LT1976EFE#PBF Datasheet LT1976BEFE#TRPBF, LT1976EFE#TRPBF, LT1976BIFE#TRPBF | P10-P12

LT1976/LT1976B

1976bfg

BLOCK DIAGRA

INTERNAL REF

UNDERVOLTAGE

LOCKOUT

THERMAL

SHUTDOWN

SOFT-START

FOLDBACK

DETECT

SLOPE

COMP

ANTISLOPE

COMP

1.2V C

CLAMP

2.4V

–

1.3V

1.25V

SHDN

SYNC

BIAS

SHDN

COMP

–

ERROR

AMP

PGFB

1.12V

–

COMP

BURST MODE

DETECT

LT1976 ONLY

200kHz

OSCILLATOR

SWITCH

LATCH

CURRENT

COMP

DRIVER

CIRCUITRY

BOOST

GND

PGND

1976 BD

CLAMP

The LT1976 is a constant frequency, current mode buck

converter. This means that there is an internal clock and two

feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscilla-

tor pulse which sets the RS latch to turn the switch on. When

switch current reaches a level set by the current compara-

tor the latch is reset and the switch turns off. Output volt-

age control is obtained by using the output of the error

amplifier to set the switch current trip point. This technique

means that the error amplifier commands current to be

delivered to the output rather than voltage. A voltage fed

system will have low phase shift up to the resonant fre-

quency of the inductor and output capacitor, then an abrupt

180° shift will occur. The current fed system will have 90°

phase shift at a much lower frequency, but will not have the

additional 90° shift until well beyond the LC resonant fre-

quency. This makes it much easier to frequency compen-

sate the feedback loop and also gives much quicker tran-

sient response.

Most of the circuitry of the LT1976 operates from an

internal 2.4V bias line. The bias regulator normally draws

Figure 1. LT1976/LT1976B Block Diagram

LT1976/LT1976B

1976bfg

BLOCK DIAGRA

power from the V

pin, but if the BIAS pin is connected to

an external voltage higher than 3V bias power will be

drawn from the external source (typically the regulated

output voltage). This improves efficiency.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing switch to be saturated.

This boosted voltage is generated with an external capaci-

tor and diode.

To further optimize efficiency, the LT1976 automatically

switches to Burst Mode operation in light load situations.

In Burst Mode operation, all circuitry associated with

controlling the output switch is shut down reducing the

input supply current to 45μA.

The only difference between the LT1976 and the LT1976B

is that the LT1976B does not shift into burst mode in light

load situations, eliminating low frequency output ripple at

the expense of light load efficiency.

The LT1976 contains a power good flag with a program-

mable threshold and delay time. A logic-level low on the

SHDN pin disables the IC and reduces input suppy current

to less than 1μA.

APPLICATIO S I FOR ATIO

WUUU

CHOOSING THE LT1976 OR LT1977

The LT1976/LT1976B and LT1977 are high voltage 1.5A

step-down switching regulators. The LT1976 and LT1977

contain circuitry which shifts into burst mode at light loads

reducing quiescent current to typically 100μA. The LT1976B

pulse skips in light load situations, eliminating low fre-

quency burst mode output ripple at the expense of light

load efficiency. The difference between the LT1976/

LT1976B and LT1977 is that the fixed switching frequency

of the LT1976/LT1976B is 200kHz versus 500kHz for the

LT1977. The switching frequency affects: inductor size,

input voltage range in continuous mode operation, effi-

ciency, thermal loss and EMI.

OUTPUT RIPPLE AND INDUCTOR SIZE

Output ripple current is determined by the input to output

voltage ratio, inductor value and switch frequency. Since

the switch frequency of the LT1977 is 2.5 times greater

than that of the LT1976/LT1976B, the inductance used in

the LT1977 application can be 2.5 times lower than the

LT1976/LT1976B while maintaining the same output ripple

current. The lower value used in the LT1977 application

allows the use of a physically smaller inductor.

