MAX1522/MAX1523/MAX1524
Simple SOT23 Boost Controllers
10 ______________________________________________________________________________________
Optional Feed-Forward
Capacitor Selection
For proper control of peak inductor current during soft-
start and for stable switching, the ripple at FB should
be greater than 25mV. Without a feed-forward capaci-
tor connected between the output and FB, the output’s
ripple must be at least 2% of V
OUT
in order to meet this
requirement. Alternatively, if a low-ESR output capacitor
is chosen to obtain small output ripple, then a feed-for-
ward capacitor should be used, and the output ripple
may be as low as 25mV. The approximate value of the
feed-forward capacitor is given by:
Do not use a feed-forward capacitor that is much larger
than this because line-transient performance will
degrade. Do not use a feed-forward capacitor at all if
the output ripple is large enough without it to provide
stable switching because load regulation will degrade.
Optional Feedback Capacitor Selection
When using a feed-forward capacitor, it is possible to
achieve too much ripple at FB. The symptoms of this
include excessive line and load regulation and possibly
high output ripple at light loads in the form of pulse
groupings or “bursts.” Fortunately, this is easy to cor-
rect by either choosing a lower-ESR output capacitor or
by adding a feedback capacitor between FB and
ground. This feedback capacitor (C
FB
), along with the
feed-forward capacitor, form an AC-coupled ripple volt-
age-divider from the output to FB:
It is relatively simple to determine a good value for C
FB
experimentally. Start with C
FB
= C
FF
to cut the FB ripple
in half; then increase or decrease C
FB
as needed. The
ideal ripple at FB is from 25mV to 40mV, which will pro-
vide stable switching, low output ripple at light and
medium loads, and reasonable line and load regula-
tion. Never use a feedback capacitor without also using
a feed-forward capacitor.
Input Capacitor Selection
The input capacitor (C
IN
) in boost designs reduces the
current peaks drawn from the input supply, increases
efficiency, and reduces noise injection. The source
impedance of the input supply largely determines the
value of C
IN
. High source impedance requires high
input capacitance, particularly as the input voltage
falls. Since step-up DC-DC converters act as “constant-
power” loads to their input supply, input current rises
as input voltage falls. Consequently, in low-input-volt-
age designs, increasing C
IN
and/or lowering its ESR
can add as many as five percentage points to conver-
sion efficiency. A good starting point is to use the same
capacitance value for C
IN
as for C
OUT
. The input
capacitor must also meet the ripple current requirement
imposed by the switching currents, which is about 30%
of I
PEAK
in CCM designs and 100% of I
PEAK
in DCM
designs.
In addition to the bulk input capacitor, a ceramic 0.1µF
bypass capacitor at V
CC
is recommended. This capaci-
tor should be located as close to V
CC
and GND as pos-
sible. In bootstrapped configuration, it is recommended
to isolate the bypass capacitor from the output capaci-
tor with a series 10Ω resistor between the output and
V
CC
.
Power MOSFET Selection
The MAX1522/MAX1523/MAX1524 drive a wide variety
of N-channel power MOSFETs (NFETs). Since the out-
put gate drive is limited to V
CC
, a logic-level NFET is
required. Best performance, especially when V
CC
is
less than 4.5V, is achieved with low-threshold NFETs
that specify on-resistance with a gate-source voltage
(V
GS
) of 2.7V or less. When selecting an NFET, key
parameters include:
1) Total gate charge (Qg)
2) Reverse transfer capacitance or charge (C
RSS
)
3) On-resistance (R
DS(ON)
)
4) Maximum drain-to-source voltage (V
DS(MAX)
)
5) Minimum threshold voltage (V
TH(MIN)
)
At high switching rates, dynamic characteristics (para-
meters 1 and 2 above) that predict switching losses
may have more impact on efficiency than R
DS(ON)
,
which predicts I
2
R losses. Qg includes all capacitances
associated with charging the gate. In addition, this
parameter helps predict the current needed to drive the
gate when switching at high frequency. The continuous
V
CC
current due to gate drive is:
Use the FET manufacturer’s typical value for Qg (see
manufacturer’s graph of Qg vs. Vgs) in the above
equation since a maximum value (if supplied) is usually
too conservative to be of any use in estimating I
GATE
.
