LTC3722-2_15 Datasheet, PDF (18/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
3. Slope compensation is required for most peak current
mode controllers in order to prevent subharmonic
oscillation of the current control loop. In general, if the
system duty cycle exceeds 50% in a fixed frequency,
continuous current mode converter, an unstable con-
dition exists within the current control loop. Any
perturbation in the current signal is amplified by the
PWM modulator resulting in an unstable condition.
Some common manifestations of this include alternate
pulse nonuniformity and pulse width jitter. Fortunately,
this can be addressed by adding a corrective slope to
the current sense signal or by subtracting the same
slope from the current command signal (error am-
plifier output). In theory, the current doubler output
configuration does not require slope compensation
since the output inductor duty cycles only approach
50%. However, transient conditions can momentarily
cause higher duty cycles and therefore, the possibility
for unstable operation.
The exact amount of required slope compensation is
easily programmed by the LTC3722-1/LTC3722-2 with
the addition of a single external resistor (see Figure 9).
The LTC3722-1/LTC3722-2 generates a current that
is proportional to the instantaneous voltage on C_T,
(33µA/V(C_T)). Thus, at the peak of C_T, this current is
approximately 74µA and is output from the CS pin. A
resistor connected between CS and the external current
sense resistor sums in the required amount of slope
compensation. The value of this resistor is dependent
on several factors including minimum V_IN, V_OUT , switch-
ing frequency, current sense resistor value and output
inductor value. An illustrative example with the design
equation for current doubler secondary follows.
Example:
V_IN = 36V to 72V
V_OUT = 3.3V
I_OUT = 40A
L = 2.2µH
Transformer turns ratio (N) =
V_{I N(M I N)}
•
D C_{M A X}
(2 • V_OUT )
=
5
R_CS = 0.05Ω
f_SW = 300kHz, i.e., transformer f =
f_{SW = 150kHz}
2
R_SLOPE
= V_O
•
R_CS
(2 • L • f_SW • 74µA • N)
= 3.3V
•
2
•
0.05
2.2µH • 300k • 74µA • 5
R_SLOPE = 338Ω, choose the next higher standard value
to account for tolerances in I_SLOPE, R_CS, N and L.
LTC3722
BRIDGE
C_T
I = ^V(CT)
33k
CS ^RSLOPE
CURRENT
ADDED
33k
SLOPE
R_CS
CURRENT SENSE
WAVEFORM
372212 F09
Figure 9. Slope Compensation Circuitry
18
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