Detailed Description

The MAX15000/MAX15001 current-mode PWM con-

trollers are ideal for isolated and nonisolated power-

supply applications. The devices offer an accurate

input startup voltage programmable through the

UVLO/EN input. This feature prevents the power supply

from entering a brownout condition in case the input

voltage sags below its minimum value. This is important

since switching power supplies increases their input

supply current as the input voltage drops to keep the

output power constant. In addition to this externally

adjustable UVLO feature, the MAX15000 also offers a

bootstrap UVLO with a large hysteresis (11.9V) and

very low startup and operating current, which result in

an efficient universal input power supply. The switching

frequency of the MAX15000/MAX15001 is programma-

ble with an external resistor.

The MAX15000 is well suited for universal input (recti-

fied 85VAC to 265VAC) or telecom (-36VDC to

-72VDC) power supplies. The MAX15001 is well suited

for low-input voltage (9.5VDC to 24VDC) power sup-

plies. The devices include an internal clamp at IN to

prevent the input voltage from exceeding the absolute

maximum rating (see Note 2 at the end of the Electrical

Characteristics table). The input is clamped when the

devices are started with a bleed resistor (R1 in Figure 1)

from a high input voltage and the UVLO/EN input is low.

The clamp can safely sink up to 2mA current.

Power supplies designed with the MAX15000 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 (265VAC).

The MAX15000/MAX15001 include a cycle-by-cycle

current limit that turns off the gate drive to the external

MOSFET whenever the internally set threshold of 1V is

exceeded. When using the MAX15000 in the boot-

strapped mode, if the power-supply output is shorted,

the tertiary winding voltage will drop below the internal-

ly set threshold causing the UVLO to turn off the gate

drive to the external power MOSFET. This will reinitiate

a startup sequence with soft-start.

Current-Mode PWM Controllers with

Programmable Switching Frequency

8

_______________________________________________________________________________________

8

9

10

3

2

1

UVLO/EN

IN

7

4

NDRV

GND

UFLG

FB

COMP

6

5

RT

CS

T1

R3

R2

R12

R1

R4

C5

C2

C2

C1

C4

D2

D1

C6

R11

0V

R13

R14

R15

Q1

MAX15000

Figure 1. Nonisolated Power Supply with Programmable Input-Supply Start Voltage