MAX761ESA Maxim Integrated Products, MAX761ESA Datasheet - Page 10

• Manufacturer Part Number: MAX761ESA
• Description: IC DC/DC CONV STEP-UP 12V 8-SOIC
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX761ESA

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 12V, 5 ~ 16.5 V
• Current - Output: 150mA
• Frequency - Switching: 300kHz
• Voltage - Input: 2 ~ 16.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 471mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

In both CCM and DCM, practical inductor values range
from 10µH to 50µH. If the inductor value is too low, the
current in the coil will ramp up to a high level before the
current-limit comparator can turn off the switch. The mini-
mum on-time for the switch (t
2.5µs, so select an inductance that allows the current to
ramp up to I
of I
er light-load efficiency and minimizing ripple. Hence, cal-
culate the minimum inductance value as:
where V
The coil’s inductance need not satisfy this criterion
exactly, as the circuit can tolerate a wide range of val-
ues. Larger inductance values tend to produce physical-
ly larger coils and increase the start-up time, but are oth-
erwise acceptable. Smaller inductance values allow the
coil current to ramp up to higher levels before the switch
can turn off, producing higher ripple at light loads. In
general, an 18µH inductor is sufficient for most applica-
tions (V
input voltages up to 3.6V, as calculated above. However,
the same 18µH coil can be used with input voltages up
to 5V with only small increases in peak current, as shown
in Figures 4a and 4b.
12V/15V or Adjustable, High-Efficiency,
Low I
Figure 5. Bootstrapped Operation with Adjustable Output
10
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
V
LIM
IN
______________________________________________________________________________________
/ 2 allows the half-size pulses to occur, giving high-
IN(max)
IN
L ≥ (V
OR
L ≥ (V
C1
≤ 5V). An 18µH inductor is appropriate for
C3
LIM
Q
/ 2 in no less than 2.5µs. Choosing a value
is in volts and L is in microhenries.
, Step-Up DC-DC Converters
18µH
L1
IN(max)
IN(max)
5
2
4
SHDN
REF
LBI
Selecting the Inductor (L)
V
I
LIM/2
REF
)(t
)(5)
MAX761
MAX762
= 1.5V NOMINAL
ON(min)
GND
LX
6
7
ON(min)
1N5817
FB
V+
)
D1
8
3
) is approximately
R2 = R1
R2
R1
(
V OUT
V REF
V
OUT
C2
C4
-1
)
Inductors with a ferrite core or equivalent are recom-
mended. The inductor’s incremental saturation-current
rating should be greater than the 1A peak current limit. It
is generally acceptable to bias the inductor into satura-
tion by approximately 20% (the point where the induc-
tance is 20% below the nominal value). For highest effi-
ciency, use a coil with low DC resistance, preferably
under 100mΩ. To minimize radiated noise, use a toroid,
a pot core, or a shielded coil.
Table 1 lists inductor types and suppliers for various
applications. The listed surface-mount inductors’ efficien-
cies are nearly equivalent to those of the larger through-
hole inductors.
The MAX761/MAX762’s high switching frequency
demands a high-speed rectifier. Use a Schottky diode
with a 1A average current rating, such as a 1N5817. For
high-temperature applications, use a high-speed silicon
diode, such as the MUR105 or the EC11FS1. These
diodes have lower high-temperature leakage than
Schottky diodes (Table 1).
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the inductor current variation and the output
filter capacitor’s ESR determines the amplitude of the
high-frequency ripple seen on the output voltage. A
33µF, 16V Sanyo OS-CON capacitor with 100mΩ ESR
typically provides 100mV ripple when stepping up from
5V to 12V at 150mA.
Because the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. The
smallest low-ESR SMT tantalum capacitors currently
available are the Sprague 595D series. Sanyo OS-CON
organic semiconductor through-hole capacitors and
Nichicon PL series also exhibit very low ESR. Table 1
lists some suppliers of low-ESR capacitors.
The input bypass capacitor, C1, reduces peak currents
drawn from the voltage source, and also reduces noise
at the voltage source caused by the MAX761/MAX762’s
switching action. The input voltage source impedance
determines the size of the capacitor required at the V+
input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
250mA, 33µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable. Bypass the IC sepa-
rately with a 0.1µF ceramic capacitor, C2, placed close
to the V+ and GND pins.
Input Bypass Capacitors
Capacitor Selection
Output Filter Capacitor
Diode Selection