LTC1430ACS8 Datasheet LTC1430ACGN#PBF, LTC1430ACS#PBF, LTC1430ACS8#PBF | P13-P15

LTC1430A

I FOR ATIO

by using a larger RC filter from the PV

pin; 22Ω and 10µF

work well here. The 10µF capacitor must be VERY close to

the part (preferably right underneath the unit) or output

regulation may suffer.

For both versions of the LTC1430A, PV

CC1

must be higher

than PV

by at least one external MOSFET V

GS(ON)

to fully

enhance the gate of Q1. This higher voltage can be

provided with a separate supply (typically 12V) which

should power up after PV

, or it can be generated with a

simple charge pump (Figure 5). The charge pump consists

of a Schottky diode from PV

to PV

CC1

and a 0.1µF

capacitor from PV

CC1

to the switching node at the drain of

Q2. This circuit provides 2PV

– V

to PV

CC1

while Q1 is

ON and PV

– V

while Q1 is OFF where V

is the ON

voltage of the Schottky diode. Ringing at the drain of Q2

can cause transients above 2PV

at PV

CC1

; if PV

higher than 7V, a 12V zener diode should be included from

CC1

to PGND to prevent transients from damaging the

circuitry at PV

CC2

or the gate of Q1.

More complex charge pumps can be constructed with the

16-lead versions of the LTC1430A to provide additional

voltages for use with standard threshold MOSFETs or very

low PV

voltages. A tripling charge pump (Figure 7) can

provide 2PV

and 3PV

voltages. These can be con-

nected to PV

CC2

and PV

CC1

respectively, allowing stan-

dard threshold MOSFETs to be used with 5V at PV

or 5V

logic level threshold MOSFETs to be used with 3.3V at

. V

can be driven from the same potential as PV

CC2

allowing the entire system to run from a single 3.3V

supply. Tripling charge pumps require the use of Schottky

diodes to minimize forward drop across the diodes at

start-up. The tripling charge pump circuit will tend to

rectify any ringing at the drain of Q2 and can provide well

more than 3PV

at PV

CC1

; all tripling (or higher multiply-

ing factor) circuits should include a 12V zener clamp diode

to prevent overvoltage at PV

CC1

3.3V Input Supply Operation

The LTC1430A can be used with input supply voltages

lower than 5V as long as a low power 5V supply is available

to power the LTC1430A itself and to provide gate drive to

the external MOSFETs. A typical 3.3V to 2.5V application

is shown in Figure 10. The circuit can supply up to 10A at

2.5V output, and draws this power from the 3.3V supply.

The 5V supply typically needs to supply about 20mA to

provide gate drive to the external MOSFETs and keep the

LTC1430A control circuits powered. For applications where

there is no 5V supply available, see the LTC1649 data

sheet.

Compensation and Transient Response

The LTC1430A voltage feedback loop is compensated at

the COMP pin; this is the output node of the internal g

error amplifier. The loop can generally be compensated

Figure 10. 3.3V to 2.5V, 10A Application

220µF

×4

OUT

330µF

×6

2.7µH/15A

CC1

FREQSET

SHDN

COMP

CC2

PGND

GND

MAX

G2NC

1430 F10

SHUTDOWN

Q1A, Q1B, Q2: INTERNATIONAL RECTIFIER IRF7801

: AVX-TPSE227M010R0100

OUT

: AVX-TPSE337M006R0100

SENSE

LTC1430A

SENSE

–

16k

100Ω

976Ω

7.5k

4700pF

220pF

MBR0530T1

0.1µF

1µF

0.01µF

0.1µF

4.7µF

1µF

2.5V

10A

Q1A, Q1B

2 IN PARALLEL

3.3V

LTC1430A

I FOR ATIO

properly with an RC network from COMP to GND and an

additional small C from COMP to GND (Figure 11). Loop

stability is affected by inductor and output capacitor

values and by other factors. Optimum loop response can

be obtained by using a network analyzer to find the loop

poles and zeros; nearly as effective and a lot easier is to

empirically tweak the R

values until the transient recovery

looks right with an output load step. Table 1 shows

recommended compensation components for 5V to 3.3V

applications based on the inductor and output capacitor

values. The values were calculated using multiple paral-

leled 330µF AVX TPS series surface mount tantalum

capacitors as the output capacitor.

Table 1. Recommended Compensation Network for 5V to 3.3V

Application Using Multiple 330µF AVX Output Capacitors

L1 (µH) C

OUT

(µF) R

(kΩ)C

(µF) C1 (pF)

1 990 1.8 0.022 820

1 1980 3.6 0.01 470

1 4950 9.1 0.0047 150

1 9900 18 0.0022 82

2.7 990 3.6 0.01 470

2.7 1980 7.5 0.0047 220

2.7 4950 18 0.0022 82

2.7 9900 39 0.001 39

5.6 990 9.1 0.0047 150

5.6 1980 18 0.0022 82

5.6 4950 47 820pF 33

5.6 9900 91 470pF 15

10 990 18 0.0022 82

10 1980 39 0.001 39

10 4950 91 470pF 15

10 9900 180 220pF 10

Output transient response is set by three major factors: the

time constant of the inductor and the output capacitor, the

ESR of the output capacitor, and the loop compensation

components. The first two factors usually have much

more impact on overall transient recovery time than the

third; unless the loop compensation is way off, more

improvement can be had by optimizing the inductor and

the output capacitor than by fiddling with the loop com-

pensation components. In general, a smaller value induc-

tor will improve transient response at the expense of ripple

LTC1430A

COMP

GND SGND

1430 F11

C1C

Figure 11. Compensation Pin Hook-Up

and inductor core saturation rating. Minimizing output

capacitor ESR will also help optimize output transient

response. See Input and Output Capacitors for more

information.

