Rainbow Electronics MAX1745 User Manual
Page 8
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controller in µMAX
8
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100% Duty Cycle and Dropout
The MAX1744/MAX1745 operate with a duty cycle up to
100%. This feature extends the input voltage range by
turning the MOSFET on continuously when the supply
voltage approaches the output voltage. This services
the load when conventional switching regulators with
less than 100% duty cycle would fail. 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. Dropout depends on the
MOSFET drain-to-source on-resistance, current-sense
resistor, and inductor series resistance, and is propor-
tional to the load current:
Regulation Control Scheme
The MAX1744/MAX1755 have a unique operating
scheme that allows PWM operation at medium and high
current, with automatic switching to pulse-skipping
mode at lower currents to improve light-load efficiency.
Figure 2 shows the simplified block diagram.
Under medium- and heavy-load operation, the inductor
current is continuous and the part operates in PWM
mode. In this mode, the switching frequency is set by
either the 1µs minimum on-time or the 2µs minimum off-
time, depending on the duty cycle. The duty cycle is
approximately the output voltage divided by the input
voltage. If the duty cycle is less than 33%, the minimum
on-time controls the frequency; and the frequency is
approximately f
≈ 1MHz
✕
D, where D is the duty cycle.
If the duty cycle is greater than 33%, the off-time sets the
frequency; and the frequency is approximately f
≈ 500kHz
✕
(1 - D).
In both cases, the voltage is regulated by the error
comparator. For low duty cycles (<33%), the MOSFET
is turned on for the minimum on-time, causing fixed-on-
time operation. During the MOSFET on-time, the output
voltage rises. Once the MOSFET is turned off, the volt-
age drops to the regulation threshold (set by the inter-
nal voltage-divider for the MAX1745 and by the external
voltage-divider for the MAX1744), at which time another
cycle is initiated. For high duty cycles (>33%), the
MOSFET remains off for the minimum off-time, causing
fixed-off-time operation. In this case, the MOSFET
remains on until the output voltage rises to the regula-
tion threshold. Then the MOSFET turns off for the mini-
mum off-time, initiating another cycle.
By switching between fixed-on-time and fixed-off-time
operation, the MAX1744/MAX1745 can operate at high
input-output ratios, yet still operate up to 100% duty
cycle for low dropout. Note that when transitioning from
fixed-on-time to fixed-off-time operation, the output volt-
age drops slightly due to the output ripple voltage. In
fixed-on-time operation, the minimum output voltage is
regulated, but in fixed-off-time operation, the maximum
output voltage is regulated. Thus, as the input voltage
drops below approximately three times the output volt-
age, a decrease in line regulation can be expected.
The drop in voltage is approximately V
DROP
≈ V
RIPPLE
/ 2.
Figure 1. Typical Application Circuit
GND
REF
VL
IN
MAX1744
VH
3/5
5V
3.3V
ON
OFF
SHDN
EXT
INPUT
4.5V TO 36V
P
D1
NIHON
EC2IQ506
M1
FAIRCHILD
NDS9407
OUT
CS
OUT
3.3V OR 5V
2A
C1
220
µF
L1
22
µH
R
SENSE
40m
Ω
C3
4.7
µF
C2
4.7
µF
LOW ESR
0.47
µF
C5
4.7
µF
C4
0.1
µF
Dropout voltage=
I
x R
+ R
+ R
OUT
DS(ON)
SENSE
INDUCTOR
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