AME, Inc.
AME5138
n
Detailed Description
The AME5138 features a constant off-time control
scheme. Operation can be best understood by referring
to Figure 3. When the voltage at the FB pin is less than
1.23V, the Enable Comp in Figure 3 enables the device
and the NMOS switch is turmed on pulling the SW pin to
ground. When the NMOS switch is on, current is sup-
plied 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
the inductor current will begin to decrease as shown in
Figure 3. During this time the energy stored in the induc-
tor 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 inductor to the output causes a stepping effect in
the output ripple.
This cycle is continued until the voltage at FB reaches
1.23V. When FB reaches this voltage, the enable com-
parator then disables the device turning off the NMOS
switch and reducing the Iq of the device to 64µA. The
load current is then supplied solely by C
OUT
indicated by
the gradually decreasing slope at the output. When the
FB pin drops slightly below 1.23V, the enable compara-
tor enables the device and begins the cycle described
previously. The EN pin can be used to turn off the
AME5138 and reduce the Iq to 0.01µA. In shutdown mode
the output voltage will be a diode drop lower than the
input voltage.
Micropower Step-Up
DC/DC Converter
n
Application Information
INDUCTOR SELECTION
The appropriate inductor for a given application is
calculated using the following equation:
V
−
V
IN(min)
+
V
D
T
OFF
L
=
OUT
I
CL
Where V
D
is the schottky diode voltage, I
CL
is the switch
current limit found in the Typical Performance Character-
istics section, and T
OFF
is the switch off time. When us-
ing this equation be sure to use in minimum input volt-
age for the application, such as for battery powered ap-
plications.
Choosing inductors with low ESR decrease power
lossed and increase efficiency.
Care should be taken when choosing an inductor. For
applications that require an input voltage that approaches
the output voltage, such as when converting a Li-ion bat-
tery voltage to 5V, the 400ns off time may not be enough
time to discharge the energy in the inductor and transfer
the energy to the output capacitor and load. This can
cause a ramping effect in the inductor current waveform
and an increased ripple on the output voltage. Using a
smaller inductor will cause the I
PK
to increase and will
increase the output voltage ripple further. This can be
solved by adding a 4.7pF capacitor across the R1 feed-
back resistor (Figure 3) and slightly increasing the out-
put capacitor. A smaller inductor can then be used to
ensure proper discharge in the 400ns off time.
DIODE SELECTION
To maintain high efficiency, the average current rating
of the schottky diode should be larger than the peak in-
ductor current, I
PK
. Schottky diodes with a low forward
drop and fast switching speeds are ideal for increasing
efficiency in portable applications. Choose a reverse break-
down of the schottky diode larger than the output volt-
age.
7
AME [ ANALOG MICROELECTRONICS ]
