Rainbow Electronics MAX8815A - Manual (Page 11)

True Shutdown
Drive ON low to place the MAX8815A in shutdown mode
and reduce supply current to 0.1µA (typ).
In shutdown, the control circuitry, internal switching
MOSFET, and synchronous rectifier turn off and LX
becomes high impedance. Drive ON high for normal
operation. The internal synchronous rectifier allows for
conversion efficiencies as high as 97%. In conventional
step-up circuits, the body diode of the synchronous rec-
tifier is forward biased in shutdown and allows current
flow from the battery to the output. If the load cannot be
shut down, an external switch is required to avoid deplet-
ing the battery during shutdown. A proprietary design in
the MAX8815A allows the synchronous rectifier to pro-
vide True Shutdown with no additional components. This
allows the output to fall to GND in shutdown and
removes any connection between the input and output.
Soft-Start
The MAX8815A has internal soft-start circuitry that con-
trols inrush current at startup, reducing transients on
the input source. Soft-start is particularly useful for
higher impedance input sources, such as Li+ and alka-
line cells. The soft-start duration is proportional to the
size of the output capacitor and load resistance. See
the
Typical Operating Characteristics section for plots
of Soft-Start Time vs. Load Current and Soft-Start Time
vs. Input Voltage.
Fault Protection
The MAX8815A has robust fault and overload protec-
tion. After power-up, the device monitors for an out-of-
regulation state such as an overload or short-circuit
condition. If the converter remains faulted for 16ms, the
output latches off until the part is reinitialized by tog-
gling ON or cycling power to the IC. If the output falls
10% below its regulation voltage or is shorted, the
device enters the fault state immediately.
If the short exists on the output before the IC is pow-
ered up, the converter goes through soft-start once and
then latches off (6ms) because the output never reach-
es regulation. The part draws about 1A of input current
during the startup period. Limiting the time under this
condition prevents thermal runaway.
Applications Information
Setting the Output Voltage
The MAX8815A has a preset output voltage of 5V. To set
other output voltages, use external feedback resistors.
To set the output voltage between 3.3V and 5V, con-
nect FB to the center of an external resistor voltage-
divider between POUT and GND, as shown in Figure
1b. Select the value of R2 no more than 500k
Ω, and
then calculate the value for R1 as follows:
R1 = R2 (V OUT /V FB - 1)
where V
FB
is the FB regulation voltage, 1.265V (typ).
MAX8815A
1A, 97% Efficiency, 30µA Quiescent Current
Step-Up Converter with True Shutdown
______________________________________________________________________________________ 11
(a) HIGH-GAIN DC LOAD REGULATION WITH POOR TRANSIENT RESPONSE
(b) VOLTAGE POSITIONING WITH DC LOAD REGULATION
9%
3%
Figure 3. Transient-Response Comparison
1ms/div
LOAD TRANSIENT
V OUT
200mV/div
5V DC OFFSET
500mV/div
MAX8815A toc18
I OUT
800mA
10mA
0A
Figure 2. Load Regulation