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參數(shù)資料
| 型號(hào)： | LTC3835IUFD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封裝： | 4 X 5 MM, PLASTIC, MO-220, QFN-20 |
| 文件頁數(shù)： | 15/30頁 |
| 文件大小： | 1775K |
| 代理商： | LTC3835IUFD |
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LTC3835
22
3835fd
APPLICATIONS INFORMATION
most critical frequency range of the feedback loop. The
output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance.
A second, more severe transient is caused by switching
in loads with large (>1F) 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
should be controlled so that the load rise time is limited
to approximately 25 CLOAD. Thus a 10F capacitor would
require a 250s rise time, limiting the charging current
to about 200mA.
Design Example
As a design example, assume VIN = 12V(nominal), VIN =
22V(max), VOUT = 1.8V, IMAX = 5A, and f = 250kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLLPF
pin to GND, generating 250kHz operation. The minimum
inductance for 30% ripple current is:
IL =
VOUT
(f)(L)
1–
VOUT
VIN
A 4.7H inductor will produce 23% ripple current and a
3.3H will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3H value. Increasing the ripple current will
also help ensure that the minimum on-time of 180ns is not
violated. The minimum on-time occurs at maxi-mum VIN:
tON(MIN) =
VOUT
VIN(MAX)f
=
1.8V
22V(250kHz)
= 327ns
The RSENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
RSENSE ≤
80mV
5.84A
≈ 0.012
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
ThepowerdissipationonthetopsideMOSFETcanbeeasily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: RDS(ON) = 0.035Ω/0.022Ω, CMILLER = 215pF. At
maximum input voltage with T(estimated) = 50°C:
PMAIN =
1.8V
22V
5
( )2 1+(0.005)(50°C – 25°C)
[
]
0.035
(
)+ 22V
( )2 5A
2
4
( ) 215pF
(
)
1
5 – 2.3
+
1
2.3
300kHz
(
) = 332mW
A short-circuit to ground will result in a folded back
current of:
ISC =
25mV
0.01
–
1
2
120ns(22V)
3.3H
= 2.1A
with a typical value of RDS(ON) and d = (0.005/°C)(20) = 0.1.
The resulting power dissipated in the bottom MOSFET is:
PSYNC =
22V – 1.8V
22V
2.1A
(
)2 1.125
(
) 0.022
(
)
= 100mW
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. COUT is
chosen with an ESR of 0.02Ω for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
VORIPPLE = RESR (IL) = 0.02Ω(1.67A) = 33mVP–P
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