MAX668EUB+T Maxim Integrated Products, MAX668EUB+T Datasheet - Page 13

• Manufacturer Part Number: MAX668EUB+T
• Description: IC PWM BST FLYBCK ISO CM 10UMAX
• Manufacturer: Maxim Integrated Products

Specifications of MAX668EUB+T

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 575kHz
• Duty Cycle: 94%
• Voltage - Supply: 3 V ~ 28 V
• Buck: No
• Boost: Yes
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Frequency-max: 575kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
The relationship between f
So a 500kHz operating frequency, for example, is set
with R
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
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
below the SYNC clock rate:
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
steps over 1024 cycles of f
with an f
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.
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.
The MAX668/MAX669 can be set to operate from
100kHz to 500kHz. Choice of operating frequency will
depend on number of factors:
OSC
R
OSC(SYNC)
OSC
= 100kΩ.
Setting the Operating Frequency
OSC
CS
of 200kHz, the complete soft-start
R
OSC
SYNC/
, to the full current-limit value, in five
OSC
______________________________________________________________________________________
OSC
should be set for a frequency 15%
= 5 x 10
, connected from FREQ to GND.
= 5 x 10
. If sync is lost with SYNC/SHDN
SHDN and FREQ Inputs
OSC
Design Procedure
OSC
10
10
and R
/ (0.85 x f
or f
/ f
OSC
SYNC
OSC
1.8V to 28V Input, PWM Step-Up
SYNC
. For example,
is:
Soft-Start
)
1) Noise considerations may dictate setting (or syn-
2) Higher frequencies allow the use of smaller value
3) Higher frequencies consume more operating power
4) Higher frequencies may exhibit poorer overall effi-
The oscillator frequency is set by a resistor, R
nected from FREQ to GND. R
whether or not the part is externally synchronized R
is in each case:
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:
where V
For most MAX668/MAX669 boost designs, the inductor
value (L
tion, which picks the optimum value for stability based
on the MAX668/MAX669’s internally set slope compen-
sation:
The MAX668/MAX669 allow significant latitude in induc-
tor selection if L
may happen if L
(such as 10µH, 22µH, etc.), or if L
be obtained with suitable resistance and saturation-cur-
rent rating in the desired size. Inductance values small-
er than L
effects; however, the peak-to-peak inductor current
(I
raising the required I
also requiring larger output capacitance to maintain a
LPP
chronizing) f
or band of frequencies, particularly in RF applica-
tions.
(hence smaller size) inductors and capacitors.
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.
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.
) will rise as L is reduced. This has the effect of
Controllers in µMAX
REF
IDEAL
R
when using an external clock, f
IDEAL
OSC(SYNC)
when not using an external clock.
L
is 1.25V.
IDEAL
) can be derived from the following equa-
R2 = R3 [(V
OSC
may be used with no adverse stability
Determining Inductance Value
R
IDEAL
IDEAL
OSC
= V
above or below a certain frequency
Setting the Output Voltage
= 5 x 10
LPK
OUT
= 5 x 10
is not a convenient value. This
is a not a standard inductance
OUT
for a given output power and
/ (4 x I
10
/ V
OSC
10
/ (0.85 x f
OUT
REF
/ f
OSC
IDEAL
must be connected
) – 1]
x f
OSC
SYNC
SYNC
is too large to
)
.
OSC
)
, con-
OSC
13