Rainbow Electronics MAX773 - Manual (Page 13)

drive outputs (EXTH and EXTL) that operate 180° out of
phase (Figures 3a and 3b). In Figure 3b, the resistor in
series with EXTH limits the base current, and EXTL (which
is connected directly to the base) turns the transistor off.
Shutdown Mode
When SHDN is high, the MAX770–MAX773 enter shut-
down mode. In this mode, the internal biasing circuit-
ry is turned off (including the reference) and V
OUT
falls to a diode drop below V IN (due to the DC path
from the input to the output). In shutdown mode, the
supply current drops to less than 5µA. SHDN is a
TTL/CMOS logic-level input. Connect SHDN to GND for
normal operation.
The MAX773’s shunt regulator is not disabled in shut-
down mode.
Low-Battery Detector
The MAX773 provides a low-battery comparator that
compares the voltage on LBI to the reference voltage.
When the LBI voltage is below V
REF, LBO (an open-
drain output) goes low. The low-battery comparator’s
20mV of hysteresis adds noise immunity, preventing
repeated triggering of LBO. Use a resistor-divider network
between V+, LBI, and GND to set the desired trip voltage
V
TRIP . LBO is high impedance in shutdown mode.
__________________Design Procedure
Setting the Output Voltage
To set the output voltage, first determine the mode of
operation, either bootstrapped or non-bootstrapped.
Bootstrapped mode provides more output current
capability, while non-bootstrapped mode reduces the
supply current (see
Typical Operating Characteristics ).
If a decaying voltage source (such as a battery) is
used, see the additional notes in the
Low Input Voltage
Operation section.
Use the MAX770/MAX771/MAX772 unless one or more
of the following conditions applies. If one or more of the
following is true, use the MAX773:
1) An NPN power transistor will be used as the power
switch
2) The LBI/LBO function is required
3) The shunt regulator must accommodate a high
input voltage
4) Preset-output non-bootstrapped operation is
desired—for example, to reduce the no-load
supply current in a 5V to 12V application.
See Table 1 for a summary of operating characteristics
and requirements for the ICs in bootstrapped and non-
bootstrapped modes.
The MAX770–MAX773’s output voltage can be adjust-
ed from very high voltages down to 3V, using external
resistors R1 and R2 configured as shown in Figure 5.
For adjustable-output operation, select feedback resis-
tor R1 in the range of 10k
Ω
to 500k
Ω
. R2 is given by:
V OUT
R2 = (R1)
(
––––– -1
)
V REF
where V REF equals 1.5V.
For preset-output operation, tie FB to GND (this
forces bootstrapped-mode operation for the
MAX770/MAX771/MAX772).
Configure the MAX773 for a preset voltage of 5V, 12V, or
15V by connecting the output to the corresponding
sense input pin (i.e., V5, V12, or V15). FB must be tied to
ground for preset-output operation. Leave all unused
sense input pins unconnected. Failure to do so will cause
an incorrect output voltage. The MAX773 can provide
a preset output voltage in both bootstrapped and non-
bootstrapped modes.
Figures 2 and 3 show various circuit configurations for
bootstrapped/non-bootstrapped, preset/adjustable
operation.
Shunt-Regulator Operation
When using the shunt regulator, connect SGND to ground
and place a 0.1µF capacitor between V+ and SGND, as
close to the IC as possible. Increase C2 to 1.0µF to
improve shunt regulators performance with heavy loads.
Select R
SHUNT such that 1mA
≤
I SHUNT
≤
20mA.
MAX770–MAX773
5V/12V/15V or Adjustable, High-Efficiency,
Low I Q , Step-Up DC-DC Controllers
______________________________________________________________________________________ 13
MAX770
MAX771
MAX772
MAX773 R1
R2
GND
FB V OUT
R1 = 10k TO 500k
V OUT
V
REF
R2 = R1
(
-1
)
V REF = 1.5V
Figure 5. Adjustable Output Circuit