MAX1832 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1832

High-Efficiency Step-Up Converters with Reverse Battery Protection

Maxim Integrated 

High-Efficiency Step-Up Converters with

Reverse Battery Protection

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 VOUT 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 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 CIN and COUT 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 100kΩ to 1MΩ

range to minimize battery drain. Calcuate R4 as follows:

R4 = R3 ✕ (VOFF / VSHDN - 1)

VOFF is the battery voltage at which the part will shut

down and VSHDN = 1.228V. Note that input ripple can

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