MAX1875-MAX1876 查看數據表（PDF） - Maxim Integrated

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MAX1875-MAX1876

Dual 180° Out-of-Phase PWM Step-Down Controllers with POR

Maxim Integrated 

Dual 180° Out-of-Phase PWM Step-

Down Controllers with POR

VL

BST_

5Ω

DH_

LX_

MAX1875

INPUT

(VIN)

Figure 3. Reducing the Switching-Node Rise Time

On the rising edge of EN both controllers enter soft-

start. Soft-start gradually ramps up to the reference

voltage seen by the error amplifier in order to control

the outputs’ rate of rise and reduce input surge cur-

rents during startup. The soft-start period is 1024 clock

cycles (1024/fSW), and the internal soft-start DAC

ramps up the voltage in 64 steps. The output reaches

regulation when soft-start is completed. On the falling

edge of EN both controllers simultaneously enter soft-

stop, which reverses the soft-start ramp. The part

enters shutdown after soft-stop is complete.

Reset Output (MAX1876 Only)

RST is an open-drain output. RST pulls low when either

output falls below 90% of its nominal regulation voltage.

Once both outputs exceed 90% of their nominal regulation

voltages and both soft-start cycles are completed, RST

goes high impedance. To obtain a logic-voltage output,

connect a pullup resistor from RST to the logic supply volt-

age. A 100kΩ resistor works well for most applications. If

unused, leave RST grounded or unconnected.

Clock Synchronization (SYNC, CKO)

SYNC serves two functions: SYNC selects the clock

output (CKO) type used to synchronize slave con-

trollers, or it serves as a clock input so the

MAX1875/MAX1876 can be synchronized with an exter-

nal clock signal. This allows the MAX1875/MAX1876 to

funtion as either a master or slave. CKO provides a

clock signal synchronized to the MAX1875/MAX1876s’

switching frequency, allowing either in-phase (SYNC =

GND) or 90° out-of-phase (SYNC = VL) synchronization

of additional DC-DC controllers (Figure 5). The

MAX1875/MAX1876 support the following three operat-

ing modes:

• SYNC = GND: The CKO output frequency equals

REG1’s switching frequency (fCKO = fDH1) and the

CKO signal is in phase with REG1’s switching fre-

quency. This provides 2-phase operation when syn-

chronized with a second slave controller.

• SYNC = VL: The CKO output frequency equals two

times REG1’s switching frequency (fCKO = 2fDH1)

and the CKO signal is phase shifted by 90° with

respect to REG1’s switching frequency. This pro-

vides 4-phase operation when synchronized with a

second MAX1875/MAX1876 (slave controller).

• SYNC Driven by External Oscillator: The controller

generates the clock signal by dividing down the

SYNC input signal, so that the switching frequency

equals half the synchronization frequency (fSW =

fSYNC/2). REG1’s conversion cycles initiate on the ris-

ing edge of the internal clock signal. The CKO output

frequency and phase match REG1’s switching fre-

quency (fCKO = fDH1) and the CKO signal is in

phase. Note that the MAX1875/MAX1876 still require

ROSC when SYNC is externally clocked and the inter-

nal oscillator frequency should be set to 50% of the

synchronization frequency (fOSC = 0.5 fSYNC).

Thermal Overload Protection

Thermal overload protection limits total power dissipation

in the MAX1875/MAX1876. When the device’s die-junc-

tion temperature exceeds TJ = +160°C, an on-chip ther-

mal sensor shuts down the device, forcing DL_ and DH_

low, allowing the IC to cool. The thermal sensor turns the

part on again after the junction temperature cools by

10°C. During thermal shutdown, the regulators shut

down, RST goes low, and soft-start is reset. If the VL lin-

ear-regulator output is short-circuited, thermal-overload

protection is triggered.

Design Procedure

Effective Input Voltage Range

Although, the MAX1875/MAX1876 controllers can oper-

ate from input supplies ranging from 4.75V to 23V, the

input voltage range can be effectively limited by the

MAX1875/MAX1876s’ duty-cycle limitations. The maxi-

mum input voltage is limited by the minimum on-time

(tON(MIN)):

VIN(MAX) ≤

VOUT

tON(MIN)fSW

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