Rainbow-electronics MAX8625A Manuel d'utilisateur télécharger pdf (Page 10)

MAX8625A

High-Efficiency, Seamless Transition,

Step-Up/Down DC-DC Converter

10 ______________________________________________________________________________________

shuttling current in and out of the output capacitor. If

SKIP is asserted high for forced-PWM mode, phase 4 is

not entered and current shuttling is allowed (and is

necessary to maintain the PWM operation frequency

when no load is present).

Step-Up Operation (V

< V

OUT

)

During medium and heavy loads when V

< V

OUT,

MOSFETs P1 and N2 turn on at the clock edge to ramp

up the inductor current. Phase 1 terminates when the

inductor current reaches the peak target current set by

the PWM comparator and N2 turns off. This is followed

by a slow-discharge phase (phase 2) instead of a

charge phase (since V

is less than V

OUT

) when P2

turns on. The slow-discharge phase terminates on a

clock edge. The converter now enters the fast-dis-

charge phase (phase 3). During phase 3, P1 turns off

and N1 turns on. At the end of the minimum time, both

P2 and N1 turn off and the cycle repeats.

If SKIP is asserted low, during light loads when inductor

current falls to zero in phase 3, the converter switches to

phase 4 (hold) to reduce power consumption and avoid

shuttling current in and out of the output. If SKIP is high

to assert forced-PWM mode, the converter never enters

phase 4 and allows negative inductor current.

Step-Up/Down Transition-Zone Operation

= V

OUT

)

When V

= V

OUT

, the converter still goes through the

three phases for moderate to heavy loads. However,

the maximum time is now spent in phase 2 where

inductor current di/dt is almost zero, since it is propor-

tional to (V

- V

OUT

). This eliminates transition glitches

Figure 2. State Diagram

FAULT

TIMEOUT

(ASYNCHRONOUS

FROM ANYWHERE)

ERROR

ON = 1

P1, P2 = OFF

N1, N2 = ON

OFF

ON = 0

P1, P2 = OFF

N1, N2 = ON

= 0μA

ON = 0

(ASYNCHRONOUS

FROM

ANYWHERE)

REFOK = 0 OR

UVLO = 0

(ASYNCHRONOUS

FROM ANYWHERE)

PHASE 2

SLOW CHARGE/

DISCHARGE

OSC = ON

P1, P2 = ON

N1, N2 = OFF

PHASE 3

FAST DISCHARGE

OSC = ON

P2, N1 = ON

P1, N2 = OFF

PHASE 1

FAST-CHARGE

OSC = ON

P1, N2 = ON

P2, N1 = OFF

PHASE 4

HOLD

OSC = OFF

N1, N2 = ON

P1, P2 = OFF

POWER-UP

ON = 1, P1, P2 = OFF, N1, N2 = ON,

OSC = ON AND REF = ON IF UVLO OK

T2-3

T3-4

T1-2

T3-1

T1-3

T4-1

TRUN

TPUP

(SKIP)

1 2 ... 5 6 7 8 9 10 11 12 13 14

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Rainbow-electronics MAX8625A Manuel d'utilisateur Page 10

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