Datasheet

32
LTC1702
1702fa
APPLICATIONS INFORMATION
WUU
U
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 R
BIAS
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
U
3.3V
IN
, 2.5V/1.8V Output Power Supply
LTC1702
PV
CC
BOOST1
BG1
TG1
SW1
I
MAX1
PGOOD1
FCB
RUN/SS
COMP1
SGND
FB1
I
MAX2
BOOST2
BG2
TG2
SW2
PGND
PGOOD2
FAULT
RUN/SS2
COMP2
FB2
V
CC
C5
1µF
R1
10Ω
C6
1µF
C8
1µF
R2 39k
R8
36k
R10
2.4k
R9 27k
1702 TA02
R3
4.3k
R4
10k
1%
C11
820pF
C9
20pF
C10
100pF
R7 68k
C15
1µF
V
OUT2
1.8V
12A
C7 1µF
R5
8.06k
1%
10k
Q2A
Q2B
Q1B
Q1A
D3
D2
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
Q3, Q4: 1/2 SILICONIX Si4966
D1
GND
L1
0.68µH
C12
180µF
×3
C4
10µF
C3
1µF
C2
1µF
C16 1µF
C1
470µF
×2
+
Q3
Q4
C19
1000pF
C17
100pF
C18
1000pF
R12
10.7k
1%
R13
4.99k
1%
L2
1µH
C20
180µF
C21
1µF
V
OUT1
2.5V
5A
GND
V
IN
V
IN
FAULT
PGOOD2
V
IN
3.3V
± 5%
+
+
PGOOD1
10k