Datasheet
Calculate the Duty Cycle
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
MIN
) and maximum (D
MAX
) duty cycle is
calculated by:
OUT D
MIN
IN(MAX) SW LIM OUT D
VV
D
V VVV V
−
−−
+
=
−+
OUT D
MAX
IN(MIN) SW LIM OUT D
VV
D
V VVV V
−
−−−
+
=
+
where V
D
is the forward drop across the output diode,
V
SW
is the drop across the external FET when on,
and V
LIM
is the current-limit threshold. To begin with,
assume V
D
= 0.5V for a Schottky diode, V
SW
= 100mV,
and V
LIM
= 100mV. Remember that V
OUT
is negative
when using this formula.
Setting the Output Voltage
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
REF
is nominally
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
REF
is in regulation (see Electrical Characteristics
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
R2
is between 50µA and 250µA.
I
R2
= V
REF
/ R2
where V
REF
= 1.25V. A typical value for R2 is 10kW.
Once R2 is selected, calculate R1 with the following
equation:
R1 = R2 x (-V
OUT
/ V
REF
)
Setting the Operating Frequency
The MAX1846/MAX1847 are capable of operating at
switching frequencies from 100kHz to 500kHz. Choice
of operating frequency depends on a number of factors:
1) Noise considerations may dictate setting (or syn-
chronizing) f
OSC
above or below a certain fre-
quency or band of frequencies, particularly in RF
applications.
Figure 1. Using an N-Channel MOSFET (MAX1847 only)
REF
COMP
0.033µF
270kΩ
SYNC
150kΩ
GND
FREQ
VL
V
IN
+12V
V
OUT
-48V AT 100mA
12µF
100V
12µF
25V
2
14
13
12
6
5
3
4
0.05Ω
0.5W
1800pF
15
0.47µF
0.1µF
10, 11
EXT
PGND
IN
CS
FB
N.C.
MAX1847
10.0kΩ
1%
383kΩ
1%
7, 9
POL
1
8
16
VP1-0190
12.2µH
1:4
CMR1U-02
470Ω
100pF
100V
SHDN
IRLL2705
www.maximintegrated.com
Maxim Integrated
│
12
MAX1846–MAX1847 High-Efciency, Current-Mode,
Inverting PWM Controller