AAT1152 Advanced Analogic Technologies, AAT1152 Datasheet - Page 14

• Manufacturer Part Number: AAT1152
• Description: 850kHz 1A Synchronous Buck DC/DC Converter
• Manufacturer: Advanced Analogic Technologies

Adjustable Output
For applications requiring an output other than the
fixed outputs available, the 1V version can be pro-
grammed externally. Resistors R3 and R4 of figure
3 force the output to regulate higher than 1 Volt.
R4 should be 100 times less than the internal 1
MegOhm resistance of the FB pin. Once R4 is
selected R3 can be calculated. For a 1.25V output
with R4 set to 10.0k, R3 is 2.55k .
Once the total losses have been determined the junction temperature can be derived from the
MSOP-8 package.
Design Example
Specifications
I
I
V
V
F
Maximum Input Capacitor Ripple:
14
I
P = ESR
OUT
RIPPLE
RMS
s
OUT
IN
= 830 kHz
R3 = (V
= 2.7 - 4.2 V (3.6V nominal)
= 1.0A
= I
= 1.5V
= 30% of full load at max V
O
COUT
O
V
- 1) R4 = 0.25 10.0k
V
INMAX
I
RMS
O
2
P
= 5m
LOSS
1-
V
=
INMAX
V
I
O
O
0.5
2
2
R
=
A = 1.25mW
DSON(H)
IN
I
2
O
= 0.5A
= 2.55k
850kHz 1A Synchronous Buck DC/DC Converter
V
O
RMS
+ R
V
IN
DSON(L)
V
IN
Layout Considerations
Figures 5 and 6 display the suggested PCB layout
for the AAT1152. The most critical aspect of the lay-
out is the placement of the input capacitor C1. For
proper operation C1 must be placed as close as
possible to the AAT1152.
Thermal Calculations
There are two types of losses associated with the
AAT1152 output switching MOSFET, switching
losses and conduction losses. The conduction
losses are associated with the Rds(on) character-
istics of the output switching device. At full load,
assuming continuous conduction mode (CCM), a
simplified form of the total losses is:
- V
O
))
+ t
sw
F I
O
V
IN
+ I
Q
V
IN
AAT1152
1152.2003.01.0.9
JA
for the