LTC4090(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4090 Datasheet PDF : 28 Pages
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LTC4090/LTC4090-5
APPLICATIONS INFORMATION
regulator does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
VC pin, as shown in Figure 1. Generally a capacitor (CC)
and a resistor (RC) in series to ground are used. In ad-
dition, there may be a lower value capacitor in parallel.
This capacitor (CF) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with the front page schematic and tune
the compensation network to optimize performance. Stabil-
ity should then be checked across all operating conditions,
including load current, input voltage and temperature. The
LTC1375 datasheet contains a more thorough discussion
of loop compensation and describes how to test the sta-
bility using a transient load. Figure 5 shows the transient
response when the load current is stepped from 500mA
to 1500mA and back to 500mA.
Low Ripple Burst Mode Operation and Pulse-Skip
Mode
The LTC4090/LTC4090-5 are capable of operating in either
low ripple Burst Mode operation or pulse-skip mode which
are selected using the SYNC pin. Tie the SYNC pin below
VSYNC,L (typically 0.5V) for low ripple Burst Mode operation
or above VSYNC,H (typically 0.8V) for pulse-skip mode.
To enhance efficiency at light loads, the LTC4090/LTC4090-5
can be operated in low ripple Burst Mode operation which
keeps the output capacitor charged to the proper voltage
while minimizing the input quiescent current. During Burst
Mode operation, the LTC4090/LTC4090-5 deliver single
cycle bursts of current to the output capacitor followed
by sleep periods where the output power is delivered to
the load by the output capacitor. Because the LTC4090/
LTC4090-5 deliver power to output with single, low current
pulses, the output ripple is kept below 15mV for a typical
application. As the load current decreases towards a no
load condition, the percentage of time that the high volt-
age regulator operates in sleep mode increases and the
average input current is greatly reduced resulting in high
efficiency even at very low loads. See Figure 6.
At higher output loads (above 70mA for the front page
application) the LTC4090/LTC4090-5 will be running at
the frequency programmed by the RT resistor, and will be
operating in standard PWM mode. The transition between
PWM and low ripple Burst Mode operation is seamless,
and will not disturb the output voltage.
If low quiescent current is not required, the LTC4090/
LTC4090-5 can operate in pulse-skip mode. The benefit
of this mode is that the LTC4090/LTC4090-5 will enter full
frequency standard PWM operation at a lower output load
current than when in Burst Mode operation. The front page
application circuit will switch at full frequency at output
loads higher than about 60mA.
IL
0.5A/DIV
VIN = 12V; FIGURE 12 SCHEMATIC
ILOAD = 10mA
FIGURE 12 SCHEMATIC
HVOUT
50mV/DIV
VSW
5V/DIV
IL
1A/DIV
25μs/DIV
4090 F05
Figure 5. Transient Load Response of the LTC4090 High Voltage
Regulator Front Page Application as the Load Current is Stepped
from 500mA to 1500mA.
VOUT
10mV/DIV
5μs/DIV
4090 F06
Figure 6. High Voltage Regulator Burst Mode Operation
4090fa
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