AAT1151IKS-1.0-T1 AnalogicTech, AAT1151IKS-1.0-T1 Datasheet - Page 8

• Manufacturer Part Number: AAT1151IKS-1.0-T1
• Description: DC to DC Converter, 850kHz 700mA Synchronous Buck DC/DC Converter, Tape and Reel
• Manufacturer: AnalogicTech

Current Limit and Over-temperature
Protection
For overload conditions the peak input current is lim-
ited. Figure 1 displays the VI current limit character-
istics. As load impedance decreases and the output
voltage falls closer to zero, more power is dissipated
internally, raising the device temperature. Thermal
protection completely disables switching when inter-
nal dissipation becomes excessive, protecting the
device from damage. The junction over-temperature
threshold is 140°C with 10°C of hysteresis.
Inductor
The output inductor is selected to limit the ripple cur-
rent to some predetermined value, typically 20-40%
of the full load current at the maximum input voltage.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
saturation characteristics. The inductor should not
show any appreciable saturation under all normal
load conditions. During over load and transient con-
ditions, the average current in the inductor can meet
or exceed the current limit point of the AAT1151.
These conditions can tolerate greater saturation in
the inductor without degradation in converter per-
formance. Some inductors may meet the peak and
average current ratings yet result in excessive loss-
es due to a high DCR. Always consider the losses
associated with the DCR and its effect on the total
converter efficiency when selecting an inductor.
For a 1.0 Amp load and the ripple set to 40% at the
maximum input voltage, the maximum peak to
peak ripple current is 280mA. The inductance
value required is 2.84µH.
The factor "k" is the fraction of full load selected for
the ripple current at the maximum input voltage. For
ripple current at 40% of the full load current the peak
current at will be 120% of full load. Selecting a stan-
8
L =
=
= 2.84µH


1A · 0.4 · 850kHz
I
O
V
· k · F
OUT
1.5V
· 1 -


V
V


OUT
IN
· 1 -




1.5V
4.2V


850kHz 700mA Synchronous Buck DC/DC Converter
dard value of 3.0µH gives 38% ripple current. A
3.0µH
CDRH5D28 series has a 24 mΩ DCR and a 2.4 DC
current rating. At full load the inductor DC loss is
24mW which amounts to a 1.6% loss in efficiency.
Input Capacitor
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1151. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize the stray inductance the capacitor should be
placed as close as possible to the IC. This keeps the
high frequency content of the input current localized;
minimizing radiated and conducted EMI while facili-
tating optimum performance of the AAT1151.
Ceramic X5R or X7R capacitors are ideal for this
function. The size required will vary depending on the
load, output voltage and input voltage source imped-
ance characteristics. A Typical value is around 10µF.
The input capacitor RMS current varies with the input
voltage and the output voltage. The equation for the
maximum RMS current in the input capacitor is:
The input capacitor RMS ripple current reaches a
maximum when V
where it is approximately one half of the load cur-
rent. Losses associated with the input ceramic
capacitor are typically minimal and not an issue.
The proper placement of the input capacitor can be
seen in the reference design layout in figure 3.
Output Capacitor
Since there are no external compensation compo-
nents, the output capacitor has a strong effect on loop
stability. Lager output capacitance will reduce the
crossover frequency with greater phase margin. For
the 1.5V 1A design using the 4.1µH inductor, a 47µF
capacitor provides a stable loop but with only 35
degrees of phase margin at a crossover frequency of
100 kHz. Doubling the capacitance to 100µF reduces
the crossover frequency to half while increasing the
phase margin to 60 degrees. In addition to assisting
stability, the output capacitor limits the output ripple
I
RMS
inductor
= I
O
·
IN
selected
V
is two times the output voltage
V
IN
O
· 1 -


from
V
V
IN
O
AAT1151


1151.2003.11.0.91
the
Sumida