MAX15001 Maxim Integrated Products, MAX15001 Datasheet - Page 12

• Manufacturer Part Number: MAX15001
• Description: (MAX15000 / MAX15001) Current-Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

The current-sense resistor (R4 in Figure 1), connected
between the source of the MOSFET and ground, sets the
current limit. The current-limit comparator has a voltage
trip level (V
culate the value of R4:
where I
transformer which also flows through the MOSFET.
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (NDRV) termi-
nates the current on-cycle within 60ns (typ). Use a
small RC network to filter out the leading-edge spikes
on the sensed waveform when needed. Set the corner
frequency between 2MHz and 10MHz.
The bypass capacitor at IN, C1, supplies current imme-
diately after the MAX15000 wakes up (see Figure 1).
The size of C1 and the connection configuration of the
tertiary winding determine the number of cycles avail-
able for startup. Large values of C1 increase the start-
up time but also supply gate charge for more cycles
during initial startup. If the value of C1 is too small, V
drops below 9.74V because NDRV does not have
enough time to switch and build up sufficient voltage
across the tertiary output which powers the device. The
device goes back into UVLO and does not start. Use a
low-leakage capacitor for C1 and C2.
Typically, offline power supplies keep startup times to
less than 500ms even in low-line conditions (85VAC
input for universal offline or 36VDC for telecom applica-
tions). Size the startup resistor, R1, to supply both the
maximum startup bias of the device (90µA) and the
charging current for C1 and C2. The bypass capacitor,
C2, must charge to 9.5V and C1 to 24V, all within the
desired time period of 500ms. Because of the internal
soft-start time of the MAX15000 (approximately 5.6ms
Current-Mode PWM Controllers with
Programmable Switching Frequency
12
______________________________________________________________________________________
Startup Time Considerations for Power
PRI
is the peak current in the primary side of the
CS
) of 1V. Use the following equation to cal-
Applications Information
Supplies Using the MAX15000
R
4
V
I
PRI
CS
Current Limit
IN
when f
deliver current to the device for at least this much time.
To calculate the approximate amount of capacitance
required, use the following formula:
where I
(2mA) after startup, Q
Q1, f
(350kHz), V
(approximately 12V) and t
time (5.6ms).
Example: I
Choose a 2.2µF standard value (assuming 350kHz
switching frequency).
Assuming C1 > C2, calculate the value of R1 as follows:
where V
the application (36V for telecom), V
strap UVLO wake-up level (23.6V max), I
supply current at startup (90µA max).
For example:
Choose a 120k standard value.
SW
SW
IN
IN(MIN)
R
is the MAX15000’s switching frequency
= 350kHz), C1 must store enough charge to
g
C
is the MAX15000’s internal supply current
1
1
= (8nC) (350kHz)
HYST
( .
0 105
(
I
R
is the minimum input supply voltage for
2
C1
1
mA
(
36
C
is the bootstrap UVLO hysteresis
I
I
C
g
1
V
mA
(
V
1
IN MIN
24
I
) (
C
2 8
(
gtot
(
Q
500
(
1
.
V
) (
I
12
V
IN
(
gtot SW
)( .
12
SUVR
500
mA
2 2
V
V
ms
90
)
is the total gate charge for
SS
V
HYST
I
I
START
f
)( .
ms
)
g
V
)
)(
5 6
C
A
F
is the internal soft-start
SUVR
t
)
2.8mA
1
)
)
SS
ms
)
0 105
123 07
)
.
SUVR
.
2 24
.
START
mA
k
is the boot-
F
is the IN