LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 26

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

LTC3857
Design Example
As a design example for one channel, assume V
(nominal), V
V
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 FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
A 4.7μH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 5.73A. Increasing the ripple
current will also help ensure that the minimum on-time
of 95ns is not violated. The minimum on-time occurs at
maximum V
The equivalent R
using the minimum value for the maximum current sense
threshold (64mV):
Choosing 1% resistors: R
an output voltage of 3.33V.
APPLICATIONS INFORMATION
26
SENSE(MAX)
R
ΔI
t
ON(MIN)
SENSE
L(NOM)
≤
=
IN
=
64mV
5.73A
= 75mV and f = 350kHz.
IN
V
:
V
IN(MAX)
ĥL
OUT
= 22V (max), V
V
SENSE
OUT
≈0.01Ω
⎛
⎜
⎝
1–
ƒ
resistor value can be calculated by
=
V
IN(NOM)
22V 350kHz
A
V
OUT
= 25k and R
(
3.3V
OUT
⎞
⎟
⎠
= 3.3V, I
)
B
=429ns
= 80.6k yields
MAX
IN
= 12V
= 5A,
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
maximum input voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only this channel is on. C
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:
IN
V
I
P
P
SC
is chosen for an RMS current rating of at least 3A at
MAIN
SYNC
ORIPPLE
=
0.01Ω
32mV
=
= 2.98
(
3.3V
22V
(
⎡
⎣ ⎢
DS(ON)
= R
0.035Ω
5V–2.3V
–
ESR
( )
)
1
5A
2
1
2
= 0.035Ω/0.022Ω, C
(
⎛
⎝ ⎜
(∆I
1.125
95ns 22V
)
2
+ 22V
⎡ ⎣
+
L
4.7μH
(
1+ 0.005
) = 0.02Ω(1.45A) = 29mV
DS(ON)
2.3V
(
)
1
(
(
0.022Ω
)
2
⎤
⎦ ⎥
and δ = (0.005/°C)(25°C)
(
5A
)
350kHz
2
⎞
⎠ ⎟
)
=2.98A
(
(
2.5Ω
50°C–25°C
)
=220mW
MILLER
)
)
=331 mW
(
215pF
= 215pF . At
)
⎤ ⎦
P-P
OUT
)
•
3857fc
is