Rainbow Electronics MAX1797 - Manual (Page 8)

MAX1795/MAX1796/MAX1797
Detailed Description
The MAX1795/MAX1796/MAX1797 compact step-up
DC-DC converters start up with voltages as low as
0.85V and operate with an input voltage down to +0.7V.
Consuming only 25µA of quiescent current, these
devices have an internal synchronous rectifier that
reduces cost by eliminating the need for an external
diode and improves overall efficiency by minimizing
losses in the circuit (see Synchronous Rectification sec-
tion for details). The internal N-channel MOSFET power
switch resistance is typically 0.17
Ω, which minimizes
losses. The LX switch current limits of the MAX1795/
MAX1796/MAX1797 are 0.25A, 0.5A, and 1A, respec-
tively.
All three devices offer Maxim’s proprietary True-
Shutdown circuitry, which disconnects the output from
the input in shutdown and puts the output in a high
impedance state. These devices also feature Maxim’s
proprietary LX-damping circuitry, which reduces EMI in
noise-sensitive applications. For additional in-system
flexibility, the LBI/LBO comparator remains active in
shutdown. (Figure 1 is a typical application circuit).
Control Scheme
A unique minimum-off-time, current-limited control
scheme is the key to the MAX1795/MAX1796/
MAX1797s’ low operating current and high efficiency
over a wide load range. The architecture combines the
high output power and efficiency of a pulse-width-mod-
ulation (PWM) device with the ultra-low quiescent cur-
rent of a traditional pulse-skipping controller (Figure 2).
Switching frequency depends upon the load current
and input voltage, and can range up to 500kHz. Unlike
conventional pulse-skipping DC-DC converters (where
ripple amplitude varies with input voltage), ripple in
these devices does not exceed the product of the
switch current limit and the filter-capacitor equivalent
series resistance (ESR).
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
8 _______________________________________________________________________________________
Figure 1. Typical Application Circuit
GND
LBO
LBI
*SEE TABLE 1 FOR COMPONENT VALUES.
BATT
FB
LX
OUT MAX1795
MAX1796
MAX1797
SHDN
V OUT = 3.3V
C OUT *
V IN
V IN
22 µH
47 µF
1M
PIN NAME FUNCTION
1 LBI
Low-Battery Comparator Input. Internally set to trip at +0.85V. This function remains operational in
shutdown.
2 FB
Dual-Mode™ Feedback Input. Connect to GND for preset 5.0V output. Connect to OUT for preset
3.3V output. Connect a resistive voltage-divider from OUT to GND to adjust the output voltage from
2V to 5.5V.
3 LBO
Low-Battery Comparator Output, Open-Drain Output. LBO is low when V LBI < 0.85V. This function
remains operational in shutdown.
4 SHDN
Shutdown Input. If SHDN is high, the device is in shutdown mode, OUT is high impedance, and
LBI/LBO are still operational. Connect shutdown to GND for normal operation.
5 GND Ground
6 LX Inductor Connection
7 OUT Power Output. OUT provides bootstrap power to the IC.
8 BATT Battery Input and Damping Switch Connection
Pin Description
Dual Mode is a trademark of Maxim Integrated Products.