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参数资料
| 型号： | MAX669EUB+T |
| 厂商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | 1.8V to 28V Input, PWM Step-Up Controllers in µMAX |
| 中文描述： | SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | COMPACT, UMAX-10 |
| 文件页数： | 5/18页 |
| 文件大小： | 289K |
| 代理商： | MAX669EUB+T |
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 = 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 ROSC. If sync is lost with SYNC/SHDN
low, the IC waits for 70s 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 10k
to 1M 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 10H, 22H, 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
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in MAX
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