MAX1745 查看數據表（PDF） - Maxim Integrated

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MAX1745

High-Voltage / Step-Down DC-DC Controller in MAX

Maxim Integrated 

High-Voltage, Step-Down DC-DC

Controllers in µMAX

A

A

B

B

C

C

10μs/div

CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 100mA

A: MOSFET DRAIN, 10V/div

B: OUT, 50mV/div, 3.3V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

Figure 3. Discontinuous-Conduction Mode, Light-Load-Current

Waveform

VL Linear Regulator

The MAX1744/MAX1745 contain a 5V low-side linear reg-

ulator (VL) that powers the internal circuit and can supply

up to 1mA to an external load. This allows the

MAX1744/MAX1745 to operate up to 36V input, while

maintaining low quiescent current and high switching fre-

quency. When the input voltage goes below 5.5V, this

regulator goes into dropout and the IN pin quiescent cur-

rent will rise. See the Typical Operating Characteristics.

Bypass VL with a 4.7μF or greater capacitor.

VH Linear Regulator

The MAX1744/MAX1745 contain a high-side linear regu-

lator (VH) that regulates its output to 5V below IN (the

positive supply input voltage). This regulator limits the

external P-channel MOSFET gate swing (EXT), allowing

high input voltage operation without exceeding the

MOSFET gate-source breakdown. Bypass VH with a

4.7μF or greater capacitor between IN and VH. Fast line

transients may drive the voltage on VH negative. The

clamp diode (D2) prevents damage to the IC during

such a condition. A Schottky diode with a minimum 40V

reverse rating such as the Nihon EP05Q04 is sufficient

for most applications.

Quiescent Current

The devices’ typical quiescent current is 90μA.

However, actual applications draw additional current to

supply MOSFET switching currents, OUT pin current,

external feedback resistors (if used), and both the diode

and capacitor leakage currents. For example, in the cir-

cuit of Figure 1, with IN at 30V and VOUT at 5V, typical

no-load supply current for the entire circuit is 100μA.

10μs/div

CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 1.5A

A: MOSFET DRAIN, 10V/div

B: OUT, 50mV/div, 3.3V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

Figure 4. Continuous-Conduction Mode, Heavy-Load-Current

Waveform

Shutdown Mode

When SHDN is low, the device enters shutdown mode. In

this mode, the internal circuitry is turned off. EXT is pulled

to IN, turning off the external MOSFET. The shutdown

supply current drops to less than 10μA. SHDN is a logic-

level input. Connect SHDN to IN for normal operation.

Reference

The 1.25V reference is suitable for driving small external

loads. It has a guaranteed 10mV maximum load regula-

tion while sourcing load currents up to 100μA. The refer-

ence is turned off during shutdown. Bypass the

reference with 0.1μF for normal operation. Place the

bypass capacitor within 0.2in (5mm) of REF, with a direct

trace to GND.

Design Information

Setting the Output Voltage

The MAX1744’s output voltage can be selected to 3.3V

or 5V under logic control by using the 3/5 pin. Connect

the 3/5 pin to GND to ensure a 3.3V output, or connect

the 3/5 pin to VL to ensure a 5V output.

The MAX1745’s output voltage is set using two resis-

tors, R2 and R3 (Figure 5), which form a voltage-divider

between the output and FB. R2 is given by:

R2= R3

x

⎛

⎝⎜

VOUT

VREF

⎞

− 1⎠⎟

where VREF = 1.25V. Since the input bias current at FB

has a maximum value of 50nA, large values (10kΩ to

200kΩ) can be used for R3 with no significant accuracy

10 ______________________________________________________________________________________

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