LTC3850-2 Datasheet, PDF (28/36 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850-2
APPLICATIONS INFORMATION
Design Example
As a design example for a two channel medium current regu-
lator, assume V_IN = 12V(nominal), V_IN = 20V(maximum),
V_OUT1 = 3.3V, V_OUT2 = 1.8V, I_MAX1,2 = 5A, and f = 500kHz
(see Figure 14).
The regulated output voltages are determined by:
V_OUT
=
0.8V
•
⎛
⎝⎜
1+
R_B
R_A
⎞
⎠⎟
Using 20k 1% resistors from both V_FB nodes to ground,
the top feedback resistors are (to the nearest 1% standard
value) 63.4k and 25.5k.
The frequency is set by biasing the FREQ/PLLFLTR pin to
1.2V (see Figure 10), using a divider from INTV_CC. This
voltage will decrease as V_IN approaches 5V, lowering the
switching frequency. If a separate 5V supply is connected to
EXTV_CC, INTV_CC will remain at 5V even if V_IN decreases.
The inductance values are based on a 35% maximum
ripple current assumption (1.75A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
L=
ƒ
•
V_OUT
ΔI_{L (MAX )}
⎛
⎜^1−
⎝
V_OUT
V_IN(MAX
)
⎞
⎟
⎠
Channel 1 will require 3.2μH, and Channel 2 will require
1.9μH. The next highest standard values are 3.3μH and 2.2μH.
At the nominal input voltage (12V), the ripple will be:
ΔI_L(NOM) =
V_OUT
ĥL
⎛
⎜^1−
⎝
V_OUT
V_IN(NOM)
⎞
⎟
⎠
Channel 1 will have 1.45A (29%) ripple, and Channel 2 will
have 1.4A (28%) ripple. The peak inductor current will be
the maximum DC value plus one-half the ripple current,
or 5.725A for Channel 1 and 5.7A for Channel 2.
The minimum on-time occurs on Channel 1 at the maximum
V_IN, and should not be less than 90ns:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )} ƒ
=
1.8V
20V(500kHz)
=
180ns
The equivalent R_SENSE resistor value can be calculated by
using the minimum value for the maximum current sense
threshold (40mV).
R_{SENSE(EQUIV )}
=
V_SENSE(MIN)
I_{LOAD(MAX )}
+
ΔI_L(NOM)
2
=
40mV
5A + ^1.5A
≅
7mΩ
2
The equivalent R_SENSE is the same for Channel 2.
The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCR_MAX
at 20°C) and HCP0703-3R3 (30mΩ DCR_MAX at 20°C) are
chosen. At 100°C, the estimated maximum DCR values are
26.4mΩ and 39.6mΩ. The divider ratios are:
R_D
= ^{RSENSE(EQUIV)}
DCR_MAX at T_L(MAX)
=
7mΩ
26.4mΩ
= 0.26;
and ^7mΩ ≅ 0.18
39.6mΩ
For each channel, 0.1μF is selected for C1.
R1||R2 =
L
= ^2.2µH
(DCR_MAX at 20°C) • C1 20mΩ • 0.1µF
= 1.1k ; and
3.3µH _{= 1.1k}
30mΩ • 0.1µF
For channel 1, the DCR_SENSE filter/divider values are:
R1= ^R1||R2 = ^1.1k ≅ 6.19k;
R_D 0.18
R2 = ^{R1 • RD} = ^{6.19k • 0.18} ≅ 1.33k
1− R_D 1− 0.18
38502f
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