SC1406G View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC1406G
Semtech Corporation
'SC1406G' PDF : 28 Pages View PDF
SC1406G
POWER MANAGEMENT
Requirements:
The critical processor requirements are:
1. V = CC650MAXDC 1.65V
2. V = CC650MINDC 1.485V
3. V = CC650MAXTRANS 1.715V
4. V = CC650MINTRANS 1.485V
5. V = CC500MAXDC 1.45V
6. V = CC500MINDC 1.25V
7. VCC500MAXTRANS = 1.45V
8. V = CC500MINTRANS 1.25V
9. ICC650MAX = 13.6A
10. ICC650SG = 2.2A
11. ICC500MAX = 9.5A
12. ICC500SG = 1.7A
13. dICC/dt = 1400A/mS (at the processor. Local decoupling
reduces the requirement at the regulator.)
Figure 3 - Hysteretic Converter Response to a Positive
Transient
The system requirements are to minimize the number of
capacitors, and:
1. VADAPTERMAX = 21V
2. VBATTMIN = 10V
In a hysteretic converter with adaptive voltage positioning, like
the SC1406, two conditions determine if you meet the positive
transient requirements:
( ) D. VIMVP ≥ ICC650MAX − ICC650SG ⋅ESR
Basic Calculations:
The 0.85% DAC output voltage accuracy accounts for an
uncertainty of 14mV at the 1.60V setting. In addition, 20mV of
resistive drop is expected in the power distribution from the
current sense resistor to the processor. A footnote in the
processor specification reads that the long-term voltage should
never exceed 1.65V. As a result, the nominal value of the no-
load voltage [V (0)] should be set accordingly.
A. VNL := VCC650MAXDC − VTOLVNL = 1.636 V
The full-load voltage (V (fl)) is set similarly, including tolerance
and DC drop.
B. VFL := VCC650MINDC + VTOL + VDIST VFL = 1.519 V
( ) E. VIMVP ≥ deltaV COUT
The first condition is easy to see if the ESR is too high, the
transient response will fail.
In the second condition, because the hysteretic converter
responds in < 100ns, the capacitor does not droop very far
before the inductor current starts ramping up. (This is not true
of control schemes where time constants in the error amplifier
cause delays.) Once the inductor current starts to rise, the
increasing DV of the capacitor is offset by reduced DV from the
ESR, so DV is constant. If the DV due to the charge taken from
the capacitor before the inductor current reaches the load
current (see the shaded area above) is less than VIMVP, then the
transient response will pass.
The regulator is to be designed with 40mV of output ripple, so
the effective IMVP voltage drop (VIMVP) is:
VRIPPLE
C. VIMVP:= VNL − VFL −
2
VIMVP = 0.098 V
(
)
Output Inductor and Capacitor Selection:
Output capacitance and ESR values are a function of transient
requirements and output inductor value. The following figure
illustrates the response of a hysteretic converter to a positive
transient:
ã 2000 Semtech Corp.
The maximum ESR requirement is:
VIMVP
( ) F. ESRMAX:= ICC650MAX − ICC650SG
ESRMAX= 8.579 × 10− 3 Ω
For the second condition, we need to know the inductor value,
which is a function of the highest desired switching frequency.
The maximum frequency occurs at the highest input voltage. As
a reasonable compromise between efficiency and component
size, a maximum switching frequency of 300kHz is desired.
15
www.semtech.com
Share Link: 