MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
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loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current.
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to V
CS
/R
SENSE
, where R
SENSE
is the value of
the current-sense resistor and V
CS
is the current-sense
threshold. V
CS
is typically 100mV. Minimizing the peak
switching current will increase efficiency and reduce
the size and cost of external components. However,
since available output current is a function of the peak
switching current, the peak current limit must not be set
too low.
Set the peak current limit to 1.3 times the maximum
load current by setting the current-sense resistor to:
Inductor Selection
The essential parameters for inductor selection are induc-
tance and current rating. The MAX1744/MAX1745 ope-
rate with a wide range of inductance values. In many
applications, values between 4.7μH and 100μH take best
advantage of the controller’s high switching frequency.
Calculate the minimum inductance value as follows:
where 1μs is the minimum on-time. Inductor values
between 2 and 10 times L
(MIN)
are recommended. With
high inductor values, the MAX1744/MAX1745 begin
continuous-conduction operation at a lower fraction of
the full load (see the
Detailed Description
section).
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to pre-
vent overheating and core saturation. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and inductance
starts to fall. The heating current rating is the maximum
DC current the inductor can sustain without overheating.
For optimum efficiency, the inductor windings’ resis-
tance should be less than the current-sense resistance.
If necessary, use a toroid, pot-core, or shielded-core
inductor to minimize radiated noise. Table 1 lists induc-
tor types and suppliers for various applications.
External Switching Transistor
The MAX1744/MAX1745 drive a P-channel enhance-
ment-mode MOSFET. The EXT output swings from VH
to IN. Be sure that the MOSFET’s on-resistance is spec-
ified for 5V gate drive or less. Table 1 recommends
MOSFET suppliers.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Q
g
), and R
DS(ON)
. The
drain-to-source breakdown voltage rating should be at
least a few volts higher than V
IN
. Choose a MOSFET
with a maximum continuous drain-current rating higher
than the peak current limit:
The Qg specification should be 80nC or less to ensure
fast drain voltage rise and fall times, and reduce power
losses during transition through the linear region. Q
g
specifies all of the capacitances associated with charging
the MOSFET gate. EXT pin rise and fall times vary with dif-
ferent capacitive loads, as shown in the
Typical Operating
Characteristics
. R
DS(ON)
should be as low as practical to
reduce power losses while the MOSFET is on. It should
be equal to or less than the current-sense resistor.
