5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
Be sure that the selected capacitor can handle the ripple
current over the required temperature range. Also locate
the input capacitor very close to the MAX724/MAX726 and
use minimum length leads (surface-mount or radial
through-hole types). In most applications, ESR is more
important than actual capacitance value since electrolytic
capacitors are mostly resistive at the MAX724/MAX726's
100kHz switching frequency.
rent and voltage appear across the switch at the same
time. t
SW
is approximately: [50ns + (3ns/A) (I
OUT
)] for
the MAX724.
Power dissipation in the MAX726 can be estimated in
exactly the same way as the MAX724, except that 1.1V
(and not 1.8V) is a more reasonable value for the nomi-
nal voltage drop across the on-board power switch.
MAX724/MAX726
__________Applications Information
Setting Output Voltage
R1 and R2 set output voltage as follows:
R1 =
V
OUT
R2
2.21V
-R2
Ground Connections
GND demands a short low-noise connection to ensure
good load regulation. Since the internal reference is
referred to GND, errors in the GND pin voltage get mul-
tiplied by the error amplifier and appear at the output.
If the MAX724/MAX726 GND pin is separated from the
negative side of the load, then high load return current
can generate significant error across a seemingly small
ground resistance. Single-point grounding is the most
effective way to eliminate these errors. A recommend-
ed ground arrangement is shown in Figure 4.
2.21V is the reference voltage, so setting R2 to 2.21kΩ
(standard 1% resistor value) results in 1mA flowing
through R1 and R2 and simplifies the above equation.
Other values will also work for R2, but should not
exceed 4kΩ.
Overload Protection
The V
SW
current is internally limited to about 6.5A in the
MAX724 and 2.6A in the MAX726. In addition, another
feature of the MAX724/MAX726's overload protection
scheme is that the oscillator frequency is reduced
when the output voltage falls below approximately half
its regulated value. This is the case during short-circuit
and heavy overload conditions.
Since the minimum on-time for the switch is about
0.6µs, frequency reduction during overload ensures
that switch duty cycle can fall to a low enough value to
maintain control of output current. At the normal
100kHz switching frequency, an on-time as short as
Synchronizing the Oscillator
The MAX724/MAX726 can be synchronized to an exter-
nal 110kHz to 160kHz source by pulsing the V
C
pin to
ground at the desired clock rate. This is conveniently
done with the collector of an external grounded-emitter
NPN transistor. V
C
should be pulled low for 300ns.
Doing this may have some impact on output regulation,
but the effect should be minimal for compensation
resistor values between 1kΩ and 4kΩ.
Power Dissipation
The MAX724/MAX726 draw about 7.5mA operating cur-
rent, which is largely independent of input voltage or
load current. They draw an additional 5mA during
switch on-time. Power dissipated in the internal V
SW
transistor is proportional to load current and depends
on both conduction losses (product of switch on-volt-
age and switch current) and dynamic switching losses
(due to switch rise and fall times). Total MAX724 power
dissipation can be calculated as follows:
P = V
IN
[7.5mA + 5mA (DC) + 2 I
OUT
t
SW
f
OSC
] + . . .
. . . DC [I
OUT
(1.8V) + 0.1Ω
DC = Duty Cycle =
(I
OUT
)
2
]
V
OUT
+ 0.5V
V
IN
- 2V
R1
MAX724
MAX726
FB
GND
R2
NEGATIVE OUTPUT
NODE WHERE LOAD
REGULATION WILL
BE MEASURED
HIGH CURRENT
RETURN PATH
t
SW
= Overlap Time = 50ns + (3ns/A) I
OUT
where t
SW
is "overlap" time. Switch dissipation is
momentarily high during overlap time because both cur-
Figure 4. Recommended Ground Connection
9
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