AAT1130 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – 2.5MHz 500mA Step-Down DC/DC Converter

DATA SHEET
AAT1130
2.5MHz 400mA Step-Down DC/DC Converter
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evalu-
ation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem. 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 should be
placed in parallel with the low ESR, ESL bypass ceramic.
This dampens the high Q network and stabilizes the sys-
tem.
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 4.7μF to
10μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL charac-
teristics necessary for low output ripple.
The internal voltage loop compensation also limits the
minimum output capacitor value to 4.7μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
C_FF
R1
FB
R2
Figure 2: AAT1130 External Resistor
Output Voltage Programming.
Feedback Resistor Selection
Resistors R1 and R2 of Figure 2 program the output to
regulate at a voltage higher than 0.6V. To limit the bias
current required for the external feedback resistor string
while maintaining good noise immunity, the minimum
suggested value for R2 is 59kΩ. Although a larger value
will further reduce quiescent current, it will also increase
the impedance of the feedback node, making it more
sensitive to external noise and interference. Table 1
summarizes the resistor values for various output volt-
ages with R2 set to either 59kΩ for good noise immunity
or 221kΩ for reduced no load input current.
R1 =
V_OUT
V_FB
-1
· R2 =
1.5V
0.6V ^{- 1 · 59kΩ = 88.5kΩ}
The AAT1130, combined with an external feedforward
capacitor (C3 in Figure 2), delivers enhanced transient
response for extreme pulsed load applications. The addi-
tion of the feedforward capacitor typically requires a
larger output capacitor C1 for stability.
V_OUT (V)
0.9
1
1.1
1.2
1.3
1.4
1.5
1.8
R2 = 59kΩ R1
(kΩ)
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
R2 = 221kΩ
R1 (kΩ)
113
150
187
221
261
301
332
442
Table 1: Feedback Resistor Values.
Thermal Calculations
There are three types of losses associated with the
AAT1130 step-down converter: switching losses, conduc-
tion losses, and quiescent current losses. Conduction
losses are associated with the R_DS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the losses is
given by:
P_TOTAL =
I_O2 · (R_DS(ON)H · V_O + R_DS(ON)L · [V_IN - V_O])
V_IN
+ (t_sw · F_S · I_O + I_Q) · V_IN
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201977B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
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