LTC1873EG#TR Linear Technology, LTC1873EG#TR Datasheet - Page 23

• Manufacturer Part Number: LTC1873EG#TR
• Description: IC REG SW 2PH DUAL SYNC 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1873EG#TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 750kHz
• Duty Cycle: 93%
• Voltage - Supply: 3 V ~ 7 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-SSOP
• Frequency-max: 750kHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
V(OUT) in degrees. Refer to your SPICE manual for details
of how to generate this plot.
*1873 modulator gain/phase
*
*this file written to run with PSpice 8.0
*may require modifications for other SPICE
simulators
*MOSFETs
rfet mod sw 0.02
*inductor
lext sw out1 1u
rl out1 out 0.005
*output cap
cout out out2 1000u
resr out2 0 0.01
*1873 internals
emod mod 0 laplace {v(comp)} =
+ {5*exp(–s*909e–9)} ;5 -> 3.3 for 3.3 VCC
*emod mod 0 comp 0 5
vstim comp 0 0 ac 1
.ac dec 100 1k 1meg
.probe
.end
With the gain/phase plot in hand, a loop crossover fre-
quency can be chosen. Usually the curves look something
like Figure 8. Choose the crossover frequency in the rising
or flat parts of the phase curve, beyond the external LC
poles. Frequencies between 10kHz and 50kHz usually
work well. Note the gain (GAIN, in dB) and phase (PHASE,
in degrees) at this point. The desired feedback amplifier
gain will be – GAIN to make the loop gain 0dB at this
frequency. Now calculate the needed phase boost, assum-
ing 60 as a target phase margin:
If the required BOOST is less than 60 , a type 2 loop can
be used successfully, saving two external components.
BOOST values greater than 60 usually require type 3
loops for satisfactory performance.
Finally, choose a convenient resistor value for R1 (10k is
usually a good value). Note that channel 1 includes R1 and
R
at 20k and R
©
B
1999 Linear Technology
BOOST = – (PHASE + 30 )
internally as part of the VID DAC circuitry. R1 is fixed
B
varies depending on the VID code selected.
U
U
;MOSFET rdson
;inductor value
;inductor series R
;capacitor value
;capacitor ESR
;use if above lines fail
;ac stimulus
W
U
Now calculate the remaining values:
(K is a constant used in the calculations)
ƒ = chosen crossover frequency
G = 10
gain)
Type 2 Loop:
Type 3 Loop:
K Tan
C
C
R
R
K Tan
C
C
R
R
C
R
1
1
2
2
B
2
2
3
3
B
(GAIN/20)
C K
C K
2
2
2
2
2
V
V
2
2
K
OUT
OUT
V
R
V
2
K
–
REF
REF
K
1
1
BOOST
GKR
GR
2
C
C
1
1
–
1
BOOST
1
1
–
K R
–
–
(this converts GAIN in dB to G in absolute
1
2
1
R
R
1
V
V
4
1
1
REF
1
REF
3
45
45
LTC1873
23