MAX8655 查看數據表（PDF） - Maxim Integrated

零件编号

产品描述 (功能)

生产厂家

MAX8655

Highly Integrated, 25A, Wide-Input, Internal MOSFET, Step-Down Regulator

Maxim Integrated 

Highly Integrated, 25A, Wide-Input,

Internal MOSFET, Step-Down Regulator

Detailed Description

DC-DC Converter Control Architecture

The MAX8655 step-down regulator uses a PWM, peak

current-mode control scheme. An internal transconduc-

tance amplifier establishes an integrated error voltage.

The heart of the PWM controller is a PWM comparator

that compares the integrated voltage-feedback signal

against the amplified current-sense signal plus an

adjustable slope-compensation ramp, which is

summed with the current signal to ensure stability. At

each rising edge of the internal clock, the internal high-

side MOSFET turns on until the PWM comparator trips

or the maximum duty cycle is reached. During this on-

time, current ramps up through the inductor, storing

energy in the output inductor while sourcing current to

the output. The current-mode feedback system regu-

lates the peak inductor current as a function of the out-

put-voltage error signal. The circuit acts as a

switch-mode transconductance amplifier and pushes

the output LC filter pole normally found in a voltage-

mode PWM to a higher frequency. Figure 1 is the func-

tional diagram.

During the second half of the cycle, the internal high-

side MOSFET turns off and the internal low-side MOS-

FET turns on. The output inductor releases the stored

energy as the current ramps down, providing current to

the load. The output capacitor stores charge when the

inductor current exceeds the required load current and

discharges when the inductor current is lower, smooth-

ing the voltage across the load. Under soft-overload

conditions, when the peak inductor current exceeds the

selected current limit (see the Current-Limit Circuit sec-

tion), the high-side MOSFET is turned off immediately

and the low-side MOSFET is turned on and remains on

to let the inductor current ramp down until the next

clock cycle. Under severe-overload or short-circuit con-

ditions, the valley foldback current limit is enabled to

reduce power dissipation of external components.

The MAX8655 operates in a forced-PWM mode. As a

result, the regulator maintains a constant switching fre-

quency, regardless of load, to allow for easier filtering

of the switching noise.

Internal Linear Regulators

The MAX8655 contains two internal LDO regulators.

The AVL regulator provides 5V for the IC’s internal cir-

cuitry, and the VL regulator provides 6.5V for the MOS-

FET gate drivers. Connect a 2.2µF ceramic capacitor

from VL to VLGND, and connect a 1µF ceramic capaci-

tor from AVL to GND. The AVL regulator input is inter-

nally connected to the VL regulator output. For 5V input

applications, connect VL directly to IN and connect a

10Ω resistor from VL to AVL.

Undervoltage Lockout

When VAVL drops below 4.03V, the MAX8655 assumes

that the supply voltage is too low to make valid deci-

sions, so the undervoltage-lockout (UVLO) circuitry

inhibits switching and turns off both internal power

MOSFETs. When VAVL rises above 4.15V, the regulator

enters the startup sequence and then resumes normal

operation.

Startup and Soft-Start

The internal soft-start circuitry gradually ramps up the

reference voltage to control the rate of rise of the output

voltage and reduce input surge currents during startup.

The soft-start period is determined by the value of the

capacitor from SS to GND. The soft-start time is

approximately (30.4ms/µF) x CSS. The MAX8655 also

features monotonic output-voltage rise; therefore, both

power MOSFETs are kept off if the voltage at FB is

higher than the voltage at SS. This allows the MAX8655

to start up into a prebiased output without pulling the

output voltage down.

Before the MAX8655 begins the soft-start and power-

up sequence, the following conditions must be met:

• VAVL exceeds the 4.15V UVLO threshold.

• EN is at logic-high.

• The thermal limit is not exceeded.

Enable

The MAX8655 features a low-power shutdown mode. A

logic-low at EN shuts down the regulator. During shut-

down, the output is high impedance. Shutdown

reduces the IN current to less than 10µA. A logic-high

at EN enables the regulator.

______________________________________________________________________________________ 11

Share Link: