MAX1595 Datasheet, PDF (4/8 Pages) Maxim Integrated Products – Regulated 3.3V/5.0V Step-Up/Step-Down Charge Pump

Regulated 3.3V/5.0V Step-Up/Step-Down
Charge Pump
PIN
MAX1595
µMAX
MAX1595
THIN QFN
1
12
2
1
3
2, 3
4
4
5
5, 6
6
7, 8
7
9
8
10, 11
NAME
AOUT
SHDN
IN
GND
PGND
CXN
CXP
OUT
Pin Description
FUNCTION
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.
Input Supply. Can range from 1.8V to 5.5V. Bypass to GND with a 1µF capacitor.
Ground
Power Ground
Negative Terminal of the Charge-Pump Transfer Capacitor
Positive Terminal of the Charge-Pump Transfer Capacitor
Output. Bypass to GND with output capacitor filter.
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 (in 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 regula-
tion point is reached. Maximum output ripple is propor-
tional 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.
IN
S1
C_IN
S2
C_X
OSC
OUT
C_OUT
Figure 1. Unregulated Voltage Doubler
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.
Applications Information
Using white LEDs to backlight LCDs is an increasingly
popular approach for portable information devices
(Figure 2). Because the forward voltage of white LEDs
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