Table 5. approximate k-factor errors – Rainbow Electronics MAX1844 User Manual
Page 20
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
20
______________________________________________________________________________________
Dropout Design Example:
V
OUT
= 2.5V
fsw = 300kHz
K = 1.8µs, worst-case K = 2.97µs
t
OFF(MIN)
= 500ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
Calculating again with h = 1 gives the absolute limit of
dropout:
V
V
V
s
s
V
V
V
IN MIN
(
)
.
.
.
.
.
.
.
=
+
(
)
µ ×
µ
+
=
2 5
0 1
1
0 5
1
2 97
0 1
0 1
3 13
-
-
V
V
V
s
s
V
V
V
IN MIN
(
)
.
.
.
.
.
.
.
.
=
+
(
)
µ ×
µ
+
=
2 5
0 1
1
0 5
1 5
2 97
0 1
0 1
3 48
-
-
AGND PLANE
PGND
PLANE
VIA TO IC OUT
POWER
GROUND
VIA TO IC
CS PIN
V
DD
BYPASS
REF
BYPASS
V
CC
BYPASS
USE AGND PLANE TO:
- BYPASS V
CC
AND REF
- TERMINATE EXTERNAL FB, ILIM,
OVP, UVP DIVIDERS
- PIN-STRAP CONTROL INPUTS
USE PGND PLANE TO:
- BYPASS V
DD
- CONNECT IC GND PIN
TO TOP-SIDE POWER GROUND
VIA TO POWER
GROUND
TOP SIDE
BOTTOM SIDE
V
OUT
L1
C1
C2
D1
Q1
V
IN
Q2
Q
VIA R
S
VIA TO PGND PLANE
AND IC GND PIN
Figure 9. Power-Stage PC Board Layout Example
Table 5. Approximate K-Factor Errors
TON
SETTING
(kHz)
APPROXIMATE
K-FACTOR
ERROR (%)
MINIMUM V
IN
AT V
OUT
= 2V
(V)
200
±10
2.6
300
±10
2.9
450
±12.5
3.2
600
±12.5
3.6
K
FACTOR
(µs)
5
3.3
2.2
1.7