Rainbow Electronics MAX1835 - Manual (Page 7)

Detailed Description
The MAX1832–MAX1835 compact, high-efficiency
step-up converters feature 4µA 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. For the MAX1832 and
MAX1833, the battery is connected to OUT through the
inductor and an internal PFET when SHDN is low. This
allows the input battery to be used as a backup or real-
time clock supply when the converter is off by eliminat-
ing the voltage drop across the PFET body diode.
The MAX1832–MAX1835 are ideal for low-power appli-
cations where ultra-small size is critical. These devices
feature built-in synchronous rectification that signifi-
cantly improves efficiency and reduces size and cost
by eliminating the need for an external Schottky diode.
Furthermore, these devices are the industry’s first boost
regulators to offer complete reverse battery protection.
This proprietary design protects the battery, IC, and the
circuitry powered by the IC in the event the input bat-
teries are connected backwards.
Control Scheme
A current-limited control scheme is a key feature of the
MAX1832–MAX1835. This scheme provides ultra-low
quiescent current and high efficiency over a wide out-
put current range. There is no oscillator. The inductor
current is limited by the 0.5A N-channel current limit or
by the 5µs switch maximum on-time. Following each
on-cycle, the inductor current must ramp to zero before
another cycle may start. When the error comparator
senses that the output has fallen below the regulation
threshold, another cycle begins.
An internal synchronous rectifier eliminates the need for
an external Schottky diode reducing cost and board
space. While the inductor discharges, the P-channel
MOSFET turns on and shunts the MOSFET body diode.
As a result, the rectifier voltage drop is significantly
reduced, improving efficiency without adding external
components.
Reverse Battery Protection
The MAX1832–MAX1835 have a unique proprietary
design that protects the battery, IC, and circuitry pow-
ered by the IC in the event that the input batteries are
connected backwards. When the batteries are connect-
ed correctly, the reverse battery protection N-channel
MOSFET is on and the device operates normally.
When the batteries are connected backwards, the
reverse battery protection N-channel MOSFET opens,
protecting the device and load (Figures 2 and 3).
Previously, this level of protection required additional
circuitry and reduced efficiency due to added compo-
nents in the battery current path.
Applications Information
Shutdown
When SHDN is low, the device is off and no current is
drawn from the battery. When SHDN is high, the device
is on. If SHDN is driven from a logic-level output, the
logic high (on) level should be referenced to V
OUT
to
avoid intermittent turn on. If SHDN is not used at all,
connect it to OUT. With SHDN connected to OUT, the
MAX1834/MAX1835 startup voltage (1.65V) is slightly
higher, due to the voltage across the PFET body diode.
The SHDN pin has reverse battery protection.
In shutdown, the MAX1832/MAX1833 connect the bat-
tery input to the output through the inductor and the
internal synchronous rectifier PFET. 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 forward drop. If the output has a residual
voltage during shutdown, a small amount of energy will
be transfered from the output back to the input immedi-
ately after shutdown. This energy transfer may cause a
slight momemntary “bump” in the input voltage. The
magnitude and duration of the input bump are related
to the ratio of C
IN
and C
OUT
and the ability of the input
to sink current. With battery input sources, the bump
will be negligible, but with power-supply inputs (that
typically cannot sink current), the bump may be 100s of
mV.
In shutdown, the MAX1834/MAX1835 do not turn on the
internal PFET and thus do not have an output-to-input
current path in shutdown. This allows a separate back-
up battery, such as a Li+ cell, to be diode-connected at
the output, without leakage current flowing to the input.
The MAX1834/MAX1835 still have the typical input-to-
output current path from the battery to the output,
through the PFET body diode, in shutdown.
Low-Battery Cutoff
The SHDN trip threshold of the MAX1832–MAX1835
can be used as a voltage detector, with a resistor-
divider, to power down the IC when the battery voltage
falls to a set level (Figure 1). The SHDN trip threshold is
1.228V. To use a resistor-divider to set the shutdown
voltage, select a value for R3 in the 100k
Ω to 1MΩ
MAX1832–MAX1835
High-Efficiency Step-Up Converters with
Reverse Battery Protection in SOT23-6
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