PEDL5501-02
OKI Semiconductor
Programming the Output Voltage
The output voltage of the ML5501 can be adjusted with an external resistor divider as shown in figure 3.
The value of the voltage on the FB pin is 1V and the maximum allowed value for the output voltage is
18V. The resistor values can be easily calculated through a resistor divider equation.
Soft start and Enable Function:
Soft-start prevents a DC-DC converter from drawing excessive current from the power source during
start up. If the soft-start time is made sufficiently long, then the output will enter regulation without
overshoot. An external capacitor from the Enss pin to the ground with an internal 10μA charging current
source set the soft-start time. At least 6.8nF capacitor is recommended on the Enss pin. The converter
starts to switch when its Enss voltage exceeds 0.9V (Typical). The peak inductor current is gradually
increased until the converter output comes into regulation. If the Enss pin is forced below 0.35V then
the soft-start capacitor will be discharged and chip will be shutdown.
Short-Circuit Protection / Shutdown mode
The ML5501 are fully protected from output short circuits through current-limiting circuitry. When the
output is shorted to ground, the output current is foldback limited to 50mA (max) and the POK pin of
ML5501 becomes high. Then the CPU will recognize this state and will drive high level for SD pin of
ML5501 and chip will goes to the shutdown mode and POK pin will be low level. The shutdown current
is less than 100uA and whenever CPU drive the low level for SD pin, ML5501 will come back to the
normal operation.
Inductor Selection
A boost converter normally requires two main passive components for storing energy during the
conversion. A boost inductor and a storage capacitor at the output are required. To select the boost
inductor, it is recommended to keep the possible peak inductor current below the current limit threshold
of the power switch in the chosen configuration. The highest peak current through the inductor and the
switch depends on the output load, the input (Vdd), and the output voltage (Vin). Estimation of the
maximum average inductor current can be done by using Equation 1:
I
L
= 1.25 * I
out
* Vin / Vdd
ML5501
(1)
The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it
is advisable to work with a ripple of less than 30% of the average inductor current. A smaller ripple
reduces the magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in
the same way, regulation time at load changes rises. In addition, a larger inductor increases the total
system costs. With those parameters, it is possible to calculate the value for the inductor by using
Equation 2:
L= V
dd
* (
V
out
– V
dd
)/(∆I
L
* f * V
out
)
(2)
Parameter
f
is the switching frequency and
ΔI
L
is the ripple current in the inductor. With the calculated
inductance and current values, it is possible to choose a suitable inductor. Also the losses in the
inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit
efficiency.
Rev 1.0
8
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13
OKI [ OKI ELECTRONIC COMPONETS ]
