Rainbow Electronics MAX889 - Manual (Page 6)

MAX889
(ESR) of the flying and output capacitors. The typical
output impedance is more accurately determined from
the Typical Operating Characteristics.
Current Limit and Soft-Start
The MAX889 features a foldback current-limit/soft-start
scheme that allows it to limit inrush currents during
startup, overload, and output short-circuit conditions.
Additionally, it permits a safe, timed recovery from fault
conditions. This protects the MAX889 and prevents
low-current or higher output impedance input supplies
(such as alkaline cells) from being overloaded at start-
up or short-circuit conditions.
The MAX889 features two current-limit/soft-start levels
with corresponding response to rising and falling out-
put voltage thresholds of -0.6V and -1.5V. When the
falling output voltage crosses -1.5V, such as during an
overload condition, the input current is immediately lim-
ited to 400mA by weakening the charge-pump switch-
es. When the falling output voltage crosses -0.6V, such
as during a short-circuit condition, the MAX889 further
weakens the charge-pump switches, immediately limit-
ing input current to 200mA.
During startup or short-circuit recovery, the MAX889
limits input current to 200mA with charge-pump switch-
es at their weakest level. Rising output voltage crossing
-0.6V initiates a 2ms timer, after which the MAX889
increases switch strength to the next level. The rising
output voltage crossing -1.5V initiates a 2ms timer, after
which the MAX889 provides full-strength operation.
Shutdown
When SHDN (a CMOS-compatible input) is driven low,
the MAX889 enters 0.1µA shutdown mode. Charge-
pump switching halts. Connect SHDN to IN or drive
high for normal operation.
Thermal Shutdown
The MAX889 features thermal shutdown with hysteresis
for added protection against fault conditions. When the
die temperature exceeds 160
°C, the internal oscillator
stops, suspending device operation. The MAX889
resumes operation when the die temperature falls 15
°C.
This prevents the device from rapidly oscillating around
the temperature trip point.
Applications Information
Resistor Selection
(Setting the Output Voltage)
The accuracy of V OUT depends on the accuracy of the
voltage biasing R1 in Figure 1. Use a separate refer-
ence voltage if greater accuracy than provided by V
IN
is desired (Figure 4). Keep the feedback node as small
as possible, with resistors mounted close to the FB pin.
High-Frequency, Regulated,
200mA, Inverting Charge Pump
6 _______________________________________________________________________________________
Figure 1. Typical Application Circuit.
MAX889T
IN
INPUT
5.0V
OUTPUT
-3.3V
C IN
4.7 µF
C OUT
4.7 µF
C FLY
1 µF
FB
R1
100k
R1
66.5k
OUT
GND
CAP+
ON
OFF
4
2
3
SHDN
CAP-
1
7
5
6
8
Figure 3. Transferring Charge on C FLY to C OUT
S2
OUT
C OUT C FLY
S1
IN
S4
S3
F OSC
CAP+
CAP-
Figure 2. Charging C FLY
S2
OUT
C OUT C FLY
S1 CAP+
CAP-
IN
S4
S3
F
OSC