ACE701D
PFM STEP-UP DC/DC Converter with High Efficiency and Low Noise
If inductor value is too small, the current ripple through it will be great. Then the current through diode
and power switch will be great. Because the power switch on chip is not ideal switch, the energy of switch
will improve. The efficiency will fall.
Thirdly，in general, smaller inductor values supply more output current while larger values start up with
lower input voltage and acquire high efficiency.
An inductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers large
output current (for example: output current is great than 50mA), large inductor value is recommended in
order to improve efficiency. If DC-DC must output very large current at low input supply voltage, small
inductor value is recommended.
The ESR of inductor will effect efficiency greatly. Suppose ESR value of inductor is rL，Rload is load
resistor，then the energy can be calculated by following expression:
Δη≈ RL / Rload (1-D)2
For example: input 1.5V, output is 3.0V, Rload=20Ω, rL=0.5Ω, The energy loss is 10%.
Consider all above，inductor value of 47uH、ESR<0.5Ω is recommended in most applications. Large
value is recommended in high efficiency applications and smaller value is recommended
(2) Capacitor Selection
Ignore ESR of capacitor，the ripple of output voltage is:
R=ΔVout / Vout=D / Rload Cf
So large value capacitor is needed to reduce ripple. But too large capacitor value will slow down system
reaction and cost will improve. So 100uF capacitor is recommended. Larger capacitor value will be used
in large output current system. If output current is small (<10mA), small value is needed.
Consider ESR of capacitor，ripple will increase：
r'=r+Imax•RESR / Vout
When current is large, ripple caused by ESR will be main factor. It may be greater than 100mV。The ESR
will affects efficiency and increase energy loss. So low-ESR capacitor (for example: tantalum capacitor) is
recommend or connect two or more filter capacitors in parallel.
(3) Diode Selection
Rectifier diode will affects efficiency greatly，Though a common diode (such as 1N4148) will work well for
light load，it will reduce about 5%~10% efficiency for heavy load，For optimum performance, a Schottky
diode (such as 1N5817、1N5819、1N5822) is recommended.
(4) Input Capacitor
If supply voltage is stable, the DC-DC circuit can output low ripple, low noise and stable voltage without
input capacitor. If voltage source is far away from DC-DC circuit, input capacitor value greater than 10uF
is recommended.
VER 1.3
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ACE [ ACE TECHNOLOGY CO., LTD. ]
