LTC1702 Linear Technology, LTC1702 Datasheet - Page 32

• Manufacturer Part Number: LTC1702
• Description: Dual 550kHz Synchronous 2-Phase Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIONS
PGOOD1
LTC1702
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 10 s, 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
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
32
TYPICAL APPLICATIONS
V
OUT2
1.8V
GND
12A
180 F
V
C12
C15
1 F
IN
10k
3
+
R4
10k
1%
R5
8.06k
1%
4.3k
0.68 H
R3
U
L1
D3
INFORMATION
C11
820pF
U
1 F
C8
Q2B
Q1B
U
Q2A
Q1A
W
C10
100pF
R7 68k
3.3V
1 F
C6
C7 1 F
IN
U
, 2.5V/1.8V Output Power Supply
R2 39k
D2
C9
20pF
1 F
C5
PV
BOOST1
BG1
TG1
SW1
I
PGOOD1
FCB
RUN/SS
COMP1
SGND
FB1
MAX1
CC
LTC1702
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
set the output voltage. The R1 resistor must stay constant
to ensure that the loop compensation is not affected.
RUN/SS2
PGOOD2
BOOST2
COMP2
10 F
FAULT
PGND
I
C4
MAX2
SW2
BG2
TG2
FB2
V
CC
10
R1
C16 1 F
1 F
D1
C3
36k
C17
100pF
R8
R9 27k
1000pF
Q3
Q4
1 F
C18
1000pF
C2
C19
+
BIAS
C1
470 F
1 H
L2
2
1702 TA02
resistor is modified to
R10
2.4k
10.7k
4.99k
R12
R13
1%
1%
+
C20
180 F
V
IN
10k
V
2.5V
5A
C21
1 F
GND
PGOOD2
FAULT
V
3.3V
OUT1
IN
5%