AME5251A Datasheet, PDF (8/19 Pages) Analog Microelectronics – Dual 1A, 1.5MHz Synchronous Step-Down Converter

AME
AME5251A
This formula has a maximum at V_IN=2V_OUT, where
IRMS=I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant devia-
tions do not offer much relief. Note that the capacitor
manufacturer ripple current ratings are often based on
2000 hours of life. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a
higher temperature than required.
The selection of C_OUT is determined by the effective
series resistance(ESR) that is required to minimize volt-
age ripple and load step transients. The output ripple,
V_OUT, is determined by:
∆V_OUT ≅ ∆I _L
ESR + ¹
8 fC_OUT
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high
ripple current, high voltage rating and low ESR make them
ideal for switching regulator applications. However, care
must be taken when these capacitors are used at the
input and output. When a ceramic capacitor is used at
the input and the power is supplied by a wall adapter
through long wires, a load step at the output can induce
ringing at the input, V_IN. At best, this ringing can couple
to the output and be mistaken as loop instability. At worst,
a sudden inrush of current through the long wires can
potentially cause a voltage spike at V_IN large enough to
damage the part.
Output Voltage Programming
The output voltage is set by an external resistive di-
vider according to the following equation:
V_OUT
= V_REF ×
1 + ^R1
R₂
Where VREF equals to 0.6V typical. The resistive di-
vider allows the FB pin to sense a fraction of the output
voltage as shown in Figure 1.
8
Dual 1A, 1.5MHz Synchronous
Step-Down Converter
0.6V VOUT 5.5V
R1
FB
AME5251 A
R2
GND
Figure 1: Setting the AME5251A Output Voltage
Thermal Considerations
In most applications the AME5251A does not dissi-
pate much heat due to its high efficiency. But, in appli-
cations where the AME5251A is running at high ambient
temperature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junction
temperature reaches approximately 160^OC, both power
switches will be turned off and the SW node will become
high impedance. To avoid the AME5251A from exceed-
ing the maximum junction temperature, the user will need
to do some thermal analysis. The goal of the thermal
analysis is to determine whether the power dissipated
exceeds the maximum junction temperature of the part.
The temperature rise is given by:
T_R = (PD)(θ_JA )
Where PD is the power dissipated by the regulator and
θ_JA is the thermal resistance from the junction of the die
to the ambient temperature.
Rev. A.04