LTC3720_15 Datasheet, PDF (18/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

LTC3720
APPLICATIO S I FOR ATIO
Design Example
As a design example, take a supply with the following
specifications: V_IN = 7V to 24V (15V nominal), V_OUT = 1.05V
to 1.825V with typical at 1.5V, I_OUT(MAX) = 15A, f = 300kHz.
First, calculate the timing resistor with V_ON = V_OUT:
( )( ) ^{RON =}
1
= 330k
300kHz 10pF
and choose the inductor for about 40% ripple current at
the maximum V_IN:
( )( )( ) ^{L =}
1.5V
300kHz 0.4
15A

^1−
1.5V ^
24V 
=
0.8µH
Selecting a standard value of 1µH results in a maximum
ripple current of:
( )( ) ^{∆IL =}
1.5V
300kHz 1µH

^1–
1.5V ^
24V 
=
4.7A
Next, choose the synchronous MOSFET switch. Because
of the narrow duty cycle and large current, a single SO-8
MOSFET will have difficulty dissipating the power lost in the
switch. Choosing two IRF7811A (R_DS(ON) = 0.013Ω, C_RSS
= 60pF, θ_JA = 50°C/W) yields a nominal sense voltage of:
V_SNS(NOM) = (15A)(0.5)(1.3)(0.012Ω) = 117mV
Tying V_RNG to INTV_CC will set the current sense voltage
range for a nominal value of 140mV with current limit
occurring at 186mV. To check if the current limit is
acceptable, assume a junction temperature of about 100°C
above a 50°C ambient with ρ_150°C = 1.6:
( )( )( ) ^{( ) ILIMIT ≥}
0.5
186mV
1.6 0.012Ω
+ ¹ 4.7A
2
= 18A
and double check the assumed T_J in the MOSFET:
( )( ) ^PBOT
=
24V – 1.5V ^ 21.7A^{ 2}
24V  2 
1.6
0.012Ω
= 2.12W
T_J = 50°C + (2.12W)(50°C/W) = 156°C
Because the top MOSFET is on for such a short time, a
single IRF7811A will be sufficient. Checking its power
dissipation at current limit with ρ_90°C = 1.3:
( ) ( )( ) ^PBOT
=
1.5V
24V
2
21.7A 1.3
0.012Ω
+
(^1.7)(^24V)²( )( ^{21.7A 60pF})(^300kHz)
= 0.46W + 0.38W = 0.84W
T_J = 50°C + (0.84W)(50°C/W) = 92°C
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary in this circuit.
C_IN is chosen for an RMS current rating of about 6A at
temperature. The output capacitors are chosen for a low
ESR of 0.005Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
∆V_OUT(RIPPLE) = ∆I_L(MAX) (ESR)
= (4.7A) (0.005Ω) = 24mV
3720f
18