Rainbow-electronics MAX1637 Manual de usuario descargar pdf (Pagina 14)

MAX1637

Miniature, Low-Voltage,

Precision Step-Down Controller

14 ______________________________________________________________________________________

make prototype troubleshooting difficult since only

20ms or 30ms elapse before the SMPS is latched off.

The overvoltage crowbar protection is disabled in out-

put undervoltage mode.

Output Overvoltage Protection

The overvoltage crowbar-protection circuit is intended

to blow a fuse in series with the battery if the main

SMPS output rises significantly higher than its standard

level (Table 4). In normal operation, the output is com-

pared to the internal precision reference voltage. If the

output goes 7% above nominal, the synchronous-recti-

fier MOSFET turns on 100% (the high-side MOSFET is

simultaneously forced off) in order to draw massive

amounts of battery current to blow the fuse. This safety

feature does not protect the system against a failure of

the controller IC itself, but is intended primarily to guard

against a short across the high-side MOSFET. A crow-

bar event is latched and can only be reset by a rising

edge on SHDN (or by removal of the V

supply volt-

age). The overvoltage-detection decision is made rela-

tive to the regulation point.

Internal Digital Soft-Start Circuit

Soft-start allows a gradual increase of the internal cur-

rent-limit level at start-up to reduce input surge cur-

rents. The SMPS contains an internal digital soft-start

circuit controlled by a counter, a digital-to-analog con-

verter (DAC), and a current-limit comparator. In shut-

down, the soft-start counter is reset to zero. When the

SMPS is enabled, its counter starts counting oscillator

pulses, and the DAC begins incrementing the compari-

son voltage applied to the current-limit comparator. The

DAC output increases from 0mV to 100mV in five equal

steps as the count increases to 512 clocks. As a result,

the main output capacitor charges up relatively slowly.

The exact time of the output rise depends on output

capacitance and load current, but it is typically 1ms

with a 300kHz oscillator.

Setting the Output Voltage

The output voltage is set via a resistor divider connect-

ed to FB (Figure 1). Calculate the output voltage with

the following formula:

OUT

= V

REF

(1 + R2 / R3)

where V

REF

= 1.1V nominal.

Recommended normal values for R3 range from 5kΩ to

100kΩ. To achieve a 1.1V nominal output, connect FB

directly to CSL. Remote output voltage sensing is pos-

sible by using the top of the external resistor divider as

the remote sense point.

__________________Design Procedure

The standard application circuit (Figure 1) contains a

ready-to-use solution for common application needs.

Use the following design procedure to optimize the

basic schematic for different voltage or current require-

ments. But before beginning a design, firmly establish

the following:

• Maximum input (battery) voltage, V

IN(MAX)

. This

value should include the worst-case conditions, such

as no-load operation when a battery charger or AC

adapter is connected but no battery is installed.

IN(MAX)

must not exceed 30V.

• Minimum input (battery) voltage, V

IN(MIN)

. This value

should be taken at full load under the lowest battery

conditions. If the minimum input-output difference is

less than 1.5V, the filter capacitance required to

maintain good AC load regulation increases (see

Low-Voltage Operation

section).

Table 4. Operating Modes

All circuit blocks offLowShutdown

REF = off, DL = lowHigh

Output

Undervoltage

Lockout

REF = off, DL = highHigh

Overvoltage

(Crowbar)

—

OUT

below 70% of

nominal after 20ms to

30ms timeout expires

OUT

greater than 7%

above regulation point

OUT

in regulation

CONDITIONSMODE

All circuit blocks activeHighRun

STATUS

SHDN

Lowest current consumption

Rising edge on SHDN exits

UVLO

Rising edge on SHDN exits

crowbar

Normal operation

NOTES

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 20

Miniature, Low-Voltage 1

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