AAT1415 Datasheet, PDF (27/32 Pages) Advanced Analog Technology, Inc. – FIVE-CHANNEL DC-DC CONVERTER WITH A 2.5V LDO

Advanced Analog Technology, Inc.
May 2008
AAT1415/AAT1415A
Table 3. Inductor Peak Current
Topology Mode
Peak Current
Boost
CCM
I_{O +} ∆I_{L ,}
(1− D) 2
∆I_L
=
V_OUT ⋅ D ⋅ (1− D) _,
f_SW ⋅ L
D =1− ^VIN
V_OUT
Buck
DCM
CCM
2 ⋅ (V_OUT − V_IN ) ⋅ I_O
L ⋅ f_SW
I_O
+
∆I_L
2
,
( ∆I_L
=
V_OUT ⋅ (1− D) _,
f_SW ⋅ L
D= ^VOUT )
V_IN
DCM
2 ⋅ (V_IN − V_OUT ) ⋅ V_OUT ⋅I_O
L ⋅ f_SW ⋅ V_IN
Inverting
CCM
I_O
+
∆I_L
2
,
( ∆I_L
=
V_IN ⋅ D
f_SW ⋅ L
,
D = ^VOUT )
V_OUT + V_IN
DCM
2 ⋅ V_OUT ⋅I_O
L ⋅ f_SW
V_IN : Input Voltage, V_OUT : Output Voltage, I_O : Output
Current, L: Inductance, f_SW : Switch Frequency, Peak
Current Unit: Ampere
Schottky Diode Selection
Choose a Schottky diode who’s maximum reverse
voltage rating is greater than the value in Table 4, and
who’s current rating is greater than the peak inductor
current.
Table 4. Diode and MOS Minimum Voltage Rating
Topology
Minimum Voltage Rating
Boost
V_OUT
Buck
V_IN
Inverting
V_IN+ V_OUT
External MOSFET Selection
The boost controller and Inverting controller drive
external logic-level MOSFETs. MOSFETs’ maximum
drain-to-source voltage ( V_DS(MAX) ) rating must be
greater than the value in Table 4. Their on-resistance
( R_DS(ON) ), total gate charge ( Q_G ) and reverse
transfer capacitance ( C_RSS ) are the lower the better.
Input Capacitor
The input current to converters are discontinuous, and
therefore input capacitors are required to supply the AC
current to converters while maintaining the DC input
voltage. A low ESR capacitor is required to keep the
noise at the IC to a minimum. Ceramic capacitors are
preferred, but tantalum or low-ESR electrolytic
capacitors may also suffice. For insuring stable
operation a bypass ceramic 0.1µF capacitor should be
placed as close to the IC VDD pin as possible.
Output Capacitor
The output capacitor is required to maintain the DC
output voltage. Low ESR capacitors are preferred to
keep the output voltage ripple low. The characteristics
of the output capacitor also affect the stability of the
regulation control system. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended. In the
case of ceramic capacitors, the impedance at the
switching frequency is dominated by the capacitance,
and so the output voltage ripple is mostly independent
of the ESR. The output voltage ripple is estimated to
be:
V_RIPPLE
≈
∆I_L
2π × f_SW × C_OUT
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