Low-Profile, 5V/12V or Adjustable, Step-Up

DC-DC Converters for Flash Memory/PCMCIA Cards

10

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When FB is connected to IN or GND, an internal volt-

age divider is configured to produce a predetermined

output. However, when the voltage at FB is between

the adjustable output mode. In this mode, the

MAX606/MAX607 output voltage is set by two external

resistors, R1 and R2 (Figure 4), which form a voltage

divider between the output and FB. Use the following

equation to determine the output voltage:

Since the input current at FB is 200nA maximum, large

values (up to 100k

Ω) can be used for R2 with no signifi-

cant loss of accuracy. For 1% error, the current through

R2 should be at least 100 times the FB input bias current.

Soft-Start

Connecting a capacitor to the Soft-Start (SS) pin

ensures a gradually increasing current limit during

power-up or when exiting shutdown, thereby reducing

initial inrush currents. This feature can be useful, for

example, when an old battery’s increased series resis-

tance limits initial inrush currents. Using the soft-start

feature in a situation like this minimizes the risk of over-

loading the incoming supply.

Soft-start timing is controlled by the value of the SS

capacitor. On power-up, the SS capacitor is charged by

the 2V reference through an internal, 45k

Ω pull-up resis-

tor. As the voltage on the SS pin increases, the voltage

at the SS clamp output also increases, which in turn

raises the current-limit threshold. The Start-Up Delay vs.

SS Capacitor graph in the

Typical Operating Charac-

teristics shows typical timing characteristics for selected

capacitor values and circuit conditions. The soft-start

capacitor is discharged each time the MAX606 or

MAX607 is put into shutdown, including during under-

voltage lockout and when powering down at IN.

If the circuit is required to start up with no load, as in

flash memory programming supplies, soft-start is not

required. Omitting the soft-start capacitor permits a

minimum output voltage rise time from the shutdown

state, improving flash memory access time.

Undervoltage Lockout

The MAX606/MAX607 monitor the supply voltage at IN

and operate for supply voltages greater than 2.8V.

When an undervoltage condition is detected, control

logic turns off the output power FET and discharges the

soft-start capacitor to ground. The control logic holds

the output power FET in an off state until the supply

voltage rises above the undervoltage threshold, at

which time a soft-start cycle begins.

Shutdown Mode

Connecting SHDN to GND will hold the MAX606/

MAX607 in shutdown mode. In shutdown, the output

power FET is off, but there is still an external path from

IN to the load via the inductor and diode. The internal

reference also turns off, which causes the soft-start

capacitor to discharge. Typical device standby current

in shutdown mode is 0.01µA. For normal operation,

connect SHDN to IN. A soft-start cycle is initiated when

the MAX606/MAX607 exit shutdown.

Applications Information

Inductor Selection

Use a 5µH inductor for the MAX606 and a 10µH induc-

tor for the MAX607. See Table 3 for a list of component

suppliers. Higher inductor values allow greater load

currents due to operation in continuous-conduction

mode, while lower inductor values lead to smaller phys-

ical size due to lower energy-storage requirements and

lower output-filter-capacitor requirements. Potential

drawbacks of using lower inductor values are

increased output ripple, lower efficiency, and lower out-

put-current capability due to operation in discontinu-

ous-conduction mode. (See the Maximum Output

Current vs. Inductor Value graph in the

Typical

Operating Characteristics.)

The inductor must have a saturation (incremental) cur-

rent rating equal to the peak switch-current limit, which

is 1.1A. For highest efficiency, minimize the inductor’s

DC resistance.

R1

10pF

R2

MAX606

MAX607

LX

FB

OUT

PGND

GND

IN

OUTPUT

INPUT

+ 1

)

R1

R2

Figure 4. Adjustable Output Voltage