CS5151
Applications Information: continued
If the input voltage rises quickly, or the regulator output is
enabled externally, output voltage will increase to the level
set by the error amplifier output more rapidly, usually
within a couple of cycles (see Figure 4).
Trace1 - Regulator Output Voltage (10V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 6: Peak-to-peak ripple on V
OUT
= 2.8V, I
OUT
= 13A (heavy load).
Trace 1 - Regulator Output Voltage (5V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 4: CS5151 demonstration board enable startup waveforms.
Normal Operation
During normal operation, switch off time is constant and
set by the C
OFF
capacitor. Switch on time is adjusted by the
V
2
™ control loop to maintain regulation. This results in
changes in regulator switching frequency, duty cycle, and
output ripple in response to changes in load and line.
Output voltage ripple will be determined by inductor rip-
ple current working into the ESR of the output capacitors
(see Figures 5 and 6).
Transient Response
The CS5151 V
2
™ control loop’s 100ns reaction time pro-
vides unprecedented transient response to changes in
input voltage or output current. Pulse by pulse adjustment
of duty cycle is provided to quickly ramp the inductor cur-
rent to the required level. Since the inductor current cannot
be changed instantaneously, regulation is maintained by
the output capacitor(s) during the time required to slew the
inductor current.
Overall load transient response is further improved
through a feature called “adaptive voltage positioning”.
This technique pre-positions the output capacitor’s voltage
to reduce total output voltage excursions during changes
in load.
Holding tolerance to 1% allows the error amplifier’s refer-
ence voltage to be targeted +40mV high without compro-
mising DC accuracy. A “droop resistor“, implemented
through a PC board trace, connects the error amplifier’s
feedback pin (V
FB
) to the output capacitors and load and
carries the output current. With no load, there is no DC
drop across this resistor, producing an output voltage
tracking the error amplifier’s, including the +40mV offset.
When the full load current is delivered, an 80mV drop is
developed across this resistor. This results in output volt-
age being offset -40mV low.
The result of adaptive voltage positioning is that additional
margin is provided for a load transient before reaching the
output voltage specification limits. When load current sud-
denly increases from its minimum level, the output capaci-
tor is pre-positioned +40mV. Conversely, when load cur-
rent suddenly decreases from its maximum level, the out-
put capacitor is pre-positioned -40mV (see Figures 7, 8, and
9). For best transient response, a combination of a number
of high frequency and bulk output capacitors are usually
used.
Trace 1 - Regulator Output Voltage (10V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 5: Peak-to-peak ripple on V
OUT
= 2.8V, I
OUT
= 0.5A (light load).
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
