LTC3857-1_15 Datasheet, PDF (26/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
APPLICATIONS INFORMATION
should be controlled so that the load rise time is limited
to approximately 25 • C_LOAD. Thus a 10μF capacitor would
require a 250μs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example for one 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:
ΔI_L(NOM) =
V_OUT
ƒ •L
⎛
⎝⎜ ^{1 –}
V_OUT ^⎞
V_{IN(NOM) ⎠⎟}
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(M AX )} ƒ
=
3.3V
22V (350kHz )
=
429ns
The equivalent 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 topside 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
( 6 A )²
⎡⎣1 +
( 0 .0 0 5 ) ( 5 0 °C
–
2 5 °C ) ⎤⎦
(0.035Ω ) + (22V )^{2 6A} (2.5Ω )(215pF ) •
2
⎡
⎣⎢
5V
1
– 2.3V
+
1
2.3V
⎤
⎦⎥
( 3 5 0kH z )
=
433mW
A short-circuit to ground will result in a folded back cur-
rent of:
I_SC
=
25mV
0.006Ω
–
1⎛
2 ⎝⎜
95ns (22V ) ⎞
3.9μH ⎠⎟
=
3.9A
with a typical value of R_DS(ON) and δ = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P_SYNC = (3.9A)² (1.125)(0.022Ω) = 376mW
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 (ΔI_L) = 0.02Ω(1.75A) = 35mV_P-P
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