MAX746 データシートの表示（PDF） - Maxim Integrated
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MAX746 Datasheet PDF : 16 Pages
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High-Efficiency, PWM, Step-Down,
N-Channel DC-DC Controller
VIN
6V TO 15V
C3
0.1µF
C2
100µF
D4
1N5817
C9
4.7µF
D2
D3
R2
2 LBI
15
V+
1N914*
CP 14
C8 1N914*
0.1µF
R1
C5
0.1µF 3 SS
HIGH 13
AV+ 8
C6
1.0µF 4 REF MAX746 CS 10
Q1
RSENSE
40mΩ
6 FB
EXT 12
Si9410DY
N L1
39µH
5 SHDN
CC 7
C7
D1
11 AGND
9
OUT
LB0 1
2.7nF
C4
0.1µF
NSQ03A03
R3
100k
GND
16
* SEE TABLE 2 FOR DIODE SELECTION.
Figure 1a. 5V Standard Application Circuit (15W)
5V
AT 3A
C1
430µF
Discontinuous-/Continuous-
Conduction Modes
The MAX746 is designed to operate in continuous-con-
duction mode (CCM) but can also operate in discontinu-
ous-conduction mode (DCM), making it ideal for variable-
load applications. In DCM, the current starts at zero and
returns to zero on each cycle. In CCM, the inductor current
never returns to zero; it consists of a small AC component
superimposed on a DC offset. This results in higher current
capability because the AC component in the inductor cur-
rent waveform is small. It also results in lower output noise,
since the inductor does not exhibit the ringing that would
occur if the current reached zero (see inductor waveforms
in the Typical Operating Characteristics). To transfer equal
amounts of energy to the load in one cycle, the peak cur-
rent level for the discontinuous waveform must be much
larger than the peak current for the continuous waveform.
Slope Compensation
Slope compensation stabilizes the inner current-feedback
loop by adding 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.
Under these conditions, the inductor must be scaled to the
current-sense resistor value.
Overcompensation adds a pole to the outer voltage feed-
back-loop response, degrading loop stability. This may cause
voltage-mode pulse-frequency-modulation instead of PWM
operation. Undercompensation results in inner current feed-
back-loop instability, and may cause the inductor current to
staircase. Ideal matching between the sense resistor and
inductor is not required; it can differ by ±30% or more.
Oscillator and EXT Control
The oscillator frequency is nominally 100kHz, and the duty
cycle varies from 5% to 96%, depending on the input/out-
put voltage ratio. EXT, which provides the gate drive for the
external logic-level N-FET, is switched between HIGH and
GND at the switching frequency. EXT is controlled by a
unique two-comparator control scheme consisting of a PWM
comparator and an idle-mode comparator (Figure 2). The
PWM comparator determines the cycle-by-cycle peak cur-
rent with heavy loads, and the idle-mode comparator sets
the light-load peak current. As VOUT begins to drop, EXT
goes high and remains high until both comparators trip.
With heavy loads, the idle-mode comparator trips first and
the PWM control comparator determines the EXT on-time;
_______________________________________________________________________________________ 7
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