LTC1873 Linear Technology, LTC1873 Datasheet - Page 29

• Manufacturer Part Number: LTC1873
• Description: Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Implementing voltage positioning is as simple as creating
an intentional resistance in the output path to generate the
required voltage drop. This resistance can be a low value
resistor, a length of PCB trace, or even the parasitic
resistance of the inductor if an appropriate filter is used. If
the LTC1873 senses the output voltage upstream from the
resistance (Figure 16c), the output voltage will move with
load as I • R
value of the voltage positioning resistor. If the feedback
network is then reset to regulate near the upper edge of the
specified tolerance, the output voltage will ride high when
I
to a traditional regulator (Figure 16a), a voltage position-
ing regulator can theoretically stand as much as twice the
ESR drop across the output capacitor while maintaining
output voltage regulation. This means smaller, cheaper
output capacitors can be used while keeping the output
voltage within acceptable limits.
LOAD
is low and will ride low when I
Figure 16c. Voltage Positioning Regulator
LTC1873
LTC1873
VP
Figure 16a. Standard Regulator
FB
FB
, where I is the load current and R
U
V
V
U
IN
IN
LOAD
POSITIONING
W
RESISTOR
VOLTAGE
+
(R
VP
is high. Compared
1873 F16a
)
+
V
OUT
1873 F16c
V
OUT
U
VP
is the
CURRENT
CURRENT
Figure 16b. Standard Regulator—Transient Response
Figure 16d. Positioning Regulator—Transient Response
LOAD
LOAD
V
V
OUT
OUT
Measurement Techniques
Measuring transient response presents a challenge in two
respects: obtaining an accurate measurement and gener-
ating a suitable transient to use to test the circuit. Output
measurements should be taken with a scope probe
directly across the output capacitor. Proper high fre-
quency probing techniques should be used. In particular,
don’t use the 6" ground lead that comes with the probe!
Use an adapter that fits on the tip of the probe and has a
short ground clip to ensure that inductance in the ground
path doesn’t cause a bigger spike than the transient signal
being measured. Conveniently, the typical probe tip ground
clip is spaced just right to span the leads of a typical output
capacitor.
Now that we know how to measure the signal, we need to
have something to measure. The ideal situation is to use
NOM
NOM
+5%
–5%
MAX
+5%
–5%
MAX
0
0
TIME
TIME
1873 F16b
1873 F16d
MAXIMUM
ALLOWABLE
TRANSIENT
MAXIMUM
ALLOWABLE
TRANSIENT
2 FIGURE 16b
LTC1873
29