Soft Start and Current Limit

The 16-lead versions of the LTC1430A include a soft start

circuit at the SS pin; this circuit is used both for initial start-

up and during current limit operation. The soft start and

current limit circuitry is disabled in the 8-lead version. SS

requires an external capacitor to GND with the value

determined by the required soft start time. An internal

12µA current source is included to charge the external

capacitor. Soft start functions by clamping the maximum

voltage that the COMP pin can swing to, thereby control-

ling the duty cycle (Figure 12). The LTC1430A will begin to

operate at low duty cycle as the SS pin rises to about 2V

below V

. As SS continues to rise, the duty cycle will

increase until the error amplifier takes over and begins to

regulate the output. When SS reaches 1V below V

the

1430 F12

COMP

LTC1430A

12µA

Figure 12. Soft Start Clamps COMP Pin

LTC1430A

I FOR ATIO

Longer overload conditions will allow the SS pin to reach

a steady level, and the output will remain at a reduced

voltage until the overload is removed. Serious overloads

will generate a larger overdrive at I

LIM

, allowing it to pull SS

down more quickly and preventing damage to the output

components.

The I

LIM

amplifier output is disabled when Q1 is OFF to

prevent the low I

voltage in this condition from activating

the current limit. It is re-enabled a fixed 170ns after Q1

turns on; this allows for the I

node to slew back high and

the I

LIM

amplifier to settle to the correct value. As the

LTC1430A goes deeper into current limit, it will reach a

point where the Q1 on-time needs to be cut to below 170ns

to control the output current. This conflicts with the

minimum settling time needed for proper operation of the

LIM

amplifier. At this point, a secondary current limit

circuit begins to reduce the internal oscillator frequency,

lengthening the off-time of Q1 while the on-time remains

constant at 170ns. This further reduces the duty cycle,

allowing the LTC1430A to maintain control over the output

current.

Under extreme output overloads or short circuits, the I

LIM

amplifier will pull the SS pin more than 2V below V

in a

single switching cycle, cutting the duty cycle to zero. At

this point all switching stops, the output current decays

through Q2 and the LTC1430A runs a partial soft start

cycle and restarts. If the short is still present the cycle will

repeat. Peak currents can be quite high in this condition,

but the average current is controlled and a properly

designed circuit can withstand short circuits indefinitely

with only moderate heat rise in the output FETs. In addi-

tion, the soft start cycle repeat frequency can drop into the

low kHz range, causing vibrations in the inductor which

provide an audible alarm that something is wrong.

Oscillator Frequency

The LTC1430A includes an onboard current controlled

oscillator which will typically free-run at 200kHz. An

internal 20µA current is summed with any current in or out

of the FREQSET pin (Pin 11), setting the oscillator fre-

quency to approximately 10kHz/µA. FREQSET is internally

servoed to the LTC1430A reference voltage (1.265V).

With FREQSET floating, the oscillator is biased from the

LTC1430A will be in full operation. An internal switch

shorts the SS pin to GND during shutdown.

The LTC1430A detects the output current by watching the

voltage at I

while Q1 is ON. The I

LIM

amplifier compares

this voltage to the voltage at I

MAX

(Figure 13). In the ON

state, Q1 has a known resistance; by calculating back-

wards, the voltage generated at I

by the maximum

output current in Q1 can be determined. As I

falls below

MAX

, I

LIM

will begin to sink current from the soft start pin,

causing the voltage at SS to fall. As SS falls, it will limit the

output duty cycle, limiting the current at the output.

Eventually the system will reach equilibrium, where the

pull-up current at the SS pin matches the pull-down

current in the I

LIM

amplifier; the LTC1430A will stay in this

state until the overcurrent condition disappears. At this

time I

will rise, I

LIM

will stop sinking current and the

internal pull-up will recharge the soft start capacitor,

restoring normal operation. Note that the I

pin requires

an external 1k series resistor to prevent voltage transients

at the drain of Q2 from damaging internal structures.

–

LIM

LTC1430A

1430 F13

IMAX

MAX

12µA

0.1µF

Figure 13. Current Limit Operation

The I

LIM

amplifier pulls current out of SS in proportion to

the difference between I

and I

MAX

. Under mild overload

conditions, the SS pin will fall gradually, creating a time

delay before current limit takes effect. Very short, mild

overloads may not trip the current limit circuit at all.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

LTC1430ACS8

Mfr. #:

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Description:

Switching Voltage Regulators LTC1430A - High Power Step-Down Switching Regulator Controller

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LTC1430ACS8

LTC1430ACS8

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