LTC2970-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2970-1' PDF : 36 Pages View PDF
LTC2970/LTC2970-1
APPLICATIO S I FOR ATIO
6. Solve for Channel 1’s tracking counter delay relative to
Channel 0, CH1_A_DELAY_TRACK().
CH1_ A _DELAY _ TRACK() =
(19)
( ) VDC1,NOM′ − VDC0,NOM′
• R31B
R21 (counts)
1μA / count •R41
Note: VDCn,NOMʹ is based on the final values of R2n and
R1n. If the result for CH1_A_DELAY_TRACK() is less than
0, apply the unsigned result to the CH0_A_DELAY_TRACK()
register.
7. Solve for the IDAC0 and IDAC1 terminal tracking codes,
Chn_a_idac_track[7:0].
Chn _ a _ idac _ track[7:0]=
(20)
255 −
VFBn
(LSB’s)
1μA / LSB•R4n
Note: This formula assumes that the Chn_a_idac_pol bit
is set to 0.
Margining Application Circuit Design Example
Consider the LTC2970 application circuit shown in Figure
1. Channel 0 is a DC/DC converter whose output needs
to be varied between 3.63V and 1.62V. VFB0 = 0.8V and
assume that IFB0 = 0A.
1. Assume values for feedback resistor R20 and the
nominal DC/DC converter output voltage VDC0,NOM, and
solve for R10.
Let VDC0,NOM = 2.625V (the average of 3.63V and 1.62V)
and assume that R20 = 10kΩ. From Equation 1:
R10 =
R20 • VFB0
=
VDC,NOM − IFB0 •R20 − VFB0
10kΩ • 0.8V = 4,384Ω
2.625V − 0.8V
Let R10 = 4.37kΩ (the nearest E192 series resistor
value).
2. Solve for the value of R30 that yields the maximum
required DC/DC converter output voltage VDC0,MAX
From Equation 2:
R30 ≤ R20 •(VFB − 10mV) =
VDC,MAX − VDC,NOM
10.0kΩ •(0.8V − 10mV) = 7,861Ω
3.63V − 2.625V
Let R30 = 7.68kΩ.
3. Solve for the value of R40 that’s needed to yield the
minimum required DC/DC converter output voltage
VDC0,MIN.
From Equation 3:
( ) R40 ≥
VDC,NOM − VDC,MIN
•
R30
R20
+
VFB
=
236μA
(2.625V − 1.62V)• 7.96kΩ + 0.8V
10kΩ
= 6,780Ω
236μA
Let R40 = 6.81kΩ.
4. Re-calculate the minimum, nominal, and maximum
DC/DC converter output voltages and the resulting mar-
gining resolution.
From Equations 4, 5, and 6:
VDC0,NOM
=
VFB
•
⎛
⎝⎜
1+
R20
R10
⎞
⎠⎟
+ IFB
• R20
=
0.8V
•
⎛
⎝⎜
1+
10kΩ ⎞
4.37kΩ ⎠⎟
=
2.631V
( ) VDC0,MIN
<
VDC0,NOM
−
R20
R30
•
236μA •R40 − VFB0
→
VDC0,MIN
<
2.631V
−
10kΩ
7.68kΩ
•
(236μA • 6.81kΩ − 0.8V − 10mV) = 1.59V
29701fc
31
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