LTC3577-4_15 Datasheet, PDF (36/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
Fixed Boost Output
Setting MD1 to 1 and MD2 to 0 selects the fixed high
voltage boost mode. This mode can be used to generate
output voltages at or greater than V_OUT. When configured
as a boost converter the I_LED pin becomes the feedback
pin, and the boost will regulate the output voltage such
that the voltage on the I_LED pin is 800mV.
Figure 12 shows a fixed 12V output generated using the
boost converter in the fixed high voltage boost mode. Any
output voltage up to 40V may be programmed by select-
ing appropriate values for the R1 and R2 voltage divider
from the equation:
V_BOOST
=
0.8V
•
⎛
⎝⎜
R1
R2
+
⎞
1_⎠⎟
Values for R2 should be kept below 24.3k to keep the pole
at the I_LED pin beyond cross over.
C1
22μF
39
V_OUT
I_{LED_FS}
3
LTC3577-3/
LTC3577-4 ₁₈
SW
SW
19
20
SW
L4
10μH
LPS4018-103ML
D12
ZLLS400
R1
I_LED ^{22 800mV VREF}
301k
R2
LED_OV ⁹
21.5k
357734 F12
BOOST
C2
10μF
10V
The boost is designed primarily as a high voltage and high
duty cycle converter. When operating with a lower boost
ratio, a larger output capacitor, 10μF, should be used. Op-
erating with a very low duty cycle will cause cycle skipping
which will increase ripple.
To keep the average steady-state inductor current below
300mA the maximum output current is reduced as pro-
grammed output voltage increases. The output current
available is given by:
I_BOOST(MAX)
=
300mA
•
V_OUT(MIN)
V_BOOST
Note that the maximum boost output current must be
set by the minimum V_OUT operating voltage. If the boost
converter is allowed to operate down to the V_OUT UVLO
then 2.5V must be assumed as the minimum operating
V_OUT voltage.
Inductor Selection
The LED boost converter is designed to work with a 10μH
inductor. The inductor must be able to handle a peak
current of 1A and should have a low ESR value for good
efficiency. Table 7 shows several inductors that work
well with the LED boost converter. These inductors offer
a good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Figure 12. Fixed 12V/75mA Boost Output Application
Table 7. Recommended Inductors for Boost Switching Reguators
INDUCTOR TYPE
LPS4018-103
L (μH)
10
MAX I_DC (A) MAX DCR (Ω)
1.1
0.200
DB62LCB
10
1.22
0.118
CDRH4D16NP-100M
10
1.05
0.155
SD18-100-R
10
1.28
0.158*
*Typical
SIZE in mm (L × W × H)
4.0 × 4.0 × 1.8
6.2 × 6.2 × 2
4.8 × 4.8 × 1.8
5.2 × 5.2 × 1.8
MANUFACTURER
Coil Craft
www.coilcraft.com
Toko
www.toko.com
Sumida
www.sumida.com
Cooper
www.cooperet.com
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