AAT2158_08 Datasheet, PDF (12/16 Pages) Advanced Analogic Technologies – 1.5A Low Noise Step-Down Converter

SwitchReg^TM
low ESR/ESL bypass ceramic capacitor. This dampens
the high Q network and stabilizes the system.
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 10μF to
22μF 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 output voltage droop due to a load transient is dom-
inated by the capacitance of the ceramic output capacitor.
During a step increase in load current, the ceramic output
capacitor alone supplies the load current until the loop
responds. Within two or three switching cycles, the loop
responds and the inductor current increases to match the
load current demand. The relationship of the output volt-
age droop during the three switching cycles to the output
capacitance can be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients.
The internal voltage loop compensation also limits the
minimum output capacitor value to 10μF. 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.
Adjustable Output Resistor Selection
The output voltage on the AAT2158 is programmed with
external resistors R3 and R4. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
gested value for R4 is 59kΩ. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensi-
tive to external noise and interference. Table 1 summa-
rizes the resistor values for various output voltages with
R4 set to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
The external resistor R3, combined with an external
100pF feed forward capacitor (C8 in Figure 1), delivers
PRODUCT DATASHEET
AAT2158
1.5A Low Noise Step-Down Converter
enhanced transient response for extreme pulsed load
applications and reduces ripple in light load conditions.
The addition of the feed forward capacitor typically
requires a larger output capacitor C3-C4 for stability. The
external resistors set the output voltage according to the
following equation:
V_OUT = 0.6V
⎛⎝_{1 +}
R3 ⎞
R4 ⎠
or
R3 =
⎡^⎛VOUT
⎣^⎝VREF
- 1^{⎞⎠⎤⎦} R4
V_OUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.0
3.3
R4 = 59kΩ
R3 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
267
R4 = 221kΩ
R3 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
Table 1: AAT2158 Resistor Values for Various
Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT2158 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 switch-
ing devices. At full load, assuming continuous conduction
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_IN
-
V_O])
+ (t_sw · F_S · I_O + I_Q) · V_IN
12
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2158.2008.02.1.2