LTC1702 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702' PDF : 36 Pages View PDF
LTC1702
APPLICATIONS INFORMATION
FAULT BEHAVIOR
Changing the Output Voltage on the Fly
Some applications use a switching scheme attached to the
feedback resistors to allow the system to adjust the
LTC1702 output voltage. The voltage can be changed on
the fly if desired, but care must be taken to avoid tripping
the overvoltage fault circuit. Stepping the voltage upwards
abruptly is safe, but stepping down quickly by more than
15% can leave the system in a state where the output
voltage is still at the old higher level, but the feedback node
is set to expect a new, substantially lower voltage. If this
condition persists for more than 10µs, the overvoltage
fault circuitry will fire and latch off the LTC1702.
The simplest solution is to disable the fault circuit by
grounding the FAULT pin. Systems that must keep the
fault circuit active should ensure that the output voltage is
never programmed to step down by more than 15% in any
single step. The safest strategy is to step the output down
by 10% or less at a time and wait for the output to settle
to the new value before taking subsequent steps.
VID Applications
Certain microprocessors specify a set of codes that corre-
spond to power supply voltages required from the regula-
tor system. If these codes are changed on the fly, the same
caveats as above apply. In addition, the switching matrix
that programs the output voltage may vary its resistance
significantly over the entire span of output voltages,
potentially changing the loop compensation if the circuit is
not designed properly. With a typical type 3 feedback loop
(Figure 8), make sure that the RBIAS resistor is modified to
set the output voltage. The R1 resistor must stay constant
to ensure that the loop compensation is not affected.
TYPICAL APPLICATIONS
D1, D2: MOTOROLA MBR0520LT1
D3: MOTOROLA MBRS320T3
C1: KEMET T510X477M006AS
C12, C20: PANASONIC EEFUE0G181R
L1: SUMIDA CEP1254712-T007
L2: SUMIDA CDRH744734-JPS023
Q1A, Q1B, Q2A, Q2B: SILICONIX Si9804 C8
Q3, Q4: 1/2 SILICONIX Si4966
1µF
VOUT2
1.8V
12A
C12 +
180µF
×3
C15
1µF
GND
VIN
10k
PGOOD1
L1
0.68µH
Q2B
D3
R3
Q1B
4.3k
R4
C11
10k
820pF
1%
R5
8.06k
1%
32
3.3VIN, 2.5V/1.8V Output Power Supply
D2
C5
1µF
C6
1µF
C4 R1
10µF 10Ω
PVCC
IMAX2
BOOST1 BOOST2
Q2A
BG1
BG2
TG1
TG2
SW1
SW2
R2 39k
LTC1702
Q1A
IMAX1
PGND
PGOOD1 PGOOD2
C7 1µF
FCB
FAULT
RUN/SS RUN/SS2
COMP1 COMP2
SGND
FB2
FB1
VCC
R7 68k
C10
C9
100pF
20pF
D1
+ C1
470µF
×2
C2
R8
1µF
36k
VIN
3.3V
± 5%
C3
1µF
C16 1µF
Q3
Q4
C19
1000pF
L2
1µH
VOUT1
2.5V
5A
R10
+ C20
2.4k
180µF
C21
R12
1µF
10.7k
1%
GND
R13
VIN
4.99k
1%
10k
PGOOD2
FAULT
R9 27k
C17
100pF
C18
1000pF
1702 TA02
1702fa
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