?
The LTC3633 isn't really the right part to use, if
you want 98mA output current.
?
1uH is not an absurdly low value, in fact it's
probably too high for 4MHz operation. The ripple current should be sized for
~40% of the max output current the chip is designed for, consistent with the
internal current-sense range - notwithstanding the actual load current. Suggest
you?look at the numerous design examples, with correctly-sized component
values.
?
----- Original Message -----
Sent: Wednesday, December 18, 2013 6:59
AM
Subject: [LTspice] Odd behaviour when
attempting to simulate an LTC3633 switching regulator.
?
I am using the ltspice simulator to simulate a design based on the
LTC3633.? I have little confidence in the efficiency
calculation.
If i design the regulator to run at 4MHz with an output of
1.1V@98mA according to the formula defined in
the LTC3633 datasheet the regulator needs a 6.4uH inductor and an output
capacitor of 1.4uF.? When I simulate this in ltspice using the LTC3633 I
get an efficiency of around 59%
If I then change the inductor to a
rather small value of 1u for example the efficiency rises to around 70%.?
I do not understand why this happens.? Why would using an adsurdly low
inductor value increase the efficiency percentage of the model?
What is
the best way to determine the efficiency of a switching regulator using the
ltspice software?
I have included the spice netlist of the model
below.
* U:\jobs\13_12_M154 regulator
simulations\simulated\LTC3633_12_to_1V1_experiment.asc
V1 IN 0 12
Rser=1
XU1 NC_01 N001 IN N001 N008 IN 0 NC_02 N007 NC_03 N001 OUT1 N005
MP_01 IN MP_02 N003 IN N001 N002 IN MP_03 N004 MP_04 OUT2 N001 NC_04 N006 0
LTC3633
C1 0 N001 2.2?
L1 N004 OUT2 1? Rser=14m
C2 N002 N004
.1?
R1 OUT2 N006 8.2k
R2 N006 0 10K
R3 N008 0 80.2K
C4 OUT2 0 47?
Irms=0 Rser=0.002 Lser=0
L2 N005 OUT1 1? Rser=14m
C5 N003 N005 .1?
R4
OUT1 N007 8.2K
R5 N007 0 10K
C6 OUT1 0 1.4? V=6.3 Irms=0 Rser=0.0008
Lser=0
C7 IN 0 47? V=6.3 Irms=0 Rser=0.002 Lser=0 mfg="TDK"
pn="C4532X5ROJ47@M" type="X5R" x2
I1 OUT1 0 0.098
I2 OUT2 0 0
.tran 0
2ms 1ms steady startup nodiscard uic
* Note:\n? If the simulation
model is not found please update with the "Sync Release" command from the
"Tools" menu.\n? It remains the customer's responsibility to verify
proper and reliable operation in the actual application.\n? Component
substitution and printed circuit board layout may significantly affect circuit
performance or reliability\n? Contact your local sales representative for
assistance. This circuit is distributed to customers only for use with LTC
parts\n? Copyright ? 2011 Linear Technology Inc. All rights
reserved.
* LTC3633? - Dual Channel 3A, 15V Monolithic Synchronous
Step-Down Regulator\n1.1V/0.9V Buck Regulator with 2.5V LDO Output\nInput:
12V?????? Output1:1V1 @
0.5A?????? Output2: 2.5V @ 2A
* 3x3mm
*
4x4mm
.lib LTC3633.sub
.backanno
.end
