Rainbow Electronics MAX5043 - Manual (Page 12)

MAX5042/MAX5043
Detailed Description
The MAX5042/MAX5043 PWM multimode power ICs
are designed for the primary side of voltage or current-
mode isolated, forward or flyback power converters.
These devices provide a high degree of integration
aimed at reducing the cost and PC board area of isolat-
ed output power supplies. Use the MAX5042/MAX5043
primarily for 24V, 42V, or 48V power bus applications.
The MAX5042/MAX5043 provide a complete system
capable of delivering up to 50W of output power. The
MAX5042 contains a hot-swap controller in addition to
the PWM and power MOSFETs. The hot-swap section
requires an external MOSFET (QHS). Figure 1 details
the MAX5042 conceptual block diagram. C
IN represents
the input bulk storage capacitance of the PWM circuit
that requires the soft-start to reduce the inrush current
from the backplane. When input power is applied,
capacitor C
IN is completely discharged and QHS is off.
An applied voltage higher than the default undervoltage
lockout threshold of the hot-swap controller (30.5V) for
more than 165ms (internal turn-on delay) causes the
gate voltage of QHS to start gradually increasing. This
results in a controlled slew-rate turn-on. The drain volt-
age of QHS falls at a rate of approximately 10V/ms,
drawing a current load from the backplane of approxi-
mately 1A for each 100µF of C
IN capacitance. The
MAX5042’s PWM block is prevented from starting up
until the QHS MOSFET is fully enhanced. After QHS
completely turns on and the voltage across capacitor
C
IN is above the default startup voltage (31V) of the
PWM section, the hot swap enables the PWM block and
the soft-start cycle begins. Soft-start limits the amount of
current initially drawn from the primary during startup
and also prevents possible output-voltage overshoots.
The MAX5043, detailed in Figure 2, does not contain an
integrated hot-swap controller. The MAX5043 begins
operating when the input voltage exceeds both of the
undervoltage lockout voltages (at UVLO and DEN pins)
for 10ms.
The MAX5042/MAX5043 support both forward and fly-
back power topologies. In forward mode, the maximum
output power is approximately 50W. In flyback mode,
the maximum output power is approximately 20W. The
amount of power dissipated by the package limits the
output power. The MAX5042/MAX5043’s QFN package
features an exposed metal pad on the bottom of the
package. Solder the exposed pad directly to the most
negative supply in the system. Use a large copper area
to improve heat dissipation. Facilitate heat transfer with
thermal vias.
Two-Switch Power ICs with Integrated
Power MOSFETs and Hot-Swap Controller
12 ______________________________________________________________________________________
Pin Description (continued)
PIN
MAX5042 MAX5043
NAME FUNCTION
47 25, 47 POSINPWM
PWM Analog Positive-Supply Input. Connect POSINPWM to the most positive input
supply rail.
48, 51, 54, 55 48, 51, 54, 55 DRNH
Drain Connection of the Internal High-Side PWM Power MOSFET. Connect DRNH to
the most positive rail of the input supply.
49, 50, 52, 53 49, 50, 52, 53 XFRMRH High-Side Connection for the Isolation Transformer
— 27 DEN
Delayed Enable Input (MAX5043 Only). DEN is the center point of the delayed
enable divider. Use an external voltage-divider or a 100k
Ω pullup resistor to the
most positive rail to override.
— 30 N.C. No Connection (MAX5043 Only). Leave unconnected.
PWM
CIRCUIT
WITH
INTEGRATED
FETs
INTEGRATED
HOT-SWAP
CONTROLLER
QH
QL
QHS
EXTERNAL
HOT-SWAP FET
PWMNEG, PWMPNEG
T1
C IN L
C OUT
+V POSINPWM
NEGIN
MAX5042 BULK STORAGE CAPACITOR
(HOT-SWAPPED CAPACITOR)
V OUT
Figure 1. Simplified Diagram of a MAX5042-Based Isolated
Power Supply