MAX15020 Datasheet, PDF (12/19 Pages) Maxim Integrated Products – 2A, 40V Step-Down DC-DC Converter with Dynamic Output-Voltage Programming

2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Applications Information
Setting the ON/OFF Threshold
When the voltage at ON/OFF rises above 1.225V, the
MAX15020 turns on. Connect a resistive divider from IN
to ON/OFF to GND to set the turn-on voltage (see
Figure 2). First select the ON/OFF to the GND resistor
(R2), then calculate the resistor from IN to ON/OFF (R1)
using the following equation:
R1
=
R2
×
⎡
⎢
⎣⎢
V_IN
V_{ON/ OFF}
⎤
− 1_⎥
⎦⎥
where V_IN is the input voltage at which the converter
turns on, V_ON/OFF = 1.225V and R2 is chosen to be
less than 600kΩ.
If ON/OFF is connected to IN directly, the UVLO feature
monitors the supply voltage at IN and allows operation
to start when V_IN rises above 7.2V.
Setting the Output Voltage
Connect a resistor-divider from OUT to FB to GND to
set the output voltage (see Figure 2). First calculate the
resistor (R7) from OUT to FB using the guidelines in the
Compensation Design section. Once R7 is known, cal-
culate R8 using the following equation:
R8 = ^R7
⎡
⎢
⎣
V_OUT
V_FB
⎤
− 1_⎥
⎦
where V_FB = REFIN and REFIN = 0 to 3.6V.
Setting the Output-Voltage Slew Rate
The output-voltage rising slew rate tracks the V_SS slew
rate, given that the control loop is relatively fast com-
pared with the V_SS slew rate. The maximum V_SS
upswing slew rate is controlled by the soft-start current
charging the capacitor connected from SS to GND
according to the formula below:
dV_{OUT =} R₇ + R_{8 ×} dV_{SS =} R₇ + R₈ I_SS
dt
R₈
dt
R₈ C_SS
when driving V_SS with a slow-rising voltage source at
REFIN, V_OUT will slowly rise according to the V_REFIN
slew rate.
The output-voltage falling slew rate is limited to the dis-
charge rate of C_SS assuming there is enough load cur-
rent to discharge the output capacitor at this rate. The
C_SS discharge current is 15µA. If there is no load, then
the output voltage falls at a slower rate based upon
leakage and additional current drain from C_OUT.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15020: inductance value (L),
peak inductor current (I_PEAK), and inductor saturation
current (I_SAT). The minimum required inductance is a
function of operating frequency, input-to-output voltage
differential, and the peak-to-peak inductor current
(∆I_L). Higher ∆I_L allows for a lower inductor value while
a lower ∆I_L requires a higher inductor value. A lower
inductor value minimizes size and cost and improves
large-signal and transient response, but reduces effi-
ciency due to higher peak currents and higher peak-to-
peak output voltage ripple for the same output
capacitor. Higher inductance increases efficiency by
reducing the ripple current. Resistive losses due to
extra wire turns can exceed the benefit gained from
lower ripple current levels especially when the induc-
tance is increased without also allowing for larger
inductor dimensions. A good compromise is to choose
∆I_P-P equal to 40% of the full load current.
Calculate the inductor using the following equation:
L = ^{(VIN − VOUT) × VOUT}
V_IN × f_SW × ∆I_L
V_IN and V_OUT are typical values so that efficiency is
optimum for typical conditions. The switching frequen-
cy (f_SW) is fixed at 300kHz or 500kHz and can vary
between 100kHz and 500kHz when synchronized to an
external clock (see the Oscillator/Synchronization Input
(SYNC) section). The peak-to-peak inductor current,
which reflects the peak-to-peak output ripple, is worst
at the maximum input voltage. See the Output
Capacitor Selection section to verify that the worst-case
output ripple is acceptable. The inductor saturating
current (I_SAT) is also important to avoid runaway cur-
rent during continuous output short circuit. Select an
inductor with an I_SAT specification higher than the max-
imum peak current limit of 4.5A.
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