AME5249 Datasheet, PDF (12/21 Pages) Analog Microelectronics – 1.5MHz, 600mA Synchronous Buck Converter

AME
AME5249
1.5MHz, 600mA
Synchronous Buck Converter
Output Capacitor Selection
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR re-
quirement for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple V_OUT is determined by
∆ V_OUT
≈
∆I
L
(ESR
+
1
8C_OUT
)
f_SW
Where f_SW=operating frequency, C_OUT=output capaci-
tance and I_L=ripple current in the inductor. For a fixed
output voltage, the output ripple is highest at maximum
input voltage since I_L increases with input voltage.
At the light load current, the device operates in Power
Saving Mode, and the output voltage ripple is indepen-
dent of the value of the output capacitor. The output ripple
is set by the internal comparator thresholds and is also
affected by the feedback capacitor C1 in figure2. Large
capacitor values can decrease the output ripple, usually
a 22pF capacitor is sufficient for most applications.
When the input and output ceramic capacitors are cho-
sen, choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acters have the best temperature and voltage character-
istics of all the ceramics for a given value and size.
Thermal Considerations
In most applications the AME5249 does not dissipate
much heat due to its high efficiency. But, in applications
where the AME5249 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum junction temperature of the part. If the junction tem-
perat ure reac hes approximat ely 160 , bot h power
switches will be turned off and the SW node will become
high impedance. To avoid the AME5249 from exceeding
the maximum junction temperature, the user will need to
do some thermal analysis. The goal of the thermal analy-
sis is to determine whether the power dissipated exceeds
the maximum junction temperature of the part. The tem-
perature rise is given by:
TR = (PD) ×θ _JA
Where PD is the power dissipated by the regulator and
θJ_A is the thermal resistance from the junction of the die
to the ambient temperature.
Output Voltage Setting
In the adjustable version, the output voltage is set by a
resistor divider according to following formula:
V_OUT
=
0.6V
× (1+
R2)
R1
The external resistor divider is connected to the output.
12
Rev.A.01