MAX1856EUB+ Maxim Integrated Products, MAX1856EUB+ Datasheet - Page 11

• Manufacturer Part Number: MAX1856EUB+
• Description: IC PS PWM SLIC SYNCH 10-UMAX
• Manufacturer: Maxim Integrated Products

Specifications of MAX1856EUB+

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 575kHz
• Duty Cycle: 94%
• Voltage - Supply: 3 V ~ 28 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: Yes
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Frequency-max: 575kHz
• Output Voltage: 0 V to - 185 V
• Output Current: 0.3 A
• Input Voltage: 3 V to 28 V
• Switching Frequency: 500 KHz
• Mounting Style: SMD/SMT
• Duty Cycle (max): 94 %
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

100kHz to 500kHz frequency, which is set by a resistor
(R
between f
Thus, a 250kHz operating frequency, for example, is
set with R
magnetic components will be smaller. Peak currents
and, consequently, resistive losses will be lower at the
higher switching frequency. However, core losses, gate
charge currents, and switching losses increase with
higher switching frequencies.
Rising clock edges on SYNC/SHDN are interpreted as
synchronization input. 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
SYNC/SHDN low, the IC waits for 50µs before shutting
down. This maintains output regulation even with inter-
mittent 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
quency 15% below the SYNC clock rate:
Set the output voltage using two external resistors form-
ing a resistive divider to FB between the output and
REF. First select a value for R3 between 3.3kΩ and
100kΩ. R1 is then given by:
For a dual output as shown in Figure 1, a split feedback
technique is recommended. Since the feedback volt-
age threshold is 0, the total feedback current is:
Since the feedback resistors are connected to the ref-
erence, I
teed to be in regulation (see Electrical Characteristics
Table). Therefore, select R3 so the total current value is
OSC
) connected from FREQ to GND. The relationship
TOTAL
OSC
OSC
R
OSC SYNC
I
and R
TOTAL
R
must be <400µA so that V
= 200kΩ. At higher frequencies, the
OSC
R
(
1
______________________________________________________________________________________
=
OSC
Setting the Output Voltage
=
=
R
OSC
3
50 Ω
I
)
R
ƒ
1
=
is:
OSC
OSC



M
+
V
. If the signal is lost with
0 85
V
OUT
REF
I
.
50
R
(
×
2
kHz
should be set for a fre-
M
ƒ
kHz
=
Wide Input Range, Synchronizable,



Ω
OSC
V
)
REF
R
×
3
kHz
(
kHz
REF
)
is guaran-
PWM SLIC Power Supply
between 200µA and 250µA as shown in Figure 1. To
ensure that the MAX1856 regulates both outputs with
the same degree of accuracy over load, select the
feedback resistors (R1 and R2) so their current ratio
(I
under full load:
Once R3 and the dual feedback currents (I
are determined from the two equations above, use the
following two equations to determine R1 and R2:
The MAX1856 PWM controller works with economical
off-the-shelf transformers. The transformer selection
depends on the input-to-output voltage ratio, output
current capacity, duty cycle, and oscillator frequency.
Table 1 shows recommended transformers for the typi-
cal applications, and Table 2 gives some recommend-
ed suppliers.
The transformer turns ratio is a function of the input-to-
output voltage ratio and maximum duty cycle. Under
steady-state conditions, the change in flux density dur-
ing the on-time must equal the return change in flux
density during the off-time (or flyback period):
For example, selecting a 50% duty cycle for the stan-
dard application circuit (Figure 1) and a +12V input
voltage, the -72V output requires a 1:6 turns ratio, and
the -24V output requires a 1:2 turns ratio. Therefore, a
transformer with a 1:2:2:2 turns ratio was selected.
The average input current at maximum load can be cal-
culated as:
where η = efficiency. For V
400mA, and V
R1
:I
R2
) equals the output power ratio (P
I
R
I
IN DC
1
IN(MIN)
=
(
V t
V
IN ON
OUT
N
)
R
I
I
R
R
=
1
P
2
1
Selecting the Transformer
= 10.8V as shown in Figure 1, this
1
V
=
OUT OUT MAX
and I
=
V
V
η
OUT OUT
OUT OUT
V
V
OUT OFF
IN MIN
I
R
OUT
Transformer Turns Ratio
N
(
2
1
2
S
I
I
t
=
(
)
V
= -24V, I
Primary inductance
OUT
1
2
R
)
2
2
OUT1
OUT(MAX)
R1
and I
:P
OUT2
R2
11
=
)
)