5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
______________________________________________________________Pin Description
PIN
1
NAME
FB
FUNCTION
Feedback Input is the error amplifier's inverting input, and controls output voltage by adjusting switch duty cycle.
Input bias current is typically 0.5µA when the error amplifier is balanced (I
OUT
= 0V). FB also aids current limiting
by reducing the oscillator frequency when the output voltage is low. (See the
Applications Information
section.)
Error-Amplifier Output. A series RC network connected to this pin compensates the MAX724/MAX726. Output
swing is limited to about 5.8V in the positive direction and -0.7V in the negative direction. V
C
can also synchro-
nize the MAX724/MAX726 to an external clock. (See the
Applications Information
section).
Ground requires a short low-noise connection to ensure good load regulation. The internal reference is referred
to GND, so errors at this pin are multiplied by the error amplifier. See the
Applications Information
section for
grounding details.
Internal Power Switch Output. The
S
witch output can swing 35V below ground and is rated for 5A (MAX724), 2A
(MAX726).
V
IN
supplies power to the MAX724/MAX726's internal circuitry and also connects to the collector. V
IN
must be
bypassed with a low-ESR capacitor, typically 200µF or 220µF.
MAX724/MAX726
2
V
C
3
GND
4
5
V
SW
V
IN
_________________Detailed Description
The MAX724/MAX726 are complete, single-chip, pulse-
width modulation (PWM), step-down DC-DC converters
(Figure 1). All oscillator (100kHz), control, and current-
limit circuitry, including a 5A power switch (2A for
MAX726), are included on-chip. The oscillator turns on
the switch (V
SW
) at the beginning of each clock cycle.
The switch turns off at a point later in the clock cycle,
which is a function of the signal provided by the error
amplifier. The maximum switch duty cycle is approxi-
mately 93% at the MAX724/MAX726's 100kHz switch-
ing frequency.
Both the input (FB) and output (V
C
) of the error
amplifier are brought out to simplify compensation.
Most applications require only a single series RC
network connected from V
C
to ground. The error
amplifier is a transconductance amplifier with a g
M
of
approximately 5000µmho. When slewing, V
C
can
source about 140µA, and sink about 1.1mA. This
asymmetry helps minimize start-up overshoot by
allowing the amplifier output to slew more quickly in
the negative direction.
Current limiting is provided by the current-limit com-
parator. If the current-limit threshold is exceeded, the
switch cycle terminates within about 600ns. The cur-
rent-limit threshold is internally set to approximately
6.5A (2.6A for MAX726). V
SW
is a power NPN, internally
driven by the PWM controller circuitry. V
SW
can swing
35V below ground and is rated for 5A (2A for MAX726).
Basic Step-Down Application
Figure 2 shows the MAX724/MAX726 in a basic step-
down DC-DC converter. Typical MAX724 waveforms
are shown in Figure 3 for V
IN
= 20V, V
OUT
= 5V, L =
50µH, and I
OUT
= 3A and 0.16A. Two sets of wave-
forms are shown. One set shows high load current (3A)
where inductor current never falls to zero during the
switch "off-cycle" (continuous-conduction mode, CCM).
The second set of waveforms, at low output current
(0.16A), shows inductor current at zero during the latter
half of the switch off-cycle (discontinuous-conduction
mode, DCM). The transition from CCM to DCM occurs
at an output current (I
DCM
) that can be derived with the
following equation:
I
DCM
= (V
OUT
+ V
D
) [(V
IN
- V
SW
) - (V
OUT
+ V
D
)]
2 (V
IN
- V
SW
) f
OSC
L
where V
D
is the diode forward voltage drop, V
SW
is the
voltage drop across the switch, and f
OSC
= 100kHz. In
most applications, the distinction between CCM and
DCM is academic since actual performance differences
are minimal. All CCM designs can be expected to exhibit
DCM behavior at some level of reduced load current.
_______________________________________________________________________________________
5
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