AAT2500B Datasheet, PDF (14/21 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

SystemPower^TM
Adjustable Output Resistor Selection
The output voltage on the step-down converter is pro-
grammed with external resistors R2 and R6. To limit the
bias current required for the external feedback resistor
string while maintaining good noise immunity, the mini-
mum suggested value for R6 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
2 summarizes the resistor values for various output volt-
ages with R6 set to either 59kΩ for good noise immu-
nity or 221kΩ for reduced no load input current.
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor (C1
in Fig.3) can be added.
V_OUT (V)
0.9*
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
2.8
3.0
3.3
R6 = 59kΩ
R2 (kΩ)
0
6.65
13.3
19.6
26.1
32.4
39.2
59.0
61.9
71.5
105
124
137
158
R6 = 221kΩ
R2 (kΩ)
0
24.3
48.7
73.2
97.6
124
147
221
232
274
392
464
511
590
Table 2: Step-Down Converter Resistor Values for
Various Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT2500B step-down converter: switching losses, con-
duction 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 LDO losses
is given by:
PRODUCT DATASHEET
AAT2500B
Adjustable 3-Channel Regulator
P_TOTAL
=
I_O2
·
(R_DSON(HS)
·
V_O
+ R_DSON(LS)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F · 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 step-down con-
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
P_TOTAL = I_O2 · R_DSON(HS) + I_Q · V_IN
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the θ_JA for the QFN34-20 pack-
age which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
LDO Linear Regulator Input Capacitor
A 1μF or larger capacitor is typically recommended for
C_IN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation; however, if the
AAT2500B is physically located more than three centi-
meters from an input power source, a C_IN capacitor will
be needed for stable operation. C_IN should be located as
closely to the device VLDO pins as practically possible.
C_IN values greater than 1μF will offer superior input line
transient response and will assist in maximizing the
highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for C_IN. There is no specific capacitor
ESR requirement for C_IN; however, ceramic capacitors
are recommended for C_IN due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as batteries in
portable devices.
* For the 0.9V output, R6 is open.
14
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2500B.2008.03.1.0