MAX17083 データシートの表示（PDF） - Maxim Integrated

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MAX17083

Low-Voltage, Internal Switch, Step-Down Regulator

Maxim Integrated 

Low-Voltage, Internal Switch,

Step-Down Regulator

For the MAX17083 system (IN) supply, ceramic capaci-

tors are preferred due to their resilience to inrush surge

currents typical of systems, and due to their low para-

sitic inductance, which helps reduce the high-frequen-

cy ringing on the IN supply when the internal MOSFETs

are turned off. Choose an input capacitor that exhibits

less than +10°C temperature rise at the RMS input cur-

rent for optimal circuit longevity.

BST Capacitors

The boost capacitor (CBST) must be selected large

enough to handle the gate charging requirements of

the high-side MOSFETs. For these low-power applica-

tions, 0.1µF ceramic capacitors work well.

Applications Information

Duty-Cycle Limits

Minimum Input Voltage

The minimum input operating voltage (dropout voltage)

is restricted by the maximum duty-cycle specification

(see the Electrical Characteristics table). For the best

dropout performance, use the slowest switching fre-

quency setting (FREQ = GND). However, keep in mind

that the transient performance gets worse as the step-

down regulators approach the dropout voltage, so bulk

output capacitance must be added (see the voltage

sag and soar equations in the SMPS Design Procedure

section). The absolute point of dropout occurs when the

inductor current ramps down during the off-time

(ΔIDOWN) as much as it ramps up during the on-time

(ΔIUP). This results in a minimum operating voltage

defined by the following equation:

( ) VIN(MIN)

=

VOUT

+

VCHG

+

⎛

h⎝⎜

1

DMAX

⎞

- 1⎠⎟

VOUT + VDIS

where VCHG and VDIS are the parasitic voltage drops in

the charge and discharge paths, respectively. A rea-

sonable minimum value for h is 1.5, while the absolute

minimum input voltage is calculated with h = 1.

Maximum Input Voltage

The MAX17083 controller includes a minimum on-time

specification, which determines the maximum input

operating voltage that maintains the selected switching

frequency (see the Electrical Characteristics table).

Operation above this maximum input voltage results in

pulse skipping to avoid overcharging the output. At the

beginning of each cycle, if the output voltage is still

above the feedback threshold voltage, the controller

does not trigger an on-time pulse, effectively skipping a

cycle. This allows the controller to maintain regulation

above the maximum input voltage, but forces the con-

troller to effectively operate with a lower switching fre-

quency. This results in an input threshold voltage at

which the controller begins to skip pulses (VIN(SKIP)):

VIN(SKIP)

=

VOUT

⎛

⎜

⎝

1

fOSCt ON(MIN)

⎞

⎟

⎠

where fOSC is the switching frequency selected by FREQ.

PCB Layout Guidelines

Careful PCB layout is critical to achieving low switching

losses and clean, stable operation. The switching power

stage requires particular attention. If possible, mount all

the power components on the top side of the board,

with their ground terminals flush against one another.

Follow the MAX17083 Evaluation Kit layout and use the

following guidelines for good PCB layout:

• Keep the high-current paths short, especially at the

ground terminals. This practice is essential for sta-

ble, jitter-free operation.

• Keep the power traces and load connections short.

This practice is essential for high efficiency. Using

thick copper PCBs (2oz vs. 1oz) can enhance full-

load efficiency by 1% or more. Correctly routing

PCB traces is a difficult task that must be

approached in terms of fractions of centimeters,

where a single milliohm of excess trace resistance

causes a measurable efficiency penalty.

• When trade-offs in trace lengths must be made, it is

preferable to allow the inductor charging path to be

made longer than the discharge path. For example,

it is better to allow some extra distance between the

input capacitors and the high-side MOSFET than to

allow distance between the inductor and the low-

side MOSFET or between the inductor and the out-

put filter capacitor.

• Route high-speed switching nodes (BST and LX)

away from sensitive analog areas (REF and FB).

14 ______________________________________________________________________________________

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