LTC3810-5 Datasheet, PDF (31/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
When load step occurs, V_OUT immediately shifts by an
amount equal to ΔI_LOAD (ESR), where ESR is the effective
series resistance of C_OUT. ΔI_LOAD also begins to charge or
discharge C_OUT generating a feedback error signal used by
the regulator to return V_OUT to its steady-state value. During
this recovery time, V_OUT can be monitored for overshoot
or ringing that would indicate a stability problem.
Design Example
As a design example, take a supply with the following
specifications: V_IN = 12V to 60V, V_OUT = 5V ±5%, I_OUT(MAX)
= 6A, f = 250kHz. First, calculate the timing resistor:
R_ON
=
2.4V
•
5V
250kHz
•
76pF
=
110k
and choose the inductor for about 40% ripple current at
the maximum V_IN:
L
=
5V
250kHz • 0.4
•
6A


1

5V
60V


=
7.6μH
With a 7.7μH inductor, ripple current will vary from 1.5A
to 2.4A (25% to 40%) over the input supply range.
Next, choose the bottom MOSFET switch. Since the
drain of the MOSFET will see the full supply voltage 60V
(max) plus any ringing, choose an 60V MOSFET. The
Si7850DP has:
BV_DSS = 60V
R_DS(ON) = 31mΩ (max)/25mΩ (nom),
δ = 0.007/°C,
C_MILLER = (8.3nC – 2.8nC)/30V = 183pF,
V_GS(MILLER) = 3.8V,
θ_JA= 22°C/W.
This yields a nominal sense voltage of:
V_SNS(NOM) = 6A • 1.3 • 0.025Ω = 195mV
To guarantee proper current limit at worst-case conditions,
increase nominal V_SNS by at least 50% to 320mV (by tying
V_RNG to 2V). To check if the current limit is acceptable at
V_SNS = 320mV, assume a junction temperature of about
55°C above a 70°C ambient (ρ_125°C = 1.7):
I_LIMIT
≥
320mV
1.7 • 0.031Ω
+
1
2
•
2.4A
=
7.3A
and double-check the assumed T_J in the MOSFET:
P_BOT
=
60V − 5V
60V
•
7.3A²
•
1.7
•
0.031Ω
=
2.6W
T_J = 70°C + 2.6W • 22°C/W = 127°C
Verify that the Si7850DP is also a good choice for the top
MOSFET by checking its power dissipation at current limit
and maximum input voltage, assuming a junction tempera-
ture of 30°C above a 70°C ambient (ρ_100°C = 1.5):
P_MAIN
=
5V
60V
•
7.3A²
(1.5
•
0.031)
+
60V²
•
7.3A
2
•
2
•
183pF
•


5V
1

3.8V
+
1
3.8V


•
250kHz
= 0.206W + 1.32W = 1.53W
T_J = 70°C + 1.53W • 22°C/W = 104°C
The junction temperature will be significantly less at
nominal current, but this analysis shows that careful at-
tention to heat sinking on the board will be necessary in
this circuit.
Since V_OUT > 4.7V, the INTV_CC voltage can be generated
from V_OUT with the internal LDO by connecting V_OUT to
the EXTV_CC pin. A small SOT23 MOSFET such as the
ZXMN10A07F can be used for the pass device if fault
timeout is enabled. Choose R_NDRV to guarantee that fault
timeout is enabled when power dissipation of M3 exceeds
0.4W (max for 70°C ambient):
I_CC = 250kHz • 2 • 18nC + 3mA = 12mA
R_NDRV
≤
0.4W
/ 0.012A
270µA
–
3V
=
112k
So, choose R_NDRV = 100k.
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