MAX1836 Maxim, MAX1836 Datasheet - Page 8

• Manufacturer Part Number: MAX1836
• Description: 24V Internal Switch / 100% Duty Cycle / Step-Down Converters
• Manufacturer: Maxim

24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
stant-frequency pulse-width-modulation (PWM) con-
trollers that switch the MOSFET unnecessarily.
When the output voltage is too low, an error comparator
sets a flip-flop, which turns on the internal P-channel
MOSFET and begins a switching cycle (Figure 3). As
shown in Figure 4, the inductor current ramps up linear-
ly, charging the output capacitor and servicing the
load. The MOSFET turns off when the current limit is
reached, or when the maximum on-time is exceeded
while the output voltage is in regulation. Otherwise, the
MOSFET remains on, allowing a duty cycle up to 100%
to ensure the lowest possible dropout voltage. Once
the MOSFET turns off, the flip-flop resets, diode D1
turns on, and the current through the inductor ramps
back down, transferring the stored energy to the output
capacitor and load. The MOSFET remains off until the
0.5µs minimum off-time expires and the inductor cur-
rent ramps down to zero, and the output voltage drops
back below the set point.
8
Figure 3. Functional Diagram
_______________________________________________________________________________________
INPUT
4.5V OR 24V
C
IN
IN
SHDN
MAX1836
MAX1837
R
S
Q
V
Q
SENSE
MAXIMUM
ON-TIME
Q
DELAY
MAXIMUM
OFF-TIME
TRIG
DELAY
TRIG
Figure 4. Discontinuous-Conduction Operation
500mA
3.3V
V
1.25V
10V
SET
0
0
100mV
CIRCUIT OF FIGURE 2, V
A. V
B. V
C. INDUCTOR CURRENT, 500mA/div
LX
OUT
OUT
GND
, 5V/div
FB
LX
= 3.3V, 20mV/div, 200mA LOAD
D1
4µs/div
IN
L1
= 12V
3.3V OR 5V
OUTPUT
C
OUT
A
B
C