LTC7899 datasheet(39/50 Pages) AD

39 / 50 page

Data Sheet

LTC7899

analog.com

Rev. 0

39 of 50

decreases, the zero frequency is kept the same, keeping the phase shift the same in the most critical frequency range

of the feedback loop. The output voltage settling behavior is related to the stability of the closed-loop system and

demonstrates the actual overall supply performance.

A second, more severe transient is caused by switching in loads with large (>1μF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can

alter its delivery of current quickly enough to prevent this sudden step change in output voltage, if the load switch

resistance is low and it is driven quickly. If the ratio of CLOAD to COUT is greater than 1:50, the switch rise time must be

controlled so that the load rise time is limited to approximately CLOAD × 25μs/μF. Therefore, a 10μF capacitor requires

a 250μs rise time, limiting the charging current to about 200mA.

Design Example

As a design example, assume the nominal input voltage VIN(NOMINAL)= –48V with a range of –60V to –36V all with respect

to ground, VOUT = 12V with respect to ground, IOUT = 8A, and fSW = 200kHz.

Take the following steps to design an application circuit:

1. Set the operating frequency. The frequency is not one of the internal preset values. Therefore, a resistor

from the FREQ pin to VIN- is required, with a value given by Equation 22, as follows:

RFREQ(in kΩ) =

37MHz

200kHz

= 185kΩ

(22)

2. Determine the inductor value. Initially, select a value based on an inductor ripple current of 30%. The

inductor value can then be calculated using Equation 23, as follows:

L =

|VIN−|

fSW∙(∆IL)

(

VOUT

|VIN−|+VOUT

) = 7.5µH

(23)

The largest ripple happens when |VIN-| = 36V, where the average maximum inductor current is

IL(MAX) = IOUT(MAX) • (1 + VOUT/|VIN-|) = 10.7A. A 10µH inductor will produce 30% ripple current. The peak inductor

current will be the maximum DC value plus one-half the ripple current, or 12.3A.

3. Verify the minimum on-time of 120ns is not violated. The minimum on-time occurs at VIN(MAX), as shown in

Equation 24, as follows:

tON(MIN) <

VOUT

(|VIN(MAX)−|+VOUT)·fSW

12V

60V·200k

= 1µs

(24)

4. This time is sufficient to satisfy the minimum on-time requirement. If the minimum on-time is violated, the

LTC7899 skips pulses at high input voltage, resulting in lower frequency operation and higher inductor

current ripple than desired. If undesirable, this behavior can be avoided by decreasing the frequency (with

the inductor value accordingly adjusted) to avoid operation near the minimum on-time.

5. Select the RSENSE resistor value. The peak inductor current is 10.7A • (1 + 0.30/2) = 12.3A in this case. The

RSENSE resistor value can then be calculated based on the minimum value for the maximum current sense

threshold (45mV for ILIM = FLOAT), given by Equation 25, as follows:

RSENSE ≤

45mV

13A

≅ 3mΩ

(25)

6. To allow for additional margin, a lower value RSENSE can be used (e.g., 2mΩ); however, be sure that the

inductor saturation current has sufficient margin above VSENSE(MAX)/RSENSE, where the maximum value of

55mV is used for VSENSE(MAX).

LTC7899 Datasheet(PDF) 39 Page - Analog Devices

LTC7899 Datasheet(HTML) 39 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Datasheet Download

Link URL