SC1480A Ver la hoja de datos (PDF) - Semtech Corporation
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SC1480A Datasheet PDF : 23 Pages
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SC1480A
POWER MANAGEMENT
Applications Information (Cont.)
VRIPPLE_VBAT(MAX) = 21mVP-P and VRIPPLE_VBAT(MIN) = 16mVP-P
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, VOUT, should be approximately 15mVP-P at minimum
VBAT, and worst case no smaller than 10mVP-P. If
VRIPPLE_VBAT(MIN) is less than 15mVP-P the above component
values should be revisited in order to improve this.
capacitor.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
I = IN(RMS)
( ) V • V − V • V I A OUT
BAT ( MIN )
OUT
OUT
BAT _ MIN
RMS
.or our DDR2 VTT example:
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIMTR,
starting from the actual static maximum, V , OUT_ST_POS when
a load release occurs:
VOUT _ ST _POS = VOUT + ERRDC V
.or our DDR2 VTT example:
VOUT_ST_POS = 0.918V
POSLIMTR = VOUT • TOL TR V
Where TOLTR is the transient tolerance. .or our DDR2
VTT example:
POSLIMTR = 1.972V
The minimum output capacitance is calculated as
follows:
( ) COUT(MIN)
=L•
IOUT
+
IRIPPLE _ VBAT(MAX)
2
2
POSLIM TR 2
−
V2
OUT _ ST _ POS
F
This calculation assumes the absolute worst case
condition of a full-load to no load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps my be
calculated by substituting the desired current for the IOUT
term.
.or our DDR2 VTT example:
IIN(RMS) = 0.95ARMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10µ.,
1210 size, 25V ceramic capacitor can handle up to 3ARMS.
Refer to manufacturers data sheets.
.inally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the valley current, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOS.ET
RDS(ON) at VGS = 4.5V for purposes of this calculation:
I = I − I 2 A VALLEY
OUT
RIPPLE _ VBAT(MIN)
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
( ) RILIM
=
IVALLEY
• 1.2
•
RDS(ON) • 1.4
10 • 10−6
Ohms
.or our DDR2 VTT example:
IVALLEY = 2.47A and RILIM = 9.12kΩ
We select the next lowest 1% resistor value: 9.09kΩ
Thermal Considerations
The junction temperature of the device may be calculated
as follows:
COUT(MIN) = 295µ..
TJ = TA + PD • θJA °C
We will select 220µ., using one 220µ., 15mΩ Where:
2003 Semtech Corp.
12
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