MAX5003 데이터 시트보기 (PDF) - Maxim Integrated

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MAX5003

High-Voltage PWM Power-Supply Controller

Maxim Integrated 

High-Voltage PWM

Power-Supply Controller

zero at 2kHz, the crossover frequency is 4kHz and the

phase margin is 50°.

Given the above considerations, RA, RB , RF, and CF

can be chosen (Figure 2). The sum of RA and RB is

chosen for low current drain. In the example, RA plus

RB is 58kΩ and draws 80µA. The following ratio sets

the output voltage:

RB / (RA + RB) = VSET / VOUT

Since VSET = 1.5V and VOUT = 5V, RA is set to 41.2kΩ

and RB to 17.4kΩ.

The midband gain is the ratio of RF/RA. RB does not

affect the gain because it is connected to a virtual

ground. For a midband gain of 5, the feedback resistor

equals 200kΩ. To set the zero at 2kHz, the capacitor

value is:

CF = 1 / (2π x RF x fz ) = 400pF

Layout Recommendations

All connections carrying pulsed currents must be very

short, be as wide as possible, and have a ground plane

behind them whenever possible. The inductance of

these connections must be kept to an absolute mini-

mum due to the high di/dt of the currents in high-

frequency switching power converters. In the develop-

ment or prototyping process, multipurpose boards, wire

wrap, and similar constructive practices are not suit-

able for these type of circuits; attempts to use them will

fail. Instead, use milled PC boards with a ground plane,

or equivalent techniques

Current loops must be analyzed in any layout pro-

posed, and the internal area kept to a minimum to

reduce radiated EMI. The use of automatic routers is

discouraged for PC board layout generation in the

board area where the high-frequency switching con-

verters are located. Designers should carefully review

the layout. In particular, pay attention to the ground

connections. Ground planes must be kept as intact as

possible. The ground for the power-line filter capacitor

and the ground return of the power switch or current-

sensing resistor must be close. All ground connections

must resemble a star system as much as practical.

“Short” and “close” are dimensions on the order of

0.25in to 0.5in (0.5cm to about 1cm).

Setting the Output Voltage

The output voltage of the converter, if using the internal

error amplifier, can easily be set by the value of the FB

pin set voltage. This value is 1.5V. A resistive divider

must be calculated from the output line to ground, with

a dividing ratio such that when the output is at the

desired value, the center-point voltage will be 1.5V. The

Thevenin equivalent of the resistors must be low

enough so the error amplifier bias current will not intro-

duce a division error. The two resistors must have simi-

lar temperature coefficients (tempcos), so the dividing

ratio will be constant with temperature.

Component Selection

CS Resistor

The CS resistor is connected in series with the source

of the N-channel MOSFET and ground, sensing the

switch current. Its value can be calculated from the fol-

lowing equation:

RCS = 100mV =

ILIM(PRI)

100mV

2 PWROUT(MAX)

×

K

TOL

LPRI × ƒSW × η

where η = efficiency and 0.5 < KTOL < 0.75.

KTOL includes the tolerance of the sensing resistor, the

dispersion of the MAX5003 CS trip point, and the

uncertainties in the calculation of the primary maximum

current.

The sensing resistor must be of the adequate power

dissipation and low tempco. It must also be noninduc-

tive and physically short. Use standard surface-mount

CS resistors. A 100Ω resistor is recommended between

the CS resistor and the CS pin. If the current surge at

the beginning of the conduction period is large and dis-

rupts the MAX5003’s operation, add a capacitor

between the CS pin and PGND, to form an RC filter.

Power Switch

The MAX5003 will typically drive an N-channel MOSFET

power switch. The maximum drain voltage, maximum

RDS(ON), and total gate switching charge are the para-

meters involved in choosing the FET. The maximum

gate switching charge is the most important factor

defining the MAX5003 internal power consumption,

since the product of the switching frequency and the

total gate charge is the IC current consumption.

RDS(ON) is the parameter that determines the total con-

duction power losses in the switch, and the choice

depends on the expected efficiency and the cooling

and mounting method. The maximum drain voltage

requirements can be different depending on the topolo-

gy used. In the flyback configuration, the maximum

voltage is the maximum supply voltage plus the reflect-

ed secondary voltage, any ringing at the end of the

conduction period, and the spike caused by the leak-

age inductance. In the case of the forward converter,

the reset time of the core will set the maximum voltage

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