LTC1475CS8 datasheet(10/20 Pages) LINER

10 / 20 page

LTC1474/LTC1475

APPLICATIONS INFORMATION

small that this loss is negligible at loads above a

milliamp but at no load accounts for nearly all of the

loss. The second component, the gate charge current,

results from switching the gate capacitance of the

internal P-channel switch. Each time the gate is switched

from high to low to high again, a packet of charge dQ

moves from VIN to ground. The resulting dQ/dt is the

current out of VIN which is typically much larger than the

DC bias current. In continuous mode, IGATECHG = fQP

where QP is the gate charge of the internal switch. Both

the DC bias and gate charge losses are proportional to

VIN and thus their effects will be more pronounced at

higher supply voltages.

2. I2R losses are predicted from the internal switch,

inductor and current sense resistor. At low supply

voltages where the switch on-resistance is higher and

the switch is on for longer periods due to higher duty

cycle, the switch losses will dominate. Keeping the peak

currents low with the appropriate RSENSE and with

larger inductance helps minimize these switch losses.

At higher supply voltages, these losses are proportional

to load and result in the flat efficiency curves seen in

Figure 1.

3. The catch diode loss is due to the VDID loss as the diode

conducts current during the off-time and is more pro-

nounced at high supply voltage where the on-time is

short. This loss is proportional to the forward drop.

However, as discussed in the Catch Diode section,

diodes with lower forward drops often have higher

leakage current, so although efficiency is improved, the

no load supply current will increase.

Adjustable Applications

For adjustable versions, the output voltage is programmed

with an external divider from VOUT to VFB (Pin 1) as shown

in Figure 4. The regulated voltage is determined by:

VOUT = 1.23 1+









(4)

To minimize no-load supply current, resistor values in the

megohm range should be used. The increase in supply

current due to the feedback resistors can be calculated

from:

∆=

















VIN

OUT

A 10pF feedforward capacitor across R2 is necessary due

to the high impedances to prevent stray pickup and

improve stability.

GND

VFB

LTC1474

LTC1475

1474/75 F04

10pF

VOUT

LBI

LBO

LTC1474/LTC1475

1474/75 F05

1.23V

REFERENCE

VIN

–

Figure 4. LTC1474/LTC1475 Adjustable Configuration

Figure 5. Low Battery Comparator

Low Battery Comparator

The LTC1474/LTC1475 have an on-chip low battery com-

parator that can be used to sense a low battery condition

when implemented as shown in Figure 5. The resistive

divider R3/R4 sets the comparator trip point as follows:

TRIP =+









123 1

The divided down voltage at the LBI pin is compared to the

internal 1.23V reference. When VLBI < 1.23V, the LBO

output sinks current. The low battery comparator is active

all the time, even during shutdown mode.

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