MAX1822
High-Side Power Supply
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Detailed Description
Charge-Pump Operation
The MAX1822 is a multistage charge-pump power sup-
ply. Although the charge pump is capable of multiply-
ing V
CC
up to four times, the output is regulated to V
CC
+ 11V by an internal feedback circuit for inputs above
4V. The charge pump typically operates at 90kHz, but
regulates by pulse skipping. When V
OUT
exceeds V
CC
+ 11V, the oscillator shuts off. As V
OUT
dips below V
CC
+ 11V, the oscillator turns on.
Power-Ready Output
The Power-Ready Output (PR) signals control circuitry
when the high-side voltage reaches a preset level. This
feature can be used to protect external FET switches
from excess dissipation and damage by preventing them
from turning on, except when adequate gate drive levels
are present. When power is applied, PR remains low until
V
OUT
reaches approximately V
CC
+ 8.5V. PR also goes
low if V
OUT
falls below this level during operation, i.e., if
the output is overloaded. The PR high level is V
CC
.
Applications Information
Quiescent Supply Current
MAX1822 quiescent supply current varies with V
CC
and
with the values of C1, C2, and C3 (Typical Operating
Characteristics). Even with no external load, the device
must still pump to overcome internal losses. Large ratios
between C3 and C1 or C2 require more charge-pump
cycles to restore V
OUT
. As V
CC
falls below 5V, quiescent
current rises fairly rapidly to about 1mA at 4V (Typical
Operating Characteristics). This rise occurs because
V
OUT
no longer pulse skips to regulate at low input volt-
ages; the oscillator runs continuously, so supply current
is higher. Figure 2 shows the test circuit for the
MAX1822 quiescent supply current.
Figure 1. MAX1822 Block Diagram
