Detailed Description
The MAX1852/MAX1853 charge pumps invert the volt-
age applied to their input. For highest performance use
low equivalent series resistance (ESR) capacitors (e.g.,
ceramic).
During the first half-cycle, switches S2 and S4 open,
switches S1 and S3 close, and capacitor C1 charges to
the voltage at IN (Figure 2). During the second half-
cycle, S1 and S3 open, S2 and S4 close, and C1 is level
shifted downward by V
IN
volts. This connects C1 in par-
allel with the reservoir capacitor C2. If the voltage across
C2 is smaller than the voltage across C1, charge flows
from C1 to C2 until the voltage across C2 reaches
-V
IN
. The actual voltage at the output is more positive
than -V
IN
since switches S1–S4 have resistance and the
load drains charge from C2.
Efficiency Considerations
The efficiency of the MAX1852/MAX1853 is dominated
by their quiescent supply current (I
Q
) at low output cur-
rent and by their output impedance (R
OUT
) at higher
output current; it is given by:
where the output impedance is roughly approximated
by:
The first term is the effective resistance of an ideal
switched-capacitor circuit (Figures 3a and 3b), and
R
SW
is the sum of the charge pump’s internal switch
resistances (typically 6Ω at V
IN
= +5V). The typical out-
put impedance is more accurately determined from the
Typical Operating Characteristics.
Shutdown
The MAX1852/MAX1853 have a logic-controlled shut-
down input. Driving SHDN low places the devices in a
low-power shutdown mode. The charge-pump switch-
ing halts, supply current is reduced to 2nA.
Driving SHDN high will restart the charge pump. The
switching frequency and capacitor values determine how
soon the device will reach 90% of the input voltage.
Applications Information
Capacitor Selection
The charge-pump output resistance is a function of the
ESR of C1 and C2. To maintain the lowest output resis-
tance, use capacitors with low ESR. (See Table 1 for a
list of recommended manufacturers.) Tables 2 and 3
suggest capacitor values for minimizing output resis-
tance or capacitor size.
Flying Capacitor (C1)
Increasing the flying capacitor’s value reduces the out-
put resistance. Above a certain point, increasing C1’s
capacitance has negligible effect because the output
resistance is then dominated by internal switch resis-
tance and capacitor ESR.
Output Capacitor (C2)
Increasing the output capacitor’s value reduces the
output ripple voltage. Decreasing its ESR reduces both
output resistance and ripple. Lower capacitance values
can be used with light loads if higher output ripple can
be tolerated. Use the following equation to calculate the
peak-to-peak ripple:
