AAT1155 Datasheet, PDF (14/17 Pages) Advanced Analogic Technologies – 1MHz 2.5A Step-Down DC/DC Converter

AAT1155
1MHz 2.5A Step-Down DC/DC Converter
AAT1155 Junction Temperature
P_ON = ^IO2
·
R_DS(ON)
V_IN
·
V_O
+ ^{⎛ tSW}
⎝
·
F·
2
I_O
+
I_{Q⎠⎞}
·
V_IN
=
2.5² · 70mΩ · 3.3V+ ⎛20ns · 1MHz · 2.5A _{+ 690µA}⎞
5V
⎝
2
⎠
0.42 Watts
T_J(MAX) = T_AMB + θ_JA · P =
70°C + 150°C/W · 0.42W = 133°C
Diode
I_DIODE =
I_O ·
⎛⎝_1-
V_{O ⎞}
V_IN ^⎠
=
2.5A · ^⎛⎝1 -
3.3V ⎞
5.25V^⎠
=
0.93A
V_FW = 0.35V
P_DIODE = V_FW · I_DIODE =
0.35V ·0.93A = 0.33A
Given an ambient thermal resistance of 120°C/W
from the manufacturer's data sheet, T_J(MAX) of the
diode is:
T_J(MAX)= T_AMB + Θ_JA · P =
70°C +120°C /W · 0.33W =
109°C
Output Capacitor
The output capacitor value required for sufficient
loop phase margin depends on the type of capaci-
tor selected. For a low ESR ceramic capacitor, a
minimum value of 200µF is required. For a low
ESR tantalum capacitor, lower values are accept-
able. While the relatively higher ESR associated
with the tantalum capacitor will give more phase
margin and a more dampened transient response,
the output voltage ripple will be higher.
The 120µF Vishay 594D tantalum capacitor has an
ESR of 85mΩ and a ripple current rating of 1.48Arms
in a C case size. Although smaller case sizes are suf-
ficiently rated for this ripple current, their ESR level
would result in excessive output ripple.
The ESR requirement for a tantalum capacitor can
be estimated by :
ESR ≤^VRIPPLE=100mV= 111mΩ
∆I 0.9A
I_RMS =
1
2·
3
· ^{(VOUT+ VFWD)· (VIN}
L · F · V_IN
-
V_OUT) ₌
1 _· 3.65V·1.7V _{= 240mArms}
2· 3 ^{1.5µH· 1MHz·5V}
Two or three 1812 X5R 100µF 6.3V ceramic
capacitors in parallel also provide sufficient phase
margin. The low ESR and ESL associated with
ceramic capacitors also reduces output ripple sig-
nificantly over that seen with tantalum capacitors.
Temperature rise due to ESR ripple current dissi-
pation is also reduced.
Input Capacitor
The input capacitor ripple is:
I_RMS = I_O ·
V_{O · ⎛1-} V_{O ⎞ = 1.82Arms}
V_IN ⎝ V_IN ⎠
In the examples shown, C1 is a ceramic capacitor
located as closely to the IC as possible. C1 pro-
vides the low impedance path for the sharp edges
associated with the input current. C4 may or may
not be required, depending upon the impedance
characteristics looking back into the source. It
serves to dampen any input oscillations that may
arise from a source that is highly inductive. For
most applications, where the source has sufficient
bulk capacitance and is fed directly to the AAT1155
through large PCB traces or planes, it is not
required. When operating the AAT1155 evaluation
board on the bench, C4 is required due to the
inductance of the wires running from the laborato-
ry power supply to the evaluation board.
14
1155.2005.11.1.6