MAX747 Maxim, MAX747 Datasheet - Page 6

• Manufacturer Part Number: MAX747
• Description: High-Efficiency PWM / Step-Down P-Channel DC-DC Controller
• Manufacturer: Maxim

High-Efficiency PWM, Step-Down
P-Channel DC-DC Controller
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.
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
Figure 1a. +5V Standard Application Circuit
6
(7.5V TO15V)
_______________________________________________________________________________________
VIN
R2
R1
C6
470pF
C4
0.1µF
C5
0.22µF
100µF
R3
100k
10
14
C2
6
2
3
5
4
1
LBO
LBI
CC
SS
REF
FB
SHDN
AGND
0.1 F
Discontinuous-/Continuous-
C3
MAX747
13
12
GND
V+
OUT
AV+
EXT
CS
Operating Principle
Conduction Modes
0.1 F
7
9
11
8
C7
Q1
SI9405DY
P
R SENSE
50m
D1
NSQ03A03
50 H
L1
430 F
430 F
C1
C1
@ 2.3A
5V
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.
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.
Figure 1b. +3.3V Standard Application Circuit
(4.5V TO 15V)
V IN
R2
R1
C4
0.1µF
C5
0.22µF
N.C.
N.C.
R3
100k
10
14
6
2
3
4
1
8
100µF
C2
LBO
LBI
OUT
CC
SS
REF
SHDN
AGND
MAX747
0.1 F
C3
13
12
GND
V+
AV+
EXT
CS
FB
Slope Compensation
C6
2.7nF
11
7
9
5
P
Q1
SI9405DY
20k
D1
NSQ03A03
R4
R SENSE
50m
33µH
L1
13k
R5
C1
880 F
@ 2.3A
3.3V