MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
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V
D
= forward voltage drop of the Schottky diode at I
LIM
current
V
OUT
= output voltage
D' = (V
IN
) / (V
OUT
+ V
D
), assuming switch voltage drop
is negligible
f = switching frequency
L1 = inductor value
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 50mΩ 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
15mΩ for stable operation. Two parallel 150µF polymer
capacitors (Panasonic EEFUE0J151R) typically exhibit
5mΩ 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 connect a 2Ω series resistor to
OUT (R2, as shown in Figure 1).
Power Dissipation
The MAX1709 output current may be more limited by
package power dissipation than by the current rating of
the on-chip switch. For pulsed loads, output currents of
4 Amps or more can be supplied with either the
MAX1709EUI+ or MAX1709ESE, but the RMS (or ther-
mal) limit of the MAX1709ESE is lower (6A
RMS
) than
that of the MAX1709EUI+ (10A
RMS
). Continuous output
current depends on the input and output voltage, oper-
ating temperature, and external components.
The major components of the MAX1709 dissipated
power (P
D
, i.e., power dissipated as heat in the IC and
NOT delivered to the load) are:
1) Internal switch conduction losses - P
SW
2) Internal switch transition losses - P
TRAN
3) Internal capacitive losses - P
CAP
These are losses that directly dissipate heat in the
MAX1709, but keep in mind that other losses, such as
those in the external diode and inductor, increase input
power by reducing overall efficiency, and so indirectly
contribute to MAX1709 heating.
Approximate equations for the loss terms are as fol-
lows. Values in {} are example values for a 3.3V input,
4V output, 4A design.
A conservative efficiency estimate for the MAX1709
boosting from 3.3V to 5V at 4A is 81%. Total estimated
power loss is then:
P
LOSS
= (P
OUT
/ 0.81) - P
OUT
{4.7W}
The total loss consists of:
Diode Loss = D’ x I
SW
x V
D
{2.5W}
Inductor Loss (resistive loss + dynamic loss
estimate) {0.58W}
External Capacitive Loss = (1 - D’) x I
SW
2
x
R
CAP-ESR
(ESR est. = 10mΩ) {0.27W}
MAX1709 Internal Loss, P
D(MAX1709)
{1.35W}
