MAX1719/MAX1720/MAX1721
SOT23, Switched-Capacitor
Voltage Inverters with Shutdown
6_______________________________________________________________________________________
Charge-Pump Output
The MAX1719/MAX1720/MAX1721 are not voltage reg-
ulators: the charge pumps’ output resistance is
approximately 23Ωat room temperature (with VIN =
+5V), and VOUT approaches -5V when lightly loaded.
VOUT will droop toward GND as load current increases.
The droop of the negative supply (VDROOP-) equals the
current draw from OUT (IOUT) times the negative con-
verter’s output resistance (RO):
VDROOP- = IOUT x RO
The negative output voltage will be:
VOUT = -(VIN - VDROOP-)
Efficiency Considerations
The efficiency of the MAX1719/MAX1720/MAX1721 is
dominated by its quiescent supply current (IQ) at low
output current and by its output impedance (ROUT) 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
RSW is the sum of the charge pump’s internal switch
resistances (typically 8Ωto 9Ωat VIN = +5V). The typi-
cal output impedance is more accurately determined
from the Typical Operating Characteristics.
Shutdown Mode
The MAX1719/MAX1720/MAX1721 have a logic-con-
trolled shutdown input. Driving SHDN low places the
MAX1720/MAX1721 in a low-power shutdown mode.
The MAX1719’s shutdown input is inverted from that of
the MAX1720/MAX1721. Driving SHDN high places the
MAX1719 in a low-power shutdown mode. The charge-
pump switching halts, supply current is reduced to
1nA, and OUT is actively pulled to ground through a 4Ω
resistance.
Applications Information
Capacitor Selection
To maintain the lowest output resistance, use capaci-
tors with low ESR (Table 1). The charge-pump output
resistance is a function of C1’s and C2’s ESR.
Therefore, minimizing the charge-pump capacitor’s
ESR minimizes the total output resistance. Table 2
gives suggested capacitor values for minimizing output
resistance or minimizing capacitor size.