AAT2786 Datasheet, PDF (24/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

SystemPower^TM
PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
Step-Down Converter Design Example
Specifications
V_OBUCK = 3.3V @ 1.5A, Pulsed Load ∆I_LOAD = 1.5A
V_OLDO = 2.5V @ 150mA
V_IN = 2.7V to 4.2V (3.6V nominal)
F_S = 1.2MHz
m = 0.75A/µs
T_AMB = 85°C in TDFN34-16 Package
3.3V Buck Output Inductor
L=
0.75 · V_O
m
=
0.75
·
3.3V
A
=
3.3µH
(see
Table
2)
0.75 _µs
For Sumida inductor CDRH4D28, 3.3µH, DCR = 49.2mΩ max.
∆I =
V_OBUCK
L1 · F_S
·1-
V_OBUCK
V_INBUCK
=
3.3V
3.3µH · 1.2MHz
·
1
-
3.3V
4.2V
= 179mA
I_PK
=
I_OBUCK
+
∆I₁
2
=
1.5A
+
0.089A
=
1.59A
P_L1
=
I²
OUTBUCK
·
DCR
=
1.5A²
·
49.2mΩ
=
110mW
3.3V Buck Output Capacitor
V_DROOP = 0.2V
C_OUT
=
3 · ∆I_LOAD
V_DROOP · F_S
=
3 · 1.5A
0.2V · 1.2MHz
= 18.8µF; use 22µF
I = _RMS(MAX)
1
2·
· ^{(VOUT) · (VIN(MAX) - VOUT)}
3
L · F_S · V_IN(MAX)
=¹
2·
3
·
3.3V · (4.2V - 3.3V)
3.3µH · 1.2MHz · 4.2V
= 52mArms
P_RMS = ESR · I_RMS2 = 5mΩ · (52mA)² = 13.3µW
3.3V Buck Input Capacitor
Input Ripple V_PP = 50mV
C_IN =
1
=
V_PP
I_OBUCK
- ESR
· 4 · F_S
1
= 7.3µF; use 10µF
50mV
1.5A
- 5mΩ
· 4 · 1.2MHz
I = _RMS(MAX)
I_OBUCK
2
= 0.75Arms
P = ESR · (I_RMS2) = 5mΩ · (0.75A)² = 3mW
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2786.2008.04.1.0