SC1406G View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC1406G
Semtech Corporation
'SC1406G' PDF : 28 Pages View PDF
SC1406G
POWER MANAGEMENT
Rearranging equation 1), we select the minimum inductor value.
( ) ( ) G.
LMIN := dMIN⋅
VINMAX− VDAC ⋅ ESRMAX+ RCS
FS⋅ VRIPPLE
LMIN = 1.426 × 10− 6 H
This value of inductance is required up to maximum load.
Inductors with a swinging choke characteristic, where the zero
current value of inductance is much less than the full load
current inductance can be used, as long as the above restriction
is met. A value of 1.5uH is used to allow tolerances. Then, the
worst-case (low input voltage) response time (the time for the
current to reach the new transient value) is:
( ) H.
LMIN⋅ ICC650MAX − ICC650SG
dT :=
VINMIN − VDAC
dT = 1.936 × 10− 6 s
Add ~100ns for the SC1405/06 response. Since the shaded
area is triangular, the total charge taken out of the capacitor =
(DI ? Dt) / 2. Q = C ? DV = (DI ? Dt) / 2, therefore;
( ) ( ) ICC650MAX − ICC650SG ⋅ dT + 100⋅10− 9⋅sec
I. CMINP :=
2⋅ VIMVP
C
= 1.186 × 10− 4 F
This condition applies only to the positive transient. For
negative load steps, the capacitance also has to be large
enough to absorb the energy in the inductance. Since:
J.
CMINN
:=
LMIN⋅
ICC650MAX2 − ICC650SG 2
VCC650MAXTRANS2 − VFL2
CMINN = 4.045 × 10− 4 F
Using Panasonic SP-Caps, the EEFUE0E221R is 220uF at 2.5V
with 15mW ESR. Two are sufficient to meet ESR requirements,
but three are required to meet the capacitance requirement,
when tolerances are considered. The Sanyo POSCAP 4TPC150
is a 150uF capacitor with a maximum ESR specification of
45mW; six are required to meet the ESR requirement. The
resulting 900mF exceeds the capacitance requirement.
Other Component Selection:
through 28) are two differential pairs connected to the current
sense resistor. In order to cancel out common-mode noise,
resistor pairs R3-R4 and R5-R6 should be equal. For the time
being, assume R3=R4=R5=R6=1kW. These values may be
adjusted later if required.
The current limit is a function of peak current and should be set
at about 125% of the peak to allow for some overshoot of
inductor current during transients. For L=1.5uH, and
F=300kHz:
( ) VINMAX− VDAC ⋅dMIN
K. IPEAK := ICC650MAX +
2⋅LMIN⋅FS
IPEAK = 15.327 A
So, set the current limit at approximately 18.5A. Using equation
10):
L. ICLMAX:= IPEAK⋅1.25 ICLMAX = 19.159A
3⋅ VREF⋅RCLOH
M. RCLSET := RCS ⋅ICLMAX
RCLSET = 8.873 × 104 Ω
Using equation 6) we calculate RDAC:
( ) N.
RDAC :=
VIMVP − ICC650MAX⋅RCS ⋅RCORE
ICC650MAX⋅RCS
RDAC = 1.397 × 103 Ω
The offset resistor, R22, is calculated from equation 8):
VNL ⋅RDAC
ROH + VDAC
O. ROFF := VNL
VDAC
−
1
ROFF = 1.068 × 105 Ω
Equation 5) is used to calculate R1 by calculating a new value of
VHYS which accounts for the IMVP and offset dividers, and then
plugging the resulting value into equation 3).
(ROFF + ROH)
P. ( ) ( ) VHYS :=
VRIPPLE ⋅
RCORE ⋅ROFF
−
ROH ⋅ RDAC
⋅
ESR
+
RCS
⋅RCORE
⋅
RCORE
⋅ROFF
−
RDAC⋅ROH
2⋅ESR⋅RCORE⋅ ROFF + ROH
V HYS = 0.027 V
As a compromise between current sensing accuracy, efficiency,
and availability, a current sense resistor of 3mW is used. The
power dissipation is I2xR, or 555mW, so choose a 1W or larger
resistor for design margin.
VREF ⋅ ROH
Q. RHYS := 2⋅ VHYS
R HYS = 1.281 × 105 Ω
The connections to CMP, CMPREF, CL, and CLREF (pins 25
ã 2000 Semtech Corp.
16
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