Rainbow Electronics MAX8569B - Manual (Page 7)

MAX8569A/MAX8569B
200mA Step-Up Converters in 6-Pin
SOT23 and TDFN
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Detailed Description
The MAX8569A and MAX8569B compact high-efficien-
cy step-up converters feature low-quiescent supply cur-
rent to ensure the highest possible efficiency over a
wide load range. With a minimum 1.5V input voltage,
these devices are well suited for applications with two
alkaline cells, two nickel-metal-hydride (NiMH) cells, or
one lithium-ion (Li+) cell. When SHDN is low, the output
is connected to the battery through the inductor and an
internal p-channel MOSFET. This allows the input bat-
tery to be used as a backup or real-time clock supply
when the converter is off by eliminating the voltage drop
across the MOSFET body diode. These devices are
ideal for low-power applications where a small footprint
is critical. The internal synchronous rectifier improves
efficiency significantly and reduces size and cost by
eliminating the need for an external Schottky diode.
Control Scheme
The MAX8569A/MAX8569B feature a current-limited
control scheme that provides ultra-low quiescent current
and high efficiency over a wide output current range.
The switching cycles are not controlled by an oscillator.
Instead, switch on-time is terminated when the inductor
current reaches the 780mA (typ) n-channel current limit,
or when the 5µs maximum n-channel switch on-time is
reached. Following each on-cycle, the synchronous rec-
tifier turns on, shunts the MOSFET body diode, and the
inductor current ramps to zero before another cycle
begins. The next cycle occurs when the error compara-
tor senses that the output has fallen below the regulation
threshold.
Applications Information
Shutdown
Drive SHDN low to shut down the MAX8569A/
MAX8569B and reduce the input current to less than
1µA. During shutdown, the battery input is connected
to the output through the inductor and the internal syn-
chronous rectifier. This allows the input battery (rather
than a separate backup battery) to provide backup
power for devices such as an idled microcontroller,
SRAM, or real-time clock, without the usual diode for-
ward drop. Drive SHDN to V
OUT (logic-high) to enable
the IC for normal operation.
MAX8569A
FB
OUT
1.5V TO 3V
INPUT
L1
10 µH
C3
22
µF
C1
10 µF
3.3V OUTPUT
R4
100k
Ω
R3
168k
Ω
LX
4
1
2 5
6
3
SHDN
BATT
GND C2
0.01 µF R2
1M Ω
R1
220k Ω
Figure 1. Typical Application Circuit for MAX8569A
MAX8569B
OUT
1.5V TO 3V
INPUT
L1
10 µH
C3
22
µF
C1
10 µF
3.3V
OUTPUT
RESET
OUTPUT
R1
100k
Ω
LX
4
1
2 5
6
3
SHDN
BATT
GND
RST
OFF
ON
Figure 2. Typical Application Circuit for MAX8569B