MAX1744_09 Datasheet, PDF (11/15 Pages) Maxim Integrated Products – High-Voltage, Step-Down DC-DC Controllers in μMAX

High-Voltage, Step-Down DC-DC
Controllers in µMAX
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:
R_CS
=
V_CS(MIN)
1.3 x I_OUT(MAX)
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:
( ) _{L(MIN) =}
V_IN - V_OUT x 1μs
V_CS(MIN)
R_CS
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.
FROM
OUTPUT
R2
TO FB
R3
Figure 5. Adjustable-Output Operation Using the MAX1745
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:
I_D(MAX) ≥ I_LIM(MAX)
=
V_CS(MAX)
R_SENSE
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.
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