MAX1771 Maxim, MAX1771 Datasheet - Page 8

• Manufacturer Part Number: MAX1771
• Description: 12V or Adjustable / High-Efficiency / Low IQ / Step-Up DC-DC Controller
• Manufacturer: Maxim

DataSheet4U.com
www.DataSheet4U.com
DataSheet
4
U
.com
12V or Adjustable, High-Efficiency,
Low I
increases at low input voltages. However, the supply
current is also reduced because V+ is at a lower volt-
age, and because less energy is consumed while
charging and discharging the external MOSFET’s gate
capacitance. The minimum input voltage is 3V when
using external feedback resistors. With supply voltages
below 5V, bootstrapped mode is recommended.
Note: When using the MAX1771 in non-boot-
strapped mode, there is no preset output operation
because V+ is also the output voltage sense point
Figure 2a. 12V Preset Output, Bootstrapped
Figure 2c. 9V Output, Bootstrapped
8
0.1 F
0.1 F
R2 = (R1)
V
C3
REF
C3
_______________________________________________________________________________________
= 1.5V
5
4
3
6
(
5
4
6
C2
0.1 F
C2
0.1 F
V
V
REF
SHDN
FB
AGND
OUT
REF
REF
SHDN
AGND
-1
Q
MAX1771
)
MAX1771
GND
V+
GND
, Step-Up DC-DC Controller
V+
2
7
2
7
EXT
CS
EXT
CS
FB
1
8
1
8
3
22 H
22 H
V
V
L1
IN
L1
IN
= 5V
= 4V
Si9410DY/
MTD20N03HDL
N
1N5817-22
28k
R
40m
1N5817-22
N
Si9410DY/
MTD20N03HDL
R
40m
R1
SENSE
SENSE
D1
D1
47 F
68 F
C1
C1
100pF
140k
C5
R2
C4
200 F
C4
300 F
V
OUT
DataSheet4U.com
@ 0.5A
V
OUT
= 12V
= 9V
for fixed-output operation. External resistors must
be used to set the output voltage. Use 1% external
feedback resistors when operating in adjustable-output
mode (Figures 2b, 2c) to achieve an overall output volt-
age accuracy of ±5%. To achieve highest efficiency,
operate in bootstrapped mode whenever possible.
The MAX1771 uses a proprietary current-limited PFM
control scheme to provide high efficiency over a wide
range of load currents. This control scheme combines the
ultra-low supply current of PFM converters (or pulse skip-
pers) with the high full-load efficiency of PWM converters.
Unlike traditional PFM converters, the MAX1771 uses a
sense resistor to control the peak inductor current. The
device also operates with high switching frequencies
(up to 300kHz), allowing the use of miniature external
components.
As with traditional PFM converters, the power transistor
is not turned on until the voltage comparator senses
the output is out of regulation. However, unlike tradition-
al PFM converters, the MAX1771 switch uses the com-
bination of a peak current limit and a pair of one-shots
that set the maximum on-time (16µs) and minimum off-
time (2.3µs); there is no oscillator. Once off, the mini-
mum off-time one-shot holds the switch off for 2.3µs.
After this minimum time, the switch either 1) stays off if
the output is in regulation, or 2) turns on again if the
output is out of regulation.
Figure 2b. 12V Output, Non-Bootstrapped
V
0.1 F
R2 = (R1)
V
68 F
IN
REF
C3
C1
= 5V
= 1.5V
5
4
6
(
V
V
OUT
REF
REF
SHDN
AGND
-1
C2
0.1 F
)
MAX1771
GND
V+
2
7
External Power-Transistor
EXT
CS
FB
1
8
3
22 H
L1
18k
MTD20N03HDL
N
1N5817-22
PFM Control Scheme
R
40m
R1
SENSE
Control Circuitry
D1
300 F
C4
127k
100pF
R2
C5
V
OUT
@ 0.5A
= 12V