LTC1704 Linear Technology, LTC1704 Datasheet - Page 21

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
*1704 modulator gain/phase
*2001 Linear Technology
*this file written to run with PSpice 9.0
*may require modifications for other SPICE
simulators
With the gain/phase plot in hand, a loop crossover fre-
quency can be chosen. Usually the curves look something
like Figure 7. 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 at 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). Now calculate the remaining values:
*MOSFETs
rfet mod sw 0.02
*inductor
lext sw out1 1u
rl out1 outsw 0.005
*output cap
cout outsw out2 1000u
resr out2 0 0.01
*1704 internals
emod mod 0 comp 0 5
vstim comp 0 0 ac 1
.ac dec 100 1k 1meg
.probe
.end
BOOST = – (PHASE + 30 )
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10
absolute gain)
(GAIN/20)
U
(this converts GAIN in dB to G in
U
;MOSFET rdson
;inductor value
;inductor series R
;capacitor value
;capacitor ESR
;3.3 for 3.3V supply
;ac stimulus
W
U
SWITCHING SUPPLY CURRENT LIMIT
PROGRAMMING
Programming the current limit on the LTC1704 switcher
supply is straightforward. The I
limit by setting the maximum allowable voltage drop
across QB (the bottom MOSFET) before the current limit
circuit engages. The voltage across QB is set by its on-
resistance and the current flowing in the inductor, which
TYPE 2 Loop:
TYPE 3 Loop:
K Tan
C
C
R
R
K Tan
C
C
R
R
C
R
1
1
2
2
2
3
3
2
4
4
C K
C K
2
2
K
2
2
V
2
V
2
2
R
OUTSW
OUTSW
K
2
1
fGKR
fGR
f KR
fC
fC
K
1
V
V
1
BOOST
1
2
1
REF
REF
1
BOOST
1
1
1
2
1
1
( )
( )
4
3
R
R
LTC1704/LTC1704B
1
1
V
V
REF
REF
45
45
MAX
pin sets the current
21
1704bfa