參數(shù)資料
型號(hào)： LTC3810IG#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PDSO28
封裝： 5.30 MM, PLASTIC, SSOP-28
文件頁(yè)數(shù)： 26/36頁(yè)
文件大?。?/td> 495K
代理商： LTC3810IG#TR
LTC3810
32
3810fb
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
load step occurs, VOUT immediately shifts by an amount
equal to
ΔILOAD (ESR), where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or dis-
charge COUT generating a feedback error signal used by the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT 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
specications: VIN = 36V to 72V (48V nominal), VOUT =
12V ±5%, IOUT(MAX) = 10A, f = 250kHz. First, calculate the
timing resistor with VON = INTVCC:
RON =
12V
2.4V 250kHz 76pF
= 263k
and choose the inductor for about 40% ripple current at
the maximum VIN:
L
=
12V
250kHz 0.4 10A
1
12V
72V
= 10μH
With a 10μH inductor, ripple current will vary from 3.2A
to 4A (32% 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 72V(max)
plus any ringing, choose an 80V MOSFET to provide a
margin of safety. The Si7852DP has:
BVDSS = 80V
RDS(ON) = 16.5mΩ(max)/13.5mΩ(nom),
δ = 0.007/°C,
CMILLER = (18.5nC – 7nC)/40V = 288pF,
VGS(MILLER) = 4.7V,
θJA= 20°C/W.
This yields a nominal sense voltage of:
VSNS(NOM) = 10A 1.3 0.0135Ω = 176mV
To guarantee proper current limit at worst-case conditions,
increase nominal VSNS by at least 50% to 320mV (by tying
VRNG to 2V). To check if the current limit is acceptable at
VSNS = 320mV, assume a junction temperature of about
80°C above a 70°C ambient (
ρ150°C = 2):
ILIMIT
320mV
2 0.0165
+
1
2
4A
= 11.7A
and double-check the assumed TJ in the MOSFET:
PBOT =
72V 12V
72V
11.7A
2 2 0.0165 = 3.8W
TJ = 70°C + 3.8W 20°C/W = 146°C
Verify that the Si7852DP 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 50°C above a 70°C ambient (
ρ120°C = 1.7):
PMAIN =
12V
72V
11.7A
2 1.7 0.0165
()
+72V2
11.7A
2
2 288pF
1
10V 4.7V
+
1
4.7V
250kHz
= 0.64W + 1.75W = 2.39W
TJ = 70°C + 2.39W 20°C/W = 118°C
The junction temperature will be signicantly 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 VOUT > 6.7V, the INTVCC/DRVCC voltage can be
generated from VOUT with the internal LDO by connecting
VOUT to the EXTVCC pin. A small SOT23 MOSFET such as
the ZXMN10A07F can be used for the pass device if fault
timeout is enabled. Choose RNDRV to guarantee that fault
timeout is enabled when power dissipation of M3 exceeds
0.4W (max for 70°C ambient). Calculate power dissipation
at VIN(MIN) = 36V:
ICC = 250kHz 2 34nC + 3mA = 20mA
PM3 = (36V – 10V)(0.02A) = 0.52W
APPLICATIONS INFORMATION
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