MAX1820 データシートの表示（PDF） - Maxim Integrated

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MAX1820

WCDMA Cellular Phone 600mA Buck Regulators

Maxim Integrated 

WCDMA Cellular Phone 600mA

Buck Regulators

VIN = 2.6V TO 5.5V

10µF

0.1µF

0.047µF

BATT

LX

SHDN MAX1821 PGND

SYNC

REF

FB

COMP

SKIP GND

4.7µH

4.7µF

VOUT = 1.5V

R1

6kΩ

RC

82kΩ

C1

330pF

R2

30kΩ

C2*

1pF

* CAN BE OMITTED IF CERAMIC OUTPUT CAPACITOR IS USED.

Figure 3. Standard Operating Circuit

the load. The duty cycle of a buck step-down converter

is ideally a ratio of the output voltage to input voltage in

steady-state condition.

The MAX1820/MAX1821 have internal switch current

limits of 1.2A (typ). If ILX exceeds this maximum, the

high-side FET turns off and the synchronous rectifier

turns on. This lowers the duty cycle and causes the out-

put voltage to droop as long as the load current

remains excessive. There is also a synchronous rectifier

current limit of -0.85A when the device is operating in

forced PWM mode (see the Forced PWM Operation sec-

tion). If the negative current limit is exceeded, the syn-

chronus rectifier is turned off, and the inductor current

continues to flow through its body diode until the begin-

ning of the next cycle or the inductor current drops to

zero. This means there is a limit on how much current

the device is allowed to shuttle in response to output

power reduction.

Normal Mode Operation

Connecting SKIP to GND enables MAX1820/MAX1821

normal operation (Figure 3). This allows automatic PWM

control at medium and heavy loads and skip mode at

light loads to improve efficiency and reduce quiescent

current to 180µA. Operating in normal mode also allows

the MAX1820/MAX1821 to pulse skip when the peak

inductor current drops below 130mA, corresponding to

a load current of approximately 65mA.

During skip operation, the MAX1820/MAX1821 switch

only as needed to service the load, reducing the

switching frequency and associated losses in the inter-

nal switch, the synchronous rectifier, and the external

inductor.

There are three steady-state operating conditions for

the MAX1820/MAX1821 in normal mode. The device

performs in continuous conduction for heavy loads in a

manner identical to forced PWM mode. The inductor

current becomes discontinuous at medium loads,

requiring the synchronous rectifier to be turned off

before the end of a cycle as the inductor current reach-

es zero. The device enters into skip mode when the

converter output voltage exceeds its regulation limit

before the inductor current reaches its skip thres-

hold level.

During skip mode, a switching cycle initiates when the

output voltage has dropped out of regulation. The P-

channel MOSFET switch turns on and conducts current

to the output-filter capacitor and load until the inductor

current reaches the skip peak current limit. Then the

main switch turns off, and the magnetic field in the

inductor collapses, while current flows through the syn-

chronous rectifier to the output filter capacitor and the

load. The synchronous rectifier is turned off when the

inductor current reaches zero. The MAX1820/ MAX1821

wait until the skip comparator senses a low output volt-

age again.

Forced PWM Operation

Connect SKIP to BATT for forced PWM operation.

Forced PWM operation is desirable in sensitive RF and

data-acquisition applications to ensure that switching

harmonics do not interfere with sensitive IF and data-

sampling frequencies. A minimum load is not required

during forced PWM operation since the synchronous

rectifier passes reverse-inductor current as needed to

allow constant-frequency operation with no load.

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