LTC3711 Datasheet, PDF (19/24 Pages) Linear Technology – 5-Bit Adjustable, Wide Operating Range, No RSENSE

LTC3711
APPLICATIO S I FOR ATIO
In the design example, Figure 9, five 0.025Ω capacitors
are required in parallel to keep the output voltage within
tolerance. Using active voltage positioning, the same
specification can be met with only three capacitors. In this
case, the load step will cause an output voltage change of:
( ) ( ) ^∆VOUT(STEP) ⁼
15A


1^
3
0.025Ω
= 125mV
By positioning the output voltage 60mV above the regula-
tion point at no load, it will only drop 65mV below the
regulation point after the load step, well within the ±100mV
tolerance.
Implementing active voltage positioning requires setting a
precise gain between the sensed current and the output
voltage. Because of the variability of MOSFET on-resis-
tance, it is prudent to use a sense resistor with active
voltage positioning. In order to minimize power lost in this
resistor, a low value is chosen of 0.003Ω. The nominal
sense voltage will now be:
V_SNS(NOM) = (0.003Ω)(15A) = 45mV
To maintain a reasonable current limit, the voltage on the
V_RNG pin is reduced to its minimum value of 0.5V, corre-
sponding to a 50mV nominal sense voltage.
Next, the gain of the LTC3711 error amplifier must be
determined. The change in I_TH voltage for a corresponding
change in the output current is:
∆I_TH
=


12V
V_RNG


R_SENSE
∆I_OUT
( )( )( ) ^{= 24 0.003Ω 15A = 1.08V}
C_SS
0.1µF
1
VID2
2
RUN/SS
24
VID1
23
VID0
R_PG
100k
3
V_ON
4
PGOOD
22
BOOST
21
TG
C_C1
470pF ^RC
20k
C_C2
100pF
5
V_RNG
20
SW
6
FCB
SENSE^{+ 19}
LTC3711
7
18
I_TH
PGND
8
SGND
17
BG
C_FB 100pF
R_ON
330k
9
I_ON
16
INTV_CC
10
V_FB
C2
15
V_IN
6.8nF ₁₁
14
V_OSENSE EXTV_CC
12
VID3
13
VID4
D_B
CMDSH-3
C_B
0.33µF
C_VCC
4.7µF
R_F
1Ω
C_F
0.1µF
M1
IRF7811A
M2
IRF7811A
×2
L1
1µH
D1
UPS840
C_IN
22µF
V_IN
7V TO 24V
50V
×3
V_OUT
1.5V
15A
+ ^COUT
270µF
2V
×5
C_IN: UNITED CHEMICON THCR70EIH226ZT
C_OUT: CORNELL DUBILIER ESRE271M02B
L1: SUMIDA CEP125-IR0MC-H
3711 F09
Figure 9. CPU Core Voltage Regulator 1.5V/15A at 300kHz
3711f
19