LTC3728 Datasheet, PDF (25/32 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIO S I FOR ATIO
Design Example
As a design example for one channel, assume V_IN =
12V(nominal), V_IN = 22V(max), V_OUT = 1.8V, I_MAX = 5A,
and f = 300kHz.
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 PLLFLTR pin
to a resistive divider using the INTV_CC pin generating 1V
for 300kHz operation. The minimum inductance for 30%
ripple current is:
∆I_L
=
V_OUT
(f)(L)
⎛
⎝⎜^1–
V_OUT
V_IN
⎞
⎠⎟
A 4.7µH inductor will produce 23% ripple current and a
3.3µH 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 3.3µH value. Increasing the ripple current
will also help ensure that the minimum on-time of 100ns
is not violated. The minimum on-time occurs at maximum
V_IN:
t_ON(MIN)
=
V_OUT
V_IN(MAX)f
=
1.8V
22V(300kHz)
=
273ns
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R_SENSE
≤
60mV
5.84A
≈
0.01Ω
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pins specified input
current.
Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in; R_DS(ON) = 0.042Ω, C_RSS = 100pF. At maximum input
voltage with T(estimated) = 50°C:
P_MAIN
=
1.8V
22V
(⁵)²[¹⁺
(0.005)(50°C
–
25°C
)_]
(^0.042Ω) ^{+ 1.7}(^22V)²(^5A)(^100pF)(^300kHz)
= 220mW
A short-circuit to ground will result in a folded back current
of:
I_SC
=
25mV
0.01Ω
+
1^⎛ 200ns(22V)^⎞
2 ⎝⎜ 3.3µH ⎠⎟
=
3.2A
with a typical value of R_DS(ON) and δ = (0.005/°C)(20) =
0.1. The resulting power dissipated in the bottom MOSFET
is:
( ) ( )( ) ^PSYNC
=
22V – 1.8V
22V
3.2A
2
1.1
0.042Ω
= 434mW
which is less than under full-load conditions.
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 voltage. The output voltage ripple due to
ESR is approximately:
V_ORIPPLE = R_ESR (∆I_L) = 0.02Ω(1.67A) = 33mV_P–P
R1_(MAX)
=
⎛
24k_⎝⎜
0.8V
2.4V – V_OUT
⎞
⎠⎟
=
⎛
24K_⎝⎜
0.8V
2.4V – 1.8V
⎞
⎠⎟
=
32k
3728fb
25