MAX669EUB/V Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX669EUB/V

1.8V to 28V Input, PWM Step-Up Controllers in µMAX

Maxim Integrated 

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

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 ROSC, connected from FREQ to GND.

The relationship between fOSC and ROSC is:

ROSC = 5 x 1010 / fOSC

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

with ROSC = 100kΩ.

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 ROSC. 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, ROSC should be set for a frequency 15%

below the SYNC clock rate:

ROSC(SYNC) = 5 x 1010 / (0.85 x fSYNC)

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 RCS, to the full current-limit value, in five

steps over 1024 cycles of fOSC or fSYNC. For example,

with an fOSC 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) fOSC 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, ROSC, con-

nected from FREQ to GND. ROSC must be connected

whether or not the part is externally synchronized ROSC

is in each case:

ROSC = 5 x 1010 / fOSC

when not using an external clock.

ROSC(SYNC) = 5 x 1010 / (0.85 x fSYNC)

when using an external clock, fSYNC.

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 10kΩ to 1MΩ range. R2 is then given by:

R2 = R3 [(VOUT / VREF) – 1]

where VREF is 1.25V.

Determining Inductance Value

For most MAX668/MAX669 boost designs, the inductor

value (LIDEAL) 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:

LIDEAL = VOUT / (4 x IOUT x fOSC)

The MAX668/MAX669 allow significant latitude in induc-

tor selection if LIDEAL is not a convenient value. This

may happen if LIDEAL is a not a standard inductance

(such as 10µH, 22µH, etc.), or if LIDEAL is too large to

be obtained with suitable resistance and saturation-cur-

rent rating in the desired size. Inductance values small-

er than LIDEAL may be used with no adverse stability

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

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

raising the required ILPK for a given output power and

also requiring larger output capacitance to maintain a

______________________________________________________________________________________ 13

Share Link: