AAT1171 Datasheet, PDF (17/22 Pages) Advanced Analogic Technologies – 600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch

AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
In applications where the input power source lead
inductance cannot be reduced to a level that does
not affect the converter performance, a high ESR
tantalum or aluminum electrolytic capacitor (C3 of
Figure 4) should be placed in parallel with the low
ESR, ESL bypass ceramic capacitor. This damp-
ens the high Q network and stabilizes the system.
DAC Programming Gain
The output voltage is dynamically controlled by the
DAC input voltage. The DAC to output gain is fixed
at 3. The typical response time for a 0.2V to 1.2V
pulsed signal on the DAC input is less than 30µs.
The DAC gain can be reduced by an external resis-
tive divider at the DAC input, as shown in the eval-
uation board schematic in Figure 2. For a DAC to
output gain of 2 and R2 at 10kΩ, R1 is 4.99kΩ.
R1 =
(3- G_DAC)R2
G_DAC
=
(3 - 2)10kΩ
2
= 4.99kΩ
Thermal Calculations
There are three types of losses associated with the
AAT1171 step-down converter: switching losses,
conduction losses, and quiescent current losses.
Conduction losses are associated with the R_DS(ON)
characteristics of the power MOSFET devices.
Switching losses are dominated by the gate charge
of the power MOSFET devices. The AAT1171 main
and synchronous power MOSFETs are sized to
have similar R_DS(ON) values that track with the input
voltage. At full load, assuming continuous conduc-
tion mode (CCM), a simplified form of the step-
down converter losses is given by:
P_TOTAL = I_O2 · R_DS(ON) + (t_SW · F_S · I_O + I_Q) · V_IN
I_Q is the step-down converter quiescent current.
The term t_sw is used to estimate the full load switch-
ing losses, which are dominated by the gate charge
losses.
GND
U1
AAT1171
1
N/C
12
LX
2
VOUT
11
PGND
3
VOUT
10
VIN
4
VCC
9
MODE/SYNC
5
AGND
8
BYPASS
6
DAC
7
EN
L1
2.2µH
C1
4.7µF
V_OUT
C2
4.7µF
GND
V_IN
DAC ^R1
R2
123
321
123
ENABLE BYPASS SYNC
Off On On Off LL PWM
L1 SD3112-2R2 or LPF2010-2R2
C1, COUT 4.7µF 10V 0805
Figure 2: AAT1171 Evaluation Board Schematic.
1171.2006.06.1.0
17