LTC1879 LINER [Linear Technology], LTC1879 Datasheet - Page 10

• Manufacturer Part Number: LTC1879
• Description: 1.2A Synchronous Step-Down Regulator with 15mA Quiescent Current
• Manufacturer: LINER [Linear Technology]

OPERATIO
APPLICATIO S I FOR ATIO
LTC1879
Slope Compensation and Inductor Peak Current
Slope compensation is required in order to prevent sub-
harmonic oscillation at high duty cycles. It is accom-
plished by internally adding a compensating ramp to the
inductor current signal at duty cycles in excess of 40%. As
a result, the maximum inductor peak current is reduced for
duty cycles > 40%. This is shown in the decrease of the
inductor peak current as a function of duty cycle graph in
Figure 3.
The basic LTC1879 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC1879. The internal nominal frequency is
550kHz, but can be externally synchronized from 350kHz
to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However, oper-
ating at a higher frequency results in lower efficiency
because of increased switching losses.
The inductor value has a direct effect on ripple current. The
ripple current I
frequency and increases with higher input voltages.
Accepting larger values of I
inductors, but results in higher output voltage ripple.
10
I
L
f L
1
V
L
OUT
U
decreases with higher inductance or
U
1–
IN
and C
U
V
V
OUT
IN
L
OUT
allows the use of smaller
.
W
U
(1)
A reasonable starting point for setting ripple current is
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
500mA. Lower inductor values (higher I
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Selection
The inductor should have a saturation current rating
greater than the peak inductor current set by the current
comparator of LTC1879. Also, consideration should be
given to the resistance of the inductor. Inductor conduc-
tion losses are directly proportional to the DC resistance
of the inductor. Manufacturers sometimes provide maxi-
mum current ratings based on the allowable losses in the
inductor.
Suitable inductors are available from Coilcraft, Cooper,
Dale, Sumida, Toko, Murata, Panasonic and other manu-
facturers.
I
L
= 0.3(I
Figure 3. Maximum Inductor Peak Current vs Duty Cycle
2400
2200
2000
1800
1600
1400
1200
1000
MAX
0
V
).
IN
= 5V
20
DUTY CYCLE (%)
40
60
80
1879 F03
L
) will cause this
100
1879f