AP5100_1011 Datasheet, PDF (7/10 Pages) Diodes Incorporated – 1.2A Step-Down Converter with 1.4MHz Switching Frequency

AP5100
1.2A Step-Down Converter with 1.4MHz Switching
Frequency
Applications Information (Continued)
Setting the Output Voltage (Continued)
Choose the inductor ripple current to be 30% of the
maximum load current. The maximum inductor peak
current is calculated from:
IL(MAX)
=
ILOAD
+
ΔIL
2
Equation 3
Peak current determines the required saturation current
rating, which influences the size of the inductor. Saturating
the inductor decreases the converter efficiency while
increasing the temperatures of the inductor, the MOSFET
and the diode. Hence choosing an inductor with
appropriate saturation current rating is important.
A 1µH to 10µH inductor with a DC current rating of at least
25% percent higher than the maximum load current is
recommended for most applications.
For highest efficiency, the inductor’s DC resistance should
be less than 200mΩ. Use a larger inductance for
improved efficiency under light load conditions.
Input Capacitor
The input capacitor reduces the surge current drawn from
the input supply and the switching noise from the device.
The input capacitor has to sustain the ripple current
produced during the on time on the upper MOSFET. It
must hence have a low ESR to minimize the losses.
Due to large dI/dt through the input capacitors, electrolytic
or ceramics should be used. If a tantalum must be used, it
must be surge protected. Otherwise, capacitor failure
could occur. For most applications, a 4.7µF ceramic
capacitor is sufficient.
Output Capacitor
The output capacitor keeps the output voltage ripple small,
ensures feedback loop stability and reduces the overshoot
of the output voltage. The output capacitor is a basic
component for the fast response of the power supply. In
fact, during load transient, for the first few microseconds it
supplies the current to the load. The converter recognizes
the load transient and sets the duty cycle to maximum, but
the current slope is limited by the inductor value.
Maximum capacitance required can be calculated from the
following equation:
Co
=
L(IOUT
+
ΔIinductor
2
)2
(Δ V + VOUT )2 − VOUT2
Equation 4
Where ΔV is the maximum output voltage overshoot.
ESR of the output capacitor dominates the output voltage
ripple. The amount of ripple can be calculated from the
equation below:
Voutcapacitor = ΔIinductor × ESR
An output capacitor with ample capacitance and low ESR
is the best option. For most applications, a 22µF ceramic
capacitor will be sufficient.
External Diode
The external diode’s forward current must not exceed the
maximum output current. Since power dissipation is a
critical factor when choosing a diode, it can be calculated
from the equation below:
Pdiode
= (1−
VOUT
VIN
) × Iout
× 0.3V
Equation 5
Note: 0.3V is the voltage drop across the schottky diode. A
diode that can withstand this power dissipation must be
chosen.
External Bootstrap Diode
It is recommended that an external bootstrap diode be
added when the input voltage is no greater than 5V or the
5V rail is available in the system. This helps improve the
efficiency of the regulator. The bootstrap diode can be a
low cost one such as IN4148 or BAT54.
5V
BST ¹
AP5100
BOOST
DIODE
10nF
6
SW
Figure 6. External Bootstrap Diode
Under Voltage Lockout (UVLO)
Under Voltage Lockout is implemented to prevent the IC
from insufficient input voltages. The AP5100 has a UVLO
comparator that monitors the internal regulator voltage. If
the input voltage falls below the internal regulator voltage,
the AP5100 will latch an under voltage fault. In this event
the output will be pulled low and power has to be re-cycled
to reset the UVLO fault.
AP5100
Document number: DS32130 Rev. 2 - 2
7 of 10
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November 2010
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