AME5296 Datasheet, PDF (8/14 Pages) Analog Microelectronics – Step Down DC/DC Converter

AME
AME5296
2A, 18V, 500KHz Synchronous
Step-Down DC/DC Converter
n Application Information
Inductor Selection
For most applications, the inductance range is chosen
based on the desired ripple current. A larger inductance
reduces ripple current; meanwhile, the output ripple volt-
age decreases. Determine inductance is to allow the peak-
to-peak ripple current to be approximately 30% of the
maximum load current. The inductance value can be cal-
culated by:
L = ^VOUT × 1− ^VOUT
f × I_L
V_IN
Where f is the oscillator frequency, V_IN is the input volt-
age, V_OUT is the output voltage, and ∆I_L is the peak-to-peak
inductor ripple current. Choose an inductor that will not
saturate under the maximum inductor peak current, cal-
culated by:
I_LPEAK
=
I _LOAD
+
V_OUT
2×f ×
I_L
×
1− ^VOUT
V_IN
Where I_LOAD is the load current. The choice of which
style inductor to use mainly depends on the price vs. size
requirements and any EMI constraints.
Input Capacitor
The input current to the buck converter is discontinu-
ous; therefore an input capacitor is required to supply the
AC current while maintaining the DC input voltage. In or-
der to prevent large voltage drop, a low ESR capacitors is
recommended for the best performance. Ceramic capaci-
tors are preferred, but tantalum or low-ESR electrolytic
capacitors will also be suggested. Choose X5R or X7R
dielectrics when using ceramic capacitors. Since the in-
put capacitor absorbs the input switching current, it re-
quires an adequate ripple current rating. The RMS current
in the input capacitor can be estimated by:
I_CIN = I_LOAD ×
V_{OUT × 1−} V_OUT
V_IN
V_IN
At V_IN = 2V_OUT, where I_CIN = I_LOAD/2 is the worst-case
condition occurs. For simplification, use an input capaci-
tor with a RMS current rating greater than half of the maxi-
mum load current. When using ceramic capacitors, make
sure that they have enough capacitance to provide suffi-
cient charge to prevent excessive voltage ripple at input.
When using electrolytic or tantalum capacitors, a high
quality, small ceramic capacitor, i.e. 1µF, should be placed
as close to the IC as possible. The input voltage ripple for
low ESR capacitors can be estimated by:
V_IN
=
I_LOAD
C_IN × f
×
V_OUT
V_IN
×
1− ^VOUT
V_IN
Where C_IN is the input capacitance value.
Output Capacitor
The output capacitor (C_OUT) is required to maintain the
DC output voltage. Ceramic, tantalum, or low ESR elec-
trolytic capacitors are recommended. Low ESR capaci-
tors are preferred to keep the output voltage ripple low.
The output voltage ripple can be estimated by:
V_OUT
=
V_OUT
f ×L
×
1− ^VOUT
V_IN
×
R _ESR
+
8×f
1
× C_OUT
Where R_ESR is the equivalent series resistance (ESR)
value of the output capacitor and C_OUT is the output capaci-
tance value.
When using ceramic capacitors, the impedance at the
switching frequency is dominated by the capacitance which
is the main cause for the output voltage ripple. For simpli-
fication, the output voltage ripple can be estimated by:
V_OUT
=
8×f
V_OUT
2 _{× L × COUT}
×
1− ^VOUT
V_IN
8
Rev. A.05