AME, Inc.
AME7720 / 7721
n
Detailed Description
The AME7720 / 7721 regulated charge pump provides a
regulated output voltages ranging from less than the output
to greater than the output. This is accomplshed by auto-
matic mode switching within the device.
Step-Up/Step-Down
60mA Regulated Charge Pump
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Efficiency
The efficiency of the charge pump regulator varies
with the output voltage version, the applied input volt-
age, the load current, and the internal operation mode
of the device.
The approximate efficiency is given by:
5V Version (V
IN
<= V
OUT
)
Efficiency (%) = V
OUT
/ 2V
IN
．
100
3.3V Version (V
IN
> V
OUT
)
Efficiency (%) = V
OUT
/ V
IN
．
100
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Short Circuit and Thermal
Protection
The AME7720 / 7721 has a thermal protection and shut-
down circuit that continuously monitors the IC junction tem-
perature. If the thermal protection circuit senses the die
temperature exceeding approximately 160
O
C, the thermal
shutdown will disable the charge pump switching cycle op-
eration, The thermal limit system has 20
O
C of system hys-
teresis before the charge pump can reset. Once the over
current event is removed from the output and the junction
temperature drops below 140
O
C, the charge pump will the
become active again. The thermal protection system will
cycle on and off if an output short circuit condition persists.
This will allow the AME7720 / 7721 to operate indefinitely a
short circuit condition without damage to the device.
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Layout
Large ripple currents flow in the V
IN
and V
OUT
traces.
To minimized both input and output ripple, keep the
capacitors as close as possible to the regulator using
short, direct circuit traces.
The trace lengths from the input and output capaci-
tors have been kept as short as possible. A star ground
system has been implemented, with GND pin as the
center of the star. No ground plane is provided in other
layers, as this will provide capacitive coupling for noise
spikes.
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Shutdown Mode
A control pin on the regulator can be used to place the
device into an energy-saving shutdown mode. In this mode,
the output is disconnected from the input as long as V
IN
is
greater than or equal to minimum V
IN
and input quiescent
current is reduced to 1µA maximum.
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Capacitor Selection
For minimum output voltage ripple, the output capacitor
C
OUT
should be a ceramic, surface-mount type. Tantalum
capacitors generally have a higher Effective Series Resis-
tance (ESR) and may contribute to higher output voltage
ripple. Leaded capacitors also increase ripple due to the
higher inductance of the package itself. To achieve best
operation with low input voltage and high load current, the
input and pump capacitors (C
IN
, C
PUMP
respecvively) should
also be surface-mount ceramic types. In all cases, X7R or
X5R dielectric are recommended.
With light loads or higher input voltage, a smaller
0.1µF pump capacitor (C
PUMP
) and smaller 1µF input
and output capacitor (C
IN
and C
OUT
, respectively) can
be used. To minimize output voltage ripple, increase
the output capacitor, C
OUT
, to 10µF or larger.
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AKM [ ASAHI KASEI MICROSYSTEMS ]
