MAX15000/MAX15001
Current Limit
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
CS
) of 1V. Use the following equation to cal-
culate the value of R4:
where I
PRI
is the peak current in the primary side of the
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.
Applications Information
Startup Time Considerations for Power
Supplies Using the MAX15000
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
IN
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
when f
SW
= 350kHz), C1 must store enough charge to
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
IN
is the MAX15000’s internal supply current
(2mA) after startup, Q
gtot
is the total gate charge for
Q1, f
SW
is the MAX15000’s switching frequency
(350kHz), V
HYST
is the bootstrap UVLO hysteresis
(approximately 12V) and t
SS
is the internal soft-start
time (5.6ms).
Example: I
g
= (8nC) (350kHz) ≅ 2.8mA
Choose a 2.2µF standard value (assuming 350kHz
switching frequency).
Assuming C1 > C2, calculate the value of R1 as follows:
where V
IN(MIN)
is the minimum input supply voltage for
the application (36V for telecom), V
SUVR
is the boot-
strap UVLO wake-up level (23.6V max), I
START
is the IN
supply current at startup (90µA max).
For example:
Choose a 120kΩ standard value.
.
(. ) ( )
.
(.)(.)
. µ
