MAX5052 데이터 시트보기 (PDF) - Maxim Integrated

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MAX5052

Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies

Maxim Integrated 

Current-Mode PWM Controllers with an Error

Amplifier for Isolated/Nonisolated Power Supplies

D1

T1

+VIN

R1

R2

C1

U2

OPTO TRANS

R5

R6

VIN

U1 NDRV

VCC MAX5052 CS

R7

COMP

GND

FB

UVLO/EN

R3

C2

D2

VOUT

C4

Q1

R8

R9

U2

OPTO LED

C3

R4

U3

TL431

R10

-VIN

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.

Soft-Start

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.

N-Channel MOSFET Switch Driver

The NDRV pin drives an external N-channel MOSFET.

The NDRV output is supplied by the internal regulator

(VCC), which is internally set to approximately 9.5V. For

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

1V/div

0V

10ms/div

Figure 4. Output Voltage Soft-Start During Initial Startup for the

Circuit of Figure 5

650mA/1000mA peak current, so select a MOSFET that

yields acceptable conduction and switching losses.

Internal Oscillator

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%.

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