MAX5053 Maxim Integrated Products, MAX5053 Datasheet - Page 9

• Manufacturer Part Number: MAX5053
• Description: (MAX5052 / MAX5053) Current-Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Figure 3. Secondary-Side Regulated, Isolated Power Supply
smaller. However, in this mode, the input voltage range
has to be roughly 2:1. Another consideration is if the bias
winding is in phase with the output, then the power sup-
ply hiccups and soft-start under output short-circuit con-
ditions. Whereas, this property is lost if the bias winding
is in phase with the MOSFET ON time.
The MAX5052/MAX5053 soft-start feature allows the load
voltage to ramp up in a controlled manner, eliminating
output voltage overshoot. Soft-start begins after UVLO is
deasserted. The voltage applied to the noninverting
node of the amplifier ramps from 0 to 1.23V in over a
60ms soft-start timeout period. Figure 4 shows the 5V
output of the power-supply circuit in Figure 5 during
startup. Note the staircase increase of the output volt-
age. This is a result of the digital soft-starting technique
used. Unlike other devices, the MAX5052/MAX5053 ref-
erence voltage to the internal amplifier is soft-started;
this method results in superior control of the output volt-
age under heavy- and light-load conditions.
The NDRV pin drives an external N-channel MOSFET.
The NDRV output is supplied by the internal regulator
(V
the universal input voltage range, the MOSFET used
must be able to withstand the DC level of the high-line
input voltage plus the reflected voltage at the primary of
the transformer. For most offline applications that use the
discontinuous flyback topology, this requires a MOSFET
rated at 600V. NDRV can source/sink in excess of the
CC
Amplifier for Isolated/Nonisolated Power Supplies
), which is internally set to approximately 9.5V. For
N-Channel MOSFET Switch Driver
OPTO TRANS
Current-Mode PWM Controllers with an Error
_______________________________________________________________________________________
U2
-V
+V
IN
IN
R6
R5
C1
R1
R7
Soft-Start
C2
V
V
COMP
FB
IN
CC
MAX5052
U1
UVLO/EN
NDRV
GND
650mA/1000mA peak current, so select a MOSFET that
yields acceptable conduction and switching losses.
The internal oscillator switches at 1.048MHz and is
divided down to 262kHz by two D flip-flops. The
MAX5052A/MAX5053A invert the Q output of the last D
flip-flop to provide a duty cycle of 50% (Figure 6). The
MAX5052B/MAX5053B perform a logic NAND opera-
tion on the Q outputs of both D flip-flops to provide a
duty cycle of 75%.
CS
Figure 4. Output Voltage Soft-Start During Initial Startup for the
Circuit of Figure 5
R2
R3
D1
Q1
0V
R4
T1
OPTO LED
D2
C4
U2
TL431
U3
R8
10ms/div
C3
R10
R9
Internal Oscillator
V
OUT
1V/div
9