Rainbow Electronics MAX17061 - Manual (Page 20)

MAX17061
To ensure the stable operation, the MAX17061 includes
slope compensation, which sets the minimum inductor
value. In continuous-conduction mode (CCM), the mini-
mum inductor value is calculated with the following
equation:
where 24.7mV is a scale factor from the slope compen-
sation, the L
CCM(MIN) is the minimum inductor value for
stable operation in CCM, and R S =12m
Ω (typ) is the
equivalent sensing scale factor from the controller’s
internal current-sense circuit.
The controller can also operate in discontinuous con-
duction mode (DCM). In this mode, the inductor value
can be lower, but the peak inductor current is higher
than in CCM. In DCM, the maximum inductor value is
calculated with the following equation:
where the L DCM(MAX) is the maximum inductor value
for DCM, η is the nominal regulator efficiency (85%),
and I
OUT(MAX)
is the maximum output current.
The output current capability of the step-up converter is
a function of current limit, input voltage, operating fre-
quency, and inductor value. Because the slope com-
pensation is used to stabilize the feedback loop, the
inductor current limit depends on the duty cycle, and is
determined with the following equation:
where 24.7mV is the scale factor from the slope com-
pensation, 1.9A is a typical current limit at 75% duty
cycle, and D is the duty cycle.
The output current capability depends on the current-
limit value and operating mode. The maximum output
current in CCM is governed by the following equation:
where I LIM is the current limit calculated above, η is the
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
where V
DIODE
is the forward voltage of the rectifier
diode and R
ON
is the internal MOSFET’s on-resistance
(0.15
Ω typ).
The maximum output current in DCM is governed by
the following equation:
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when selecting
an inductor. These factors affect the converter’s operating
mode, efficiency, maximum output load capability, tran-
sient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance minimizes the current rip-
ple, and therefore reduces the peak current, which
decreases core losses in the inductor and I
2 R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increases physical size and I
2 R copper
losses. Low inductor values decrease the physical size,
but increase the current ripple and peak current.
Finding the best inductor involves the compromises
among circuit efficiency, inductor size, and cost.
In choosing an inductor, the first step is to determine
the operating mode: continuous conduction mode
(CCM) or discontinuous conduction mode (DCM). The
MAX17061 has a fixed internal slope compensation that
requires minimum inductor value. When CCM mode is
chosen, the ripple current and the peak current of the
inductor can be minimized. If a small-size inductor is
required, DCM mode can be chosen. In DCM mode,
the inductor value and size can be minimized, but the
inductor ripple current and peak current are higher
than those in CCM. The controller can be stable, inde-
pendent of the internal slope compensation mode, but
there is a maximum inductor value requirement to
ensure the DCM operating mode.
I
L I f
V V V
OUT DCM MAX
LIM OSC
OUT DIODE IN
_ ( )
=
Ч Ч Ч
Ч + −
( )
2
2
η
D
V V V
V I R V
OUT IN DIODE
OUT LIM ON DIODE
=
− +
− × +
I I
D V
f L
OUT CCM MAX LIM
IN
OSC
_ ( )
.
= −
Ч Ч
Ч
⎛
⎝⎜ ⎞
⎠⎟
0 5
I A
mV D
R
LIM
S
= +
× −
( )
1 9
24 7 0 75
.
. .
LDCM
V
V V
V
f V I
MAX
IN MIN
OUT MAX DIODE
IN MIN
OSC MAX OUT MAX OUT MAX
( )
( )
( )
( )
( ) ( ) ( )
= −
+
⎛
⎝
⎜
⎞
⎠
⎟
Ч
Ч
Ч Ч Ч
1
2
2 η
L
V V V R
mV f
CCM MIN
OUT MAX DIODE IN MIN S
OSC MIN
( )
( ) ( )
( )
.
=
+ − Ч
( )
Ч
Ч Ч
2
2 24 7
8-String White LED Driver with
SMBus for LCD Panel Applications
20 ______________________________________________________________________________________