LTC3859AL Datasheet, PDF (34/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28μA Burst Mode IQ

LTC3859AL
Applications Information
Buck Design Example
As a design example for one of the buck channels channel,
assume V_IN = 12V_(NOMINAL), V_IN = 22V_(MAX), V_OUT = 3.3V,
I_MAX = 6A, V_SENSE(MAX) = 50mV, and f = 350kHz.
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:
∆I_L
=
V_OUT
(f)(L)

^1−
V_OUT
V_{IN(NOM INAL )}


A 3.9µH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 6.88A. 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_IN:
t_ON(MIN)
=
V_OUT
V_IN(MAX)(f)
=
3.3V
22V(350kHz)
=
429ns
The R_SENSE resistor value can be calculated by using the
minimum value for the maximum current sense threshold
(43mV):
R_SENSE
≤
43mV
6.88A
=
0.006Ω
Choosing 1% resistors: R_A = 25k and R_B = 80.6k yields
an output voltage of 3.33V.
The power dissipation on the top side MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R_DS(ON) = 0.035Ω/0.022Ω, C_MILLER = 215pF.
At maximum input voltage with T(estimated) = 50°C:
P_MAIN
=
3.3V
22V
(6A)² {1+
(0.005)(50°C
−
25°C)}
(0.035Ω) + (22V)²6 ^5A (2.5Ω)(215pF) •
2
 1 ₊ 1 _{(350kHz) = 433mW}
5V − 2.3V 2.3V 
A short-circuit to ground will result in a folded back cur-
rent of:
I_SC
=
20 mV
0.006Ω
−
1 ^ 95ns(22V) ^
2  3.9µH 
=
3.07A
with a typical value of R_DS(ON) and z = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P_SYNC = (2.23A)²(1.125)(0.022Ω) = 233mW
which is less than under full-load conditions.
The input capacitor to the buck regulator C_IN is chosen
for an RMS current rating of at least 3A at temperature
assuming only this channel is on. C_OUT 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 volt-
age. The output voltage ripple due to ESR is approximately:
V_ORIPPLE = R_ESR (DI_L) = 0.02Ω(1.75A) = 35mV_P-P
34
For more information www.linear.com/3859AL
3859alf