INPUT VOLTAGE RANGE

The minimum on and off times for all versions of the ic are

equivalent. This results in a narrower range of continuous

mode operation for the LT1977. Typical minimum and

maximum duty cycles are 6% to 92% for the LT1976/

LT1976B and 15% to 90% for the LT1977. Both parts will

regulate up to an input voltage of 60V but the LT1977 will

transistion into pulse-skipping/Burst Mode operation when

the input voltage is above 30V for a 5V output. At outputs

above 10V the LT1977’s input range will be similar to the

LT1976/LT1976B. Lowering the input voltage below the

maximum duty cycle limitation will cause a dropout in

regulation.

Table 1. LT1976/LT1976B/LT1977 Comparison

PARAMETER ADVANTAGE

Minimum Duty Cycle LT1976/LT1976B

Maximum Duty Cycle LT1976/LT1976B

Inductor Size LT1977

Output Capacitor Size LT1977

Efficiency LT1976

EMI LT1976B

Input Range LT1976/LT1976B

Output Ripple LT1977

Light Load Output Ripple LT1976B

LT1976/LT1976B

1976bfg

APPLICATIO S I FOR ATIO

WUUU

FEEDBACK PIN FUNCTIONS

The feedback (FB) pin on the LT1976 is used to set output

voltage and provide several overload protection features.

The first part of this section deals with selecting resistors

to set output voltage and the remaining part talks about

frequency foldback and soft-start features. Please read

both parts before committing to a final design.

Referring to Figure 2, the output voltage is determined by

a voltage divider from V

OUT

to ground which generates

1.25V at the FB pin. Since the output divider is a load on the

output care must be taken when choosing the resistor

divider values. For light load applications the resistor

values should be as large as possible to achieve peak

efficiency in Burst Mode operation. Extremely large values

for resistor R1 will cause an output voltage error due to the

50nA FB pin input current. The suggested value for the

output divider resistor (see Figure 2) from FB to ground

(R2) is 100k or less. A formula for R1 is shown below. A

table of standard 1% values is shown in Table 2 for

common output voltages.

RnA

OUT

125

125 2 50

•

–.

.•

For LT1976B aplications, the suggested value for R2 is 10k

or less, eliminating output voltage errors due to feedback

pin current and reducing noise susceptibility.

More Than Just Voltage Feedback

The FB pin is used for more than just output voltage

sensing. It also reduces switching frequency and con-

trols the soft-start voltage ramp rate when output voltage

is below the regulated level (see the Frequency Foldback

and Soft-Start Current graphs in Typical Performance

Characteristics).

Frequency foldback is done to control power dissipation in

both the IC and in the external diode and inductor during

short-circuit conditions. A shorted output requires the

switching regulator to operate at very low duty cycles. As

a result the average current through the diode and induc-

tor is equal to the short-circuit current limit of the switch

(typically 2.4A for the LT1976). Minimum switch on time

limitations would prevent the switcher from attaining a

sufficiently low duty cycle if switching frequency were

maintained at 200kHz, so frequency is reduced by about

4:1 when the FB pin voltage drops below 0.4V (see

Frequency Foldback graph). In addition, if the current in

the switch exceeds 1.5 times the current limitations speci-

fied by the V

pin, due to minimum switch on time, the

LT1976 will skip the next switch cycle. As the feedback

voltage rises, the switching frequency increases to 200kHz

with 0.95V on the FB pin. During frequency foldback,

external syncronization is disabled to prevent interference

with foldback operation. Frequency foldback does not

affect operation during normal load conditions.

In addition to lowering switching frequency the soft-start

ramp rate is also affected by the feedback voltage. Large

SOFT-START

FOLDBACK

DETECT

200kHz

OSCILLATOR

–

ERROR

AMP

1.25V

OUT

LT1976

1976 F02

Figure 2. Feedback Network

Table 2

OUTPUT R1 OUTPUT

VOLTAGE R2 NEAREST (1%) ERROR

(V) (kΩ, 1%) (kΩ)(%)

2.5 100 100 0

3 100 140 0

3.3 100 165 0.38

5 100 300 0

6 100 383 0.63

8 100 536 –0.63

10 100 698 – 0.25

12 100 866 0.63

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LT1976EFE#PBF

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Description:

Switching Voltage Regulators 1.5A, 200kHz uP HV Step-down Converter

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LT1976EFE#PBF

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