough for the output to drop out of regulation. For a
step-down converter with 100% duty cycle, dropout
depends on the MOSFET drain-to-source on-resistance
and inductor series resistance; therefore, it is propor-
tional to the load current:
V
DROPOUT
= I
OUT
✕
(R
DS(ON) +
R
INDUCTOR
)
Figure 2. Simplified Functional Diagram
OUTPUT
D1
L1
C
OUT
MAX1776
C
IN
GND
LX
FB
OUT
MINIIMUM
OFF-TIME
DELAY
MAXIMUM
ON-TIME
DELAY
V
SET
1.25V
100mV
RQ
S
ILIM
SET
ILIM2
ILIM
SHDN
D
Figure 3. Discontinuous-Conduction Operation
LX WAVEFORM, CIRCUIT 1
V
IN
= 15V, I
LOAD
= 500mA
V
LX
I
L
V
OUT
1A
10V
0
2µs/div
50mV/div
0