Rainbow Electronics MAX726 - Manual (Page 7)

MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________________________________________________________________________________ 7
Inductor Selection
Although most MAX724 designs perform satisfactorily
with 50
µ H inductors (100µH for the MAX726), the
MAX724/MAX726 are able to operate with values rang-
ing from 5
µ
H to 200
µ
H. In some cases, inductors other
than 50
µ
H may be desired to minimize size (lower
inductance), or reduce ripple (higher inductance). In
any case, inductor current must at least be rated for the
desired output current.
In high-current applications, pay particular attention to
both the RMS and peak inductor ratings. The induc-
tor's peak current is limited by core saturation.
Exceeding the saturation limit actually reduces the
coil's inductance and energy storage ability, and
increases power loss. Inductor RMS current ratings
depend on heating effects in the coil windings.
The following equation calculates maximum output cur-
rent as a function of inductance and input conditions:
I
OUT
= I
SW
-
V
OUT
(V
IN
- V
OUT
)
2 f
OSC
V
IN
L
where I
SW
is the maximum switch current (5.5A for
MAX724), V
IN
is the maximum input voltage, V
OUT
is the
output voltage, and f
OSC
is the switching frequency.
For the MAX724 example in Figure 2, with L = 50
µ
H
and V
IN
= 25V,
I
OUT
= 5.5A -
5V (25V - 5V)
= 5.1A
2 (10 5 Hz) 25V (50 x 10 -6 H)
Note that increasing or decreasing inductor value pro-
vides only small changes in maximum output current
(100
µ
H = 5.3A, 20
µ
H = 4.5A). The equation shows that
output current is mostly a function of the
MAX724/MAX726 current-limit value. Again, a 50
µ
H
inductor works well in most applications and provides
5A with a wide range of input voltages.
Catch Diode
D1 provides a path for inductor current when V
SW
turns
off. Under normal load conditions, the average diode
current may only be a fraction of load current; but dur-
ing short-circuit or current-limit, diode current is higher.
Conservative design dictates that the diode average
current rating be 2 times the desired output current. If
operation with extended short-circuit or overload time is
expected, then the diode current rating must exceed
the current limit (6.5A = MAX724, 2.6A = MAX726), and
heat sinking may be necessary.
Under normal operating conditions (not shorted), power
dissipated in the diode P
D
is calculated by:
P
D
= I
OUT
(V
IN
- V
OUT
) V
D
V
IN
where V
D
is forward drop of the diode at a current
equal to I
OUT
. In nearly all circuits, Schottky diodes
provide the best performance and are recommended
due to their fast switching times and low forward voltage
drop. Standard power rectifiers such as the 1N4000
series are too slow for DC-DC conversion circuits and are
not recommended.
Output Filter Capacitor
For most MAX724/MAX726 applications, a high-quality,
low-ESR, 470
µ F or 500 µ F output filter capacitor will suf-
fice. To reduce ripple, minimize capacitor lead length
and connect the capacitor directly to the GND pin.
Capacitor suppliers are listed in Table 1. Output ripple
is a function of inductor value and output capacitor
effective series resistance (ESR). In continuous-con-
duction mode:
V
CR(p-p)
=
ESR (V
OUT
) (1 - V
OUT
/V
IN
)
L f
OSC
It is interesting to note that input voltage (V
IN
), and not
load current, affects output ripple in CCM. This is
because only the DC, and not the peak-to-peak, induc-
tor current changes with load (see Figure 3).
In discontinuous-conduction mode, the equation is dif-
ferent because the peak-to-peak inductor current does
depend on load:
V
DR(p-p)
= ESR
√
2 I
OUT
V
OUT
(V
IN
- V
OUT
)
L f
OSC
V
IN
where output ripple is proportional to the square root of
load current. Refer to the earlier equation for I
DCM
to
determine where DCM occurs and hence when the
DCM ripple equation should be used.
Input Bypass Capacitor
An input capacitor (200
µ
F or 220
µ
F) is required for step-
down converters because the input current, rather than
being continuous (like output current), is a square wave.
For this reason the capacitor must have low ESR and a
ripple-current rating sufficiently large so that its ESR and
the AC input current do not conspire to overheat the
capacitor. In CCM, the capacitor's RMS ripple current is:
I
R(RMS)
= I
OUT
√
V
OUT
(V
IN
- V
OUT
)
V
IN
2
The power dissipated in the input capacitor is then P
C
:
P
C
= I
R(RMS)
2 (ESR)