Datasheet

LTC4090/LTC4090-5
21
4090fc
Figure 5. Transient Load Response of the LTC4090 High
Voltage Regulator Front Page Application as the Load
Current is Stepped from 500mA to 1500mA.
Figure 6. High Voltage Regulator Burst Mode Operation
High Voltage Regulator Frequency Compensation
The LTC4090/LTC4090-5 high voltage regulator uses
current mode control to regulate the output. This simpli-
fi es loop compensation. In particular, the high voltage
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
V
C
pin, as shown in Figure 1. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In ad-
dition, there may be a lower value capacitor in parallel.
This capacitor (C
F
) is not part of the loop compensation
but is used to fi lter 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 data sheet 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-Skipping Mode
The LTC4090/LTC4090-5 are capable of operating in either
low ripple Burst Mode operation or pulse-skipping mode
which are selected using the SYNC pin. Tie the SYNC pin
below V
SYNC,L
(typically 0.5V) for low ripple Burst Mode
operation or above V
SYNC,H
(typically 0.8V) for pulse-
skipping mode.
To enhance effi ciency 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
effi ciency 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 R
T
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.
APPLICATIONS INFORMATION
25μs/DIV
HVOUT
50mV/DIV
I
L
1A/DIV
4090 F05
FIGURE 12 SCHEMATIC
4090 F06
I
L
0.5A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5μs/DIV
V
IN
= 12V; FIGURE 12 SCHEMATIC
I
LOAD
= 10mA