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MAX17499

Current-Mode PWM Controllers with Programmable Switching Frequency

Maxim Integrated 

Current-Mode PWM Controllers with

Programmable Switching Frequency

Soft-Start

The devices’ soft-start feature allows the output voltage

to ramp up in a controlled manner, eliminating voltage

overshoot. The devices’ reference generator that is

internally connected to the error amplifier soft-starts to

achieve superior control of the output voltage under

heavy- and light-load conditions. Soft-start begins after

UVLO is deasserted (VIN is above 21.6V for the

MAX17500, VIN is above 9.5V for the MAX17499, and

the voltage on UVLO/EN is above 1.23V). The voltage

applied to the noninverting node of the amplifier ramps

from 0 to 1.23V in 1984 NDRV switching cycles. Use the

following formula to calculate the soft-start time (tSS):

tSS

=

1984

fNDRV

where fNDRV is the switching frequency at the NDRV

output. Figure 4 shows the soft-start regulated output of

a power supply using the MAX17500 during startup.

n-Channel MOSFET Switch Driver

The NDRV output drives an external n-channel MOSFET.

The internal regulator output (VCC), set to approximately

9V, drives NDRV. For the universal input voltage range,

the MOSFET used must withstand the DC level of the

high-line input voltage plus the reflected voltage at the

primary of the transformer. Most applications that use the

discontinuous flyback topology require a MOSFET rated

at 600V. NDRV can source/sink in excess of 650mA/

1000mA peak current; therefore, select a MOSFET that

mA yields acceptable conduction and switching losses.

Oscillator/Switching Frequency

Use an external resistor at RT to program the devices’

internal oscillator frequency between 50kHz and 2.5MHz.

The MAX17499A/MAX17500A output switching frequen-

cy is one-half the programmed oscillator frequency with a

50% duty cycle. The MAX17499B/MAX17500B output

switching frequency is one-quarter of the programmed

oscillator frequency with a 75% duty cycle.

The MAX17499A/MAX17500A and MAX17499B/

MAX17500B have programmable output switching fre-

quencies from 25kHz to 625kHz and 12.5kHz to

625kHz, respectively. Use the following formulas to

determine the appropriate value of resistor R12 (see

Figure 1) needed to generate the desired output

switching frequency (fSW) at the NDRV output:

R12 = 1010 for the MAX17499A MAX17500A.

2fSW

R12 = 1010 for the MAX17499B MAX17500B.

4fSW

MAX17499/MAX17500 fig04

VOUT

2V/div

100mA LOAD ON/VOUT1

100mA LOAD ON/VOUT2

2ms/div

Figure 4. Primary-Side Output Voltage Soft-Start During Initial

Startup for the Circuit in Figure 6

where R12 is the resistor connected from RT to GND

(see Figure 1).

Connect an RC network in parallel with R12 as shown in

Figure 1. The RC network should consist of a 100nF

capacitor, C6, (for stability) in series with resistor R15,

which serves to further minimize jitter. Use the following

formula to determine the value of R15:

1

R15 = 88.9 × (R12)4

For example, if R12 is 4kΩ, R15 becomes 707Ω.

Internal Error Amplifier

The devices include an internal error amplifier to regu-

late the output voltage in the case of a nonisolated

power supply (see Figure 1). For the circuit in Figure 1,

calculate the output voltage using the following equation:

VOUT

=

⎛⎝⎜1+

R13 ⎞

R14 ⎠⎟

VREF

where VREF = 1.23V. The amplifier’s noninverting input

is internally connected to a digital soft-start circuit that

gradually increases the reference voltage during start-

up applied to this input. This forces the output voltage

to come up in an orderly and well-defined manner

under all load conditions.

The error amplifier may also be used to regulate the ter-

tiary winding output, which implements a primary-side-

regulated, isolated power supply (see Figure 6). For the

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