LTC1704_15 Datasheet, PDF (21/28 Pages) Linear Technology – 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller

LTC1704/LTC1704B
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
*MOSFETs
rfet mod sw 0.02
;MOSFET rdson
*inductor
lext sw out1 1u
rl out1 outsw 0.005
;inductor value
;inductor series R
*output cap
cout outsw out2 1000u ;capacitor value
resr out2 0 0.01
;capacitor ESR
*1704 internals
emod mod 0 comp 0 5
vstim comp 0 0 ac 1
.ac dec 100 1k 1meg
.probe
.end
;3.3 for 3.3V supply
;ac stimulus
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:
BOOST = – (PHASE + 30°)
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:
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10^(GAIN/20) (this converts GAIN in dB to G in
absolute gain)
TYPE 2 Loop:
K
=
Tan^{}
BOOST
2
+
45°
C2 = ¹
2πfGKR1
( ) ^{C1= C2 K2 − 1}
R4 = ^K
2πfC1
R2 = ^VREF(R1)
V_OUTSW − V_REF
TYPE 3 Loop:
K
=
Tan^{2 }
BOOST
4
+
45°
C2
=
1
2πfGR1
C1= C2₍K − 1₎
R4 = ^K
2πfC1
R3 = ^R1
K−1
C3 = ¹
2πf KR3
R2 = ^VREF(R1)
V_OUTSW − V_REF
SWITCHING SUPPLY CURRENT LIMIT
PROGRAMMING
Programming the current limit on the LTC1704 switcher
supply is straightforward. The I_MAX pin sets the current
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
1704bfa
21