LTC3850_15 Datasheet, PDF (28/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
2.2Ω
1µF
V_IN
7V TO
22µF
20V
50V
V_OUT1
3.3V
5A
L1
3.3µH
6.19k
1%
33pF
4.7µF
M1
0.1µF
1.33k
1%
0.1µF
D3 _{VIN PGOOD EXTVCC INTVCC}
TG1
TG2
BOOST1
SW1
BG1
LTC3850
BOOST2
SW2
BG2
MODE/PLLIN
PGND
I_LIM
FREQ/PLLFLTR
SENSE1⁺
SENSE2⁺
SENSE1^–
RUN1
SENSE2^–
RUN2
D4
0.1µF
10k, 1%
0.1µF
M2
L2
2.2µH
4.12k
1%
1.5k
1%
33pF
63.4k
1%
C_OUT1
20k
100µF
1%
X2
1800pF
4.75k
1%
V_FB1
I_TH1
100pF ^TK/SS1
0.1µF
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C_OUT1, C_OUT2: TAIYO YUDEN JMK325BJ107MM
SGND
V_FB2
I_TH2
TK/SS2
0.1µF
3.16k
1%
2200pF
5.49k
1%
100pF
25.5k
1%
20k
1%
38501 F14
Figure 14. High Efficiency Dual 500kHz 3.3V/1.8V Step-Down Converter
V_OUT2
1.8V
5A
C_OUT2
100µF
X2
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=
f
V_OUT
• ∆I_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:
∆I_L(NOM)
=
V_OUT
f •L

^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) f
=
1.8V
20V(500kHz)
=
180ns
With I_LIM floating, 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 )}
+
∆I_L(NOM)
2
=
40mV
5A + ^1.5A
≅ 7mΩ
2
The equivalent R_SENSE is the same for Channel 2.
38501fc
28