AME, Inc.
AME5170
n
Detailed Description
The AME5170 features a constant off-time control
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
OUT
. Once the current
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
OUT
and the load. After the 400ns off-
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.
This cycle is continued until the voltage at FB pin
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 65µA typical. The load current is then sup-
plied solely by C
OUT
indicated by the gradually decreas-
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 AME5170 and reduce the I
Q
to
0.1µA. In shutdown mode the output voltage will be a
diode drop lower than the input voltage.
DIODE SELECTION
To maintain high efficiency, the average current rating
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
CAPACITOR SELECTION
Choose low ESR capacitors for the output to minimize
output voltage ripple. Multilayer ceramic capacitors are
the best choice. For most applications, a 1µF 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 AME5170. Multilayer ceramic capacitors
are a good choice for this as well. A 4.7µF 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
The input bypass capacitor C
IN
, as shown in Figure 3,
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
IN
to shunt any
high frequency noise to ground. The output capacitor,
C
OUT
, should also be placed close to the IC. Any copper
trace connections for the C
capacitor can increase
OUT
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.
Low Cost Micro Power
Boost DC/DC Converter
Rev.A.02
7
AME [ ANALOG MICROELECTRONICS ]
