High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

6

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Operating Principle

Figure 2 is the MAX747 block diagram. The MAX747

regulates using an inner current-feedback loop and an

outer voltage-feedback loop. The current loop is

stabilized by a slope compensation scheme and the

voltage loop is stabilized by the dominant pole formed

by the filter output capacitor and the load.

Discontinuous-/Continuous-

Conduction Modes

The MAX747 operates in continuous-conduction mode

(CCM)

under

heavy

loads,

but

operates

in

discontinuous-conduction mode (DCM) at light loads,

making it ideal for variable load applications. In DCM,

the inductor current starts and ends at zero on each

cycle. In CCM, the inductor current never returns to zero.

It is composed of a small AC component superimposed

on a DC level, which results in higher load-current

capability and lower output noise. Output noise is

reduced because the inductor does not exhibit the

ringing that occurs when the inductor current reaches

zero, and because there is a smaller AC component in

the inductor-current waveform (see inductor waveforms

in the

Typical Operating Characteristics section). Note

that to transfer equal amounts of energy to the load in

one cycle, the peak current level for the discontinuous

waveform must be much larger than the continuous

waveform peak current.

Slope Compensation

Stability of the inner current-feedback loop is provided

by a slope-compensation scheme that adds a ramp

signal to the current-sense amplifier output. Ideal slope

compensation can be achieved by adding a linear

ramp with the same slope as the declining inductor

current to the rising inductor current-sense voltage.

Therefore, the inductor must be scaled to the current-

sense resistor value.

Overcompensation adds a pole to the outer voltage-

feedback loop response that degrades loop stability.

This may cause voltage-mode pulse-frequency-

modulation instead of PWM operation. Under-

compensation results in inner current-feedback loop

instability, and may cause the inductor current to

staircase. Ideal matching between the sense resistor

and inductor is not required. The matching can be

±30% or more.

AV+

GND

MAX747

VIN

(7.5V TO15V)

C2

100µF

V+

C6

470pF

C4

0.1µF

C5

0.22µF

CC

SS

REF

FB

SHDN

AGND

OUT

C1

430

µF

5V

@ 2.3A

CS

RSENSE

50m

Ω

P

EXT

L1

50

µH

LBO

LBI

R1

R2

R3

100k

6

2

3

5

4

10

14

1

12

7

9

11

8

D1

NSQ03A03

C3

0.1

µF

Q1

SI9405DY

13

C7

0.1

µF

C1

430

µF

AV+

GND

MAX747

VIN

(4.5V TO 15V)

C2

100µF

V+

C4

0.1µF

C5

0.22µF

CC

SS

REF

SHDN

AGND

FB

3.3V

@ 2.3A

CS

RSENSE

50m

Ω

P

EXT

L1

33µH

LBO

LBI

OUT

R1

R2

R3

100k

6

2

3

4

10

14

1

8

12

7

9

11

5

D1

NSQ03A03

C3

0.1

µF

Q1

SI9405DY

13

C6

2.7nF

R4

20k

R5

13k

N.C.

N.C.

C1

880

µF

Figure 1b. +3.3V Standard Application Circuit

Figure 1a. +5V Standard Application Circuit