MAX17499AAUB 查看數據表（PDF） - Maxim Integrated

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MAX17499AAUB

Current-Mode PWM Controllers with Programmable Switching Frequency

Maxim Integrated 

Current-Mode PWM Controllers with

Programmable Switching Frequency

Power supplies designed with the MAX17500 use a

high-value startup resistor, R1, that charges a reservoir

capacitor, C1 (see Figure 1). During this initial period,

while the voltage is less than the internal bootstrap

UVLO threshold, the device typically consumes only

50μA of quiescent current. This low startup current and

the large bootstrap UVLO hysteresis help to minimize

the power dissipation across R1 even at the high end of

the universal AC input voltage (265V AC).

The devices include a cycle-by-cycle current limit that

turns off the gate drive to the external MOSFET when-

ever the internally set threshold of 1V is exceeded.

When using the MAX17500 in bootstrapped mode, if

the power-supply output is shorted, the tertiary winding

voltage drops below the internally set threshold caus-

ing the UVLO to turn off the gate drive to the external

power MOSFET. This reinitiates a startup sequence

with soft-start.

Current-Mode Control Loop

The advantages of current-mode control over voltage-

mode control are twofold. First, there is the feed-for-

ward characteristic brought on by the controller’s

ability to adjust for variations in the input voltage on a

cycle-by-cycle basis. Secondly, the stability require-

ments of the current-mode controller are reduced to

that of a single-pole system unlike the double pole in

voltage-mode control.

The devices use a current-mode control loop where the

output of the error amplifier (COMP) is compared to the

current-sense voltage at CS. When the current-sense

signal is lower than the noninverting input of the CPWM

comparator, the output of the CPWM comparator is low

and the switch is turned on at each clock pulse. When

the current-sense signal is higher than the inverting input

of the CPWM, the output of the CPWM comparator goes

high and the switch is turned off.

Undervoltage Lockout

The devices provide a UVLO/EN input. The threshold

for UVLO is 1.23V with 60mV hysteresis. Before any

operation can commence, the voltage on UVLO/EN has

to exceed 1.23V. The UVLO circuit keeps the CPWM

comparator, ILIM comparator, oscillator, and output dri-

ver shut down to reduce current consumption (see the

Functional Diagram).

Use this UVLO/EN input to program the input-supply

start voltage. For example, a reasonable start voltage

for a 36V to 72V telecom range is usually 34V.

Calculate the resistor-divider values, R2 and R3 (see

Figure 1) by using the following formulas:

R3 ≅

VULR2 VIN

500 IUVLO (VIN − VULR2)

R2 = VIN − VULR2 R3

VULR2

where IUVLO is the UVLO/EN input current (50nA max),

and VULR2 is the UVLO/EN wake-up threshold (1.23V).

VIN is the value of the input-supply voltage where the

power supply must start. The value of R3 is calculated

to minimize the voltage-drop error across R2 as a result

of the input bias current of the UVLO/EN input.

MAX17500 Bootstrap UVLO

In addition to the externally programmable UVLO func-

tion offered in both devices, the MAX17500 includes an

internal bootstrap UVLO that is very useful when

designing high-voltage power supplies (see the

Functional Diagram). This allows the device to bootstrap

itself during initial power-up. The MAX17500 attempts to

start when VIN exceeds the bootstrap UVLO threshold

of 21.6V. During startup, the UVLO circuit keeps the

CPWM comparator, ILIM comparator, oscillator, and

output driver shut down to reduce current consumption.

Once VIN reaches 21.6V, the UVLO circuit turns on the

CPWM and ILIM comparators, the oscillator, and allows

the output driver to switch. If VIN drops below 1.17V, the

UVLO circuit shuts down the CPWM comparator, ILIM

comparator, oscillator, and output driver returning the

MAX17500 to the low-current startup mode.

Startup Operation

The MAX17499 starts up when the voltage at IN

exceeds 9.5V and the UVLO/EN input is greater than

1.23V. However, the MAX17500 requires that, in addi-

tion to meeting the specified startup conditions for the

MAX17499, the voltage at IN exceeds the bootstrap

UVLO threshold of 21.6V.

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