Rainbow Electronics MAX15001 - Manual (Page 8)

MAX15000/MAX15001
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
V CC
NDRV
GND
UFLG
FB
COMP
6 5
RT CS
T1
R3
R2
R12
R1
R4
C5
C2
C2 C1
C4
D2
D1
C6
R11
V SUPPLY
0V
R13
R14
V OUT
R15
Q1
MAX15000
Figure 1. Nonisolated Power Supply with Programmable Input-Supply Start Voltage