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Design procedure – Rainbow Electronics MAX669 User Manual

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SYNC/

SHDN and FREQ Inputs

The SYNC/SHDN pin provides both external-clock syn-
chronization (if desired) and shutdown control. When
SYNC/SHDN is low, all IC functions are shut down. A
logic high at SYNC/SHDN selects operation at a fre-
quency set by R

OSC

, connected from FREQ to GND.

The relationship between f

OSC

and R

OSC

is:

R

OSC

= 5 x 10

10

/ f

OSC

So a 500kHz operating frequency, for example, is set
with R

OSC

= 100k

.

Rising clock edges on SYNC/SHDN are interpreted as
synchronization inputs. If the sync signal is lost while
SYNC/SHDN is high, the internal oscillator takes over at
the end of the last cycle and the frequency is returned
to the rate set by R

OSC

. If sync is lost with SYNC/SHDN

low, the IC waits for 70µs before shutting down. This
maintains output regulation even with intermittent sync
signals. When an external sync signal is used, Idle
Mode switchover at the 15mV current-sense threshold
is disabled so that Idle Mode only occurs at very light
loads. Also, R

OSC

should be set for a frequency 15%

below the SYNC clock rate:

R

OSC(SYNC)

= 5 x 10

10

/ (0.85 x f

SYNC

)

Soft-Start

The MAX668/MAX669 feature a “digital” soft start which
is preset and requires no external capacitor. Upon
start-up, the peak inductor increments from 1/5 of the
value set by R

CS

, to the full current-limit value, in five

steps over 1024 cycles of f

OSC

or f

SYNC

. For example,

with an f

OSC

of 200kHz, the complete soft-start

sequence takes 5ms. See the

Typical Operating

Characteristics for a photo of soft-start operation. Soft-
start is implemented: 1) when power is first applied to
the IC, 2) when exiting shutdown with power already
applied, and 3) when exiting undervoltage lockout. The
MAX669’s soft-start sequence does not start until LDO
reaches 2.5V.

Design Procedure

The MAX668/MAX669 can operate in a number of DC-
DC converter configurations including step-up, SEPIC
(single-ended primary inductance converter), and fly-
back. The following design discussions are limited to
step-up, although SEPIC and flyback examples are
shown in the

Application Circuits section.

Setting the Operating Frequency

The MAX668/MAX669 can be set to operate from
100kHz to 500kHz. Choice of operating frequency will
depend on number of factors:

1) Noise considerations may dictate setting (or syn-

chronizing) f

OSC

above or below a certain frequency

or band of frequencies, particularly in RF applica-
tions.

2) Higher frequencies allow the use of smaller value

(hence smaller size) inductors and capacitors.

3) Higher frequencies consume more operating power

both to operate the IC and to charge and discharge
the gate of the external FET. This tends to reduce
efficiency at light loads; however, the MAX668/
MAX669’s Idle Mode feature substantially increases
light-load efficiency.

4) Higher frequencies may exhibit poorer overall effi-

ciency due to more transition losses in the FET;
however, this shortcoming can often be nullified by
trading some of the inductor and capacitor size
benefits for lower-resistance components.

The oscillator frequency is set by a resistor, R

OSC

, con-

nected from FREQ to GND. R

OSC

must be connected

whether or not the part is externally synchronized R

OSC

is in each case:

R

OSC

= 5 x 10

10

/ f

OSC

when

not using an external clock.

R

OSC(SYNC)

= 5 x 10

10

/ (0.85 x f

SYNC

)

when using an external clock, f

SYNC

.

Setting the Output Voltage

The output voltage is set by two external resistors (R2
and R3, Figures 2, 3, 4, and 5). First select a value for
R3 in the 10k

to 1M

range. R2 is then given by:

R2 = R3 [(V

OUT

/ V

REF

) – 1]

where V

REF

is 1.25V.

Determining Inductance Value

For most MAX668/MAX669 boost designs, the inductor
value (L

IDEAL

) can be derived from the following equa-

tion, which picks the optimum value for stability based
on the MAX668/MAX669’s internally set slope compen-
sation:

L

IDEAL

= V

OUT

/ (4 x I

OUT

x f

OSC

)

The MAX668/MAX669 allow significant latitude in induc-
tor selection if L

IDEAL

is not a convenient value. This

may happen if L

IDEAL

is a not a standard inductance

(such as 10µH, 22µH, etc.), or if L

IDEAL

is too large to

be obtained with suitable resistance and saturation-cur-
rent rating in the desired size. Inductance values small-
er than L

IDEAL

may be used with no adverse stability

effects; however, the peak-to-peak inductor current
(I

LPP

) will rise as L is reduced. This has the effect of

raising the required I

LPK

for a given output power and

also requiring larger output capacitance to maintain a

MAX668/MAX669

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

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