ame5170a AME, Inc., ame5170a Datasheet - Page 7

• Manufacturer Part Number: ame5170a
• Description: Low Cost Micro Power Boost Dc/dc Converter
• Manufacturer: AME, Inc.

AME5170A
n Detailed Description
Rev.A.01
scheme. Operation can be best understood by referring
to Figure 5. When the voltage at the FB pin is less than
1.23V, the Enable Comp in Figure.5 enables the device
and the NMOS switch is turned on, pulling the SW pin to
ground. When the NMOS switch is on, load current is
supplied by the output capacitor C
in the inductor reaches the peak current limit, the 400ns
One Shot turns off the NMOS switch. The SW voltage
will then rise to the output voltage plus a diode drop and
inductor current will begin to decrease as shown in Fig-
ure3. During this time the energy stored in the inductor
is transferred to C
time the NMOS switch is turned on and energy is stored
in the inductor again. This energy transfer from the in-
ductor to the output causes a stepping effect in the out-
put ripple.
reaches 1.23V. When FB pin reaches this voltage, the
enable comparator then disables the device turning off
the NMOS switch and reducing the quiescent current of
the device to 65 A typical. The load current is then sup-
plied solely by C
ing slope at the output. When the FB pin drops slightly
below 1.23V, the enable comparator enables the device
and begins the cycle described previously. The EN pin
can be used to turn off the AME5170A and reduce the I
to 0.1 A. In shutdown mode the output voltage will be a
diode drop lower than the input voltage.
the NMOS power switch is turned off when the current
limit is reached. There is approximately a 200ns delay
from the time the current limit is reached in the NMOS
power switch and when the internal logic actually turns
off the switch. During this 200ns delay, the peak inductor
current will increase. This increase in inductor current de-
mands a larger saturation current rating for the inductor.
This saturation current can be approximated by the fol-
lowing equation:
I
The AME5170A features a constant off-time control
This cycle is continued until the voltage at FB pin
For the AME5170A constant-off time control scheme,
PK
I
CL
AME
V
OUT
IN
OUT
L
(max)
indicated by the gradually decreas-
and the load. After the 400ns off-
200
ns
OUT
. Once the current
Q
of the schottky diode should be larger than the peak in-
ductor current. Schottky diodes with a low forward drop
and fast switching speeds are ideal for increasing effi-
ciency in portable applications. Choose a reverse break-
down of the schottky diode larger than the output voltage
DIODE SELECTION
CAPACITOR SELECTION
output voltage ripple. Multilayer ceramic capacitors are
the best choice. For most applications, a 1 F ceramic
capacitor is sufficient. For some applications a reduc-
tion in output voltage ripple can be achieved by increas-
ing the output capacitor. Local bypassing for the input is
needed on the AME5170A. Multilayer ceramic capaci-
tors are a good choice for this as well. A 4.7 F capacitor
is sufficient for most applications. For additional bypass-
ing, a 100nF ceramic capacitor can be used to shunt
high frequency ripple on the input.
LAYOUT CONSIDERATIONS
must be placed close to the IC. This will reduce copper
trace resistance which effects input voltage ripple of the
IC. For additional input voltage filtering, a 100nF bypass
capacitor can be placed in parallel with C
high frequency noise to ground. The output capacitor,
C
trace connections for the C
the series resistance, which directly effects output volt-
age ripple. The feedback network, resistors R1 and R2,
should be kept close to the FB pin to minimize copper
trace connections that can inject noise into the system.
The ground connection for the feedback resistor network
should connect directly to an analog ground plane. The
analog ground plane should tie directly to the GND pin. If
no analog ground plane is available, the ground connec-
tion for the feedback network should tie directly to the
GND pin. Trace connections made to the inductor and
schottky diode should be minimized to reduce power
dissipation and increase overall efficiency.
OUT
To maintain high efficiency, the average current rating
Choose low ESR capacitors for the output to minimize
The input bypass capacitor C
, should also be placed close to the IC. Any copper
Boost DC/DC Converter
Low Cost Micro Power
OUT
IN
, as shown in Figure 3,
capacitor can increase
IN
to shunt any
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