Applications information – Rainbow Electronics MAX1709 User Manual
Page 11
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
______________________________________________________________________________________
11
I
LIM
= minimum value of switch current limit from Elec-
trical Characteristics or set by R
SET/LIM
.
Diode Selection (D1)
The MAX1709’s high switching frequency demands a
high-speed rectifier. Schottky diodes, such as the
MBRD1035CTL or STPS8L30B (Table 3), are recom-
mended. The diode’s current rating must exceed the
maximum load current, and its breakdown voltage must
exceed V
OUT
. The diode must be placed within 10mm
of the LX switching node and the output filter capacitor.
The diode also must be able to dissipate the power cal-
culated by the following equation:
P
DIODE
= I
OUT
✕
V
D
where I
OUT
is the average load current and V
D
is the
diode forward voltage at the peak switch current.
Capacitor Selection
Input Bypass Capacitors (C1, C2)
Two 150µF, low-ESR tantalum input capacitors will
reduce peak currents and reflected noise due to induc-
tor current ripple. Lower ESR allows for lower input rip-
ple current, but combined ESR values up to 50m
Ω are
acceptable. Smaller ceramic capacitors may also be
used for light loads or in applications that can tolerate
higher input current ripple.
Output Filter Capacitors (C6, C7)
The output filter capacitor ESR must be kept under
15m
Ω for stable operation. Two parallel 150µF polymer
capacitors (Panasonic EEFUE0J151R) typically exhibit
5m
Ω of ESR. This translates to approximately 35mV of
output ripple at 7A switch current. Bypass the
MAX1709 IC supply input (OUT) with a 0.1µF ceramic
capacitor to GND and a 2
Ω series resistor (R2, as
shown in Figure 1).
MAX1709 IC Power Dissipation
The major components of MAX1709 dissipated power
are switch conductance loss (P
SW
), capacitive loss
(P
CAP
), and switch transition loss (P
TRAN
). Throughout
the formulas, numerical examples are provided in {},
corresponding to the following condition:
{V
IN
= 3.3V, V
OUT
= 5V, V
D
= 0.5V, I
OUT
= 4A}
An important parameter to compute the power dissipat-
ed in the MAX1709 is the approximate peak switch cur-
rent (I
SW
):
P
D
(MAX1709) = P
SW
+ P
CAP
+ P
TRAN
{0.83W}
P
SW
= (1 - D') I
SW
2 ✕
R
SW
{0.59W}
where:
R
SW
= switch resistance {33m
Ω}
P
CAP
= (C
DIO
+ CD
SW
+ CG
SW
) (V
OUT
+ V
D
)
2
f {0.09W}
C
DIO
= catch-diode capacitance {1000pF}
CD
SW
= switch drain capacitance {2500pF}
CG
SW
= switch gate capacitance {1500pF}
f = switching frequency {600kHz}
P
TRAN
= (V
OUT
+ V
D
) I
SW
✕
t
SW
✕
f / 3 {0.15W}
where t
SW
= is switch turn-on or turn-off time {20ns}.
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn
the MAX1709 on and off. As shown in Figure 5, when
ONA is pulled low and ONB is pulled high, the part is
off. When the momentary switch is pressed, ONB is
pulled low and the regulator turns on. The switch
should be on long enough for the microcontroller to exit
reset. The controller issues a logic high to ONA, which
guarantees that the part will stay on regardless of the
subsequent switch state. To turn the regulator off,
press the switch long enough for the controller to read
the switch status and pull ONA low. When the switch is
released, ONB pulls high and the regulator turns off.
Layout Considerations
Due to high inductor current levels and fast switching
waveforms, proper PC board layout is essential. Protect
sensitive analog grounds by using a star ground config-
I
I
D
A
D
V
V
V
SW
OUT
IN
OUT
D
'
{ .
}
'
{ . }
=
=
+
6 67
0 6
µC
270k
ONB
ONA
0.1
µF
270k
ON/OFF
MAX1709
V
DD
I/O
I/O
Figure 5. Momentary Pushbutton On-Off Switch