AAT2315_07 Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – Dual 600mA Step-Down Converter with Synchronization

Adjustable Output Resistor Selection
Resistors R1 through R4 of Figure 1 program the out-
put to regulate at a voltage higher than 0.6V. To limit
the bias current required for the external feedback
resistor string, the minimum suggested value for R2
and R4 is 59kΩ. Although a larger value will reduce
the quiescent current, it will also increase the imped-
ance of the feedback node, making it more sensitive
to external noise and interference. Table 2 summa-
rizes the resistor values for various output voltages
with R2 and R4 set to either 59kΩ for good noise
immunity or 221kΩ for reduced no load input current.
R1
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R2
=
⎛ ^1.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
88.5kΩ
With an external feedforward capacitor (C4 and C5 of
Figure 2) the AAT2513 delivers enhanced transient
response for extreme pulsed load applications. The
addition of the feedforward capacitor typically requires
a larger output capacitor (C1 and C2) for stability.
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
3.3
R2, R4 = 59kΩ
R1, R3 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
265
R2, R4 = 221kΩ
R1, R3 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 2: Feedback Resistor Values.
Thermal Calculations
There are three types of losses associated with the
AAT2513 converter: switching losses, conduction
losses, and quiescent current losses. The conduction
AAT2513
Dual 600mA Step-Down
Converter with Synchronization
losses are associated with the R_DS(ON) characteristics
of the power output switching devices. The switching
losses are dominated by the gate charge of the
power output switching devices. At full load, assum-
ing continuous conduction mode (CCM), a simplified
form of the dual converter losses is given by:
P_TOTAL
=
I_O12
·
(R_DSON(HS)
·
V_O1
+ R_DSON(LS)
V_IN
·
[V_IN
-V_O1])
+ ^{IO22 · (RDSON(HS) · VO2 + RDSON(LS) · [VIN -VO2])}
V_IN
+ (t_sw · F · [I_O1 + I_O2] + 2 · I_Q) · V_IN
I_Q is the AAT2513 quiescent current for one chan-
nel and t_SW is used to estimate the full load switch-
ing losses.
For the condition where channel one is in dropout
at 100% duty cycle the total device dissipation
reduces to:
P_TOTAL = I_O12 · R_DSON(HS)
+ ^{IO22 · (RDSON(HS) · VO2 + RDSON(LS) · [VIN -VO2])}
V_IN
+ (t_sw · F · I_O2 + 2 · I_Q) · V_IN
Since R_DS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θ_JA for the
QFN33-12 package which is 28°C/W to 50°C/W
minimum.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
14
2513.2007.04.1.1