Datasheet

MAX1513/MAX1514
TFT-LCD Power-Supply Controllers
20 ______________________________________________________________________________________
Because V
SENSE
would be greater than 100mV, the cir-
cuit in Figure 9 should be used and the scale factor is:
Calculate R
S1
and R
S2
:
The closest 1% standard values for R
S1
and R
S2
are
866Ω and 1.13kΩ, respectively.
If the 2.2µH inductor used in the typical operating cir-
cuit (Figures 1 and 2) has a typical DCR of 10mΩ and a
maximum DCR of 14mΩ, the RC time constant of the
sense network is:
Select C
S
= 0.1µF, R
S
is:
Assuming ∆T is 40°C and TC is 0.5%, the worst-case
high sense voltage over temperature is:
Because V
SENSE
would be much less than 80mV, the
circuit in Figure 10 can be used to improve the current-
limit accuracy. Calculate R
S3
and R
S4
:
The closest 1% standard values for R
S3
and R
S4
are
2.61kΩ and 14.0Ω, respectively.
Output-Capacitor Selection
The output capacitor and its equivalent series resistance
(ESR) affect the circuit’s stability, output voltage ripple,
and transient response. The Output-Capacitor Stability
Requirement section discusses the output capacitance
requirement based on the loop stability. This section
deals with how to determine the output capacitance
according to the ripple voltage and load-transient
requirements.
The total output voltage ripple has two components: the
ohmic ripple due to the capacitor’s equivalent series
resistance (ESR), and the capacitive ripple caused by
the charging and discharging of the output capacitance:
where V
MAIN
is the output voltage of the step-up regu-
lator, I
MAIN
is the output current, C
OUT
is the output
capacitance, R
ESR
is the ESR of the output capacitor,
f
OSC
is the switching frequency, and I
PEAK
is the peak
inductor current (see the Inductor Selection section).
VV V
VIR
V
I
C
VV
Vf
RIPPLE RIPPLE ESR RIPPLE C
RIPPLE ESR PEAK ESR
RIPPLE C
MAIN
OUT
MAIN IN
MAIN OSC
() ()
()
()
=+
≈×
≈×
×
⎛
⎝
⎜
⎞
⎠
⎟
-
R
VV
V
R
S
S
3
4
15 4 5
1 4 0 0 044
2600 2614
2614 2600 14
.
.
=
+
×=
==
-
5V - .5V - .1V
-
ΩΩ
ΩΩΩ
VAm CmV
SENSE
=× +×°
()
=2 6 14 1 0 005 40 44. .Ω
R
s
F
S
.
==
220
01
2200
µ
µ
Ω
τ
µ
µ
.
==
22
10
220
H
m
s
Ω
R
R
S
S
1
2
489
0 571
856
856 0 571
1 0 571
1139
.
.
.
==
=
×
=
Ω
Ω
Ω
-
SF
mV
mV
.==
100
175
0 571
VAm CmV
SENSE
=× + ×°
()
=2 6 56 1 0 005 40 175. .Ω
MAX1513
MAX1514
L
R
S3
C
S
V
IN
V
MAIN
C
S+
C
S-
GATE
GND
FB
R
S4
Figure 10. Lossless Current Sensing with V
SENSE
< 80mV