MAX15000BEUB 查看數據表（PDF） - Maxim Integrated

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MAX15000BEUB

Current-Mode PWM Controllers with Programmable Switching Frequency

Maxim Integrated 

Current-Mode PWM Controllers with

Programmable Switching Frequency

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 (VCS) of 1V. Use the following equation to cal-

culate the value of R4:

R4 = VCS

IPRI

where IPRI 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, VIN

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 fSW = 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:

Ig = Qgtot fSW

C1= (IIN + Ig)(tSS)

VHYST

where IIN is the MAX15000’s internal supply current

(2mA) after startup, Qgtot is the total gate charge for

Q1, fSW is the MAX15000’s switching frequency

(350kHz), VHYST is the bootstrap UVLO hysteresis

(approximately 12V) and tSS is the internal soft-start

time (5.6ms).

Example: Ig = (8nC) (350kHz) ≅ 2.8mA

C1= (2mA + 2.8mA)(5.6ms) = 2.24µF

12V

Choose a 2.2µF standard value (assuming 350kHz

switching frequency).

Assuming C1 > C2, calculate the value of R1 as follows:

IC1

=

VSUVR C1

(500ms)

R1≅ VIN(MIN) − VSUVR

IC1 + ISTART

where VIN(MIN) is the minimum input supply voltage for

the application (36V for telecom), VSUVR is the boot-

strap UVLO wake-up level (23.6V max), ISTART is the IN

supply current at startup (90µA max).

For example:

IC1

=

(24V)(2.2µF)

(500ms)

=

0.105mA

R1≅ (36V) − (12V) = 123.07kΩ

(0.105mA) + (90µA)

Choose a 120kΩ standard value.

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