Detailed Description
The MAX1595 charge pump provides either a 3.3V or 5V
regulated output. It delivers a maximum 125mA load cur-
rent. In addition to boost regulating from a lower supply,
it is also capable of buck regulating from supplies that
exceed the regulated output by a diode drop or more.
Designed specifically for compact applications, a com-
plete regulator circuit requires only three small external
capacitors. An innovative control scheme provides con-
stant frequency operation from medium to heavy loads,
while smoothly transitioning to low-power mode at light
loads to maintain optimum efficiency. In buck mode,
switch S1 (Figure 1) is switched continuously to IN, while
switch S2 alternates between IN and OUT. An amount of
charge proportional to the difference between the output
voltage and the supply voltage is stored on C
X
, which
gets transferred to the output when the regulation point is
reached. Maximum output ripple is proportional to the
difference between the supply voltage and the output
voltage, as well as to the ratio of the transfer capacitor
(C
X
) to the output capacitor (C
OUT
).
The MAX1595 consists of an error amplifier, a 1.23V
bandgap reference, internal resistive feedback network,
oscillator, high-current MOSFET switches, and shutdown
and control logic. Figure 1 shows an idealized unregulat-
ed charge-pump voltage doubler. The oscillator runs at a
50% duty cycle. During one half of the period, the trans-
fer capacitor (C
X)
charges to the input voltage. During
the other half, the doubler transfers the sum of C
X
and
input voltage to the output filter capacitor (C
OUT
). Rather
than doubling the input voltage, the MAX1595 provides a
regulated output voltage of either 3.3V or 5.0V.
Shutdown
Driving SHDN low places the device in shutdown mode.
The device draws 0.1μA of supply current in this mode.
When driven high, the MAX1595 enters a soft-start
mode. Soft-start mode terminates when the output volt-
age regulates, or after 2ms, whichever comes first. In
shutdown, the output disconnects from the input.
Undervoltage Lockout
The MAX1595 has an undervoltage-lockout that deacti-
vates the devices when the input voltage falls below 1.6V.
Below UVLO, hysteresis holds the device in shutdown until
the input voltage rises 40mV above the lockout threshold.
Analog Power and Sense Input for Error Amplifier/Comparator. Connect to OUT at
output filter capacitor.
Shutdown Input. When SHDN = low, the device turns off; when SHDN = high, the device
activates. In shutdown, OUT is disconnected from IN.
Exposed Pad. Internally connected to GND. Connect to a large ground plane to
maximize thermal performance. Not intended as an electrical connection point (TQFN
package only).
