MAX1847EEE Maxim Integrated Products, MAX1847EEE Datasheet - Page 12

• Manufacturer Part Number: MAX1847EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1847EEE

• Case: ssop
• Dc: 05+
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

High-Efficiency, Current-Mode,
Inverting PWM Controller
Figure 1. Using an N-Channel MOSFET (MAX1847 only)
The duty cycle is the ratio of the on-time of the MOSFET
switch to the oscillator period. It is determined by the
ratio of the input voltage to the output voltage. Since
the input voltage typically has a range of operation, a
minimum (D
calculated by:
where V
V
V
assume V
and V
when using this formula.
The output voltage is set using two external resistors to
form a resistive-divider to FB between the output and
REF (refer to R1 and R2 in Figure 1). V
12
SW
LIM
D
______________________________________________________________________________________
D
is the drop across the external FET when on, and
is the current-limit threshold. To begin with,
MAX
LIM
MIN
D
D
= 100mV. Remember that V
is the forward drop across the output diode,
=
=
= 0.5V for a Schottky diode, V
MIN
V
V
IN MAX
IN MIN
) and maximum (D
(
(
Setting the Output Voltage
0.033µF
)
)
−
−
V
V
−
SW
−
SW
V
V
0.47µF
OUT
OUT
−
Calculate the Duty Cycle
−
150kΩ
V
V
270kΩ
LIM
+12V
LIM
+
V
+
IN
V
−
V
MAX
D
−
D
V
V
OUT
OUT
REF
OUT
16
4
3
5
8
) duty cycle is
REF
SW
COMP
FREQ
0.1µF
SHDN
SYNC
VL
2
is nominally
+
is negative
+
POL
MAX1847
= 100mV,
V
1
V
D
D
IN
15
GND
PGND
10, 11
N.C.
EXT
CS
FB
12µF
25V
12
14
13
7, 9
6
VP1-0190
IRLL2705
1.25V and the regulation voltage for FB is nominally 0.
The load presented to the reference by the feedback
resistors must be less than 500µA to guarantee that
V
Table ). Conversely, the current through the feedback
resistors must be large enough so that the leakage cur-
rent of the FB input (50nA) is insignificant. Therefore,
select R2 so that I
where V
Once R2 is selected, calculate R1 with the following
equation:
The MAX1846/MAX1847 are capable of operating at
switching frequencies from 100kHz to 500kHz. Choice
of operating frequency depends on a number of fac-
tors:
1)
0.05Ω
12.2µH
0.5W
REF
chronizing) f
quency or band of frequencies, particularly in RF
applications.
Noise considerations may dictate setting (or syn-
is in regulation (see Electrical Characteristics
REF
1800pF
1:4
CMR1U-02
= 1.25V. A typical value for R2 is 10kΩ.
470Ω
100pF
100V
Setting the Operating Frequency
R1 = R2 x (-V
R2
OSC
I
R2
is between 50µA and 250µA.
383kΩ
1%
10.0kΩ
1%
= V
above or below a certain fre-
12µF
100V
V
-48V AT 100mA
OUT
REF
OUT
/ R2
/ V
REF
)