MAX1833 Maxim, MAX1833 Datasheet - Page 7

• Manufacturer Part Number: MAX1833
• Description: High-Efficiency Step-Up Converters with Reverse Battery Protection
• Manufacturer: Maxim

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.
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.
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,
High-Efficiency Step-Up Converters with
_______________________________________________________________________________________
Reverse Battery Protection
Detailed Description
Control Scheme
Reverse Battery Protection
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.
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
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 pro-
vide backup power for devices such as an idled micro-
controller, 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 immediately
after shutdown. This energy transfer may cause a slight
momemntary “bump” in the input voltage. The magni-
tude and duration of the input bump are related to the
ratio of C
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.
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 100kΩ to 1MΩ
range to minimize battery drain. Calcuate R4 as follows:
V
down and V
OFF
is the battery voltage at which the part will shut
IN
and C
SHDN
R4 = R3
Applications Information
OUT
= 1.228V. Note that input ripple can
and the ability of the input to sink
✕
(V
OFF
/ V
Low-Battery Cutoff
SHDN
- 1)
Shutdown
OUT
to
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