LTC4090 datasheet(21/28 Pages) LINER

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LTC4090/LTC4090-5

4090fa

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

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(typically0.5V)forlowrippleBurstModeoperation

or above VSYNC,H (typically 0.8V) for pulse-skip mode.

Toenhanceefﬁciencyatlightloads,theLTC4090/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.

APPLICATIONS INFORMATION

μs/DIV

HVOUT

50mV/DIV

1A/DIV

4090 F05

FIGURE 12 SCHEMATIC

4090 F06

0.5A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

μs/DIV

VIN = 12V; FIGURE 12 SCHEMATIC

ILOAD = 10mA

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