LTC3770_15 Datasheet, PDF (13/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
When there is no R_ON resistor connected to the I_ON pin,
the on-time t_ON is theoretically infinite, which in turn could
damage the converter. To prevent this, the LTC3770 will
detect this fault condition and provide a minimum I_ON
current of 5μA to 10μA.
Changes in the load current magnitude will cause frequency
shift. Parasitic resistance in the MOSFET switches and
inductor reduce the effective voltage across the inductance,
resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be maintained.
This is accomplished with a resistive divider from the I_TH pin
to the V_ON pin and V_OUT. The values required will depend
on the parasitic resistances in the specific application. A
good starting point is to feed about 25% of the voltage
change at the I_TH pin to the V_ON pin as shown in Figure
3a. Place capacitance on the V_ON pin to filter out the I_TH
variations at the switching frequency. The resistor load on
I_TH reduces the DC gain of the error amp and degrades
load regulation, which can be avoided by using the PNP
emitter follower of Figure 3b.
Minimum Off-Time and Dropout Operation
The minimum off-time t_OFF(MIN) is the smallest amount of
time that the LTC3770 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
V_OUT
R_VON1
30k
R_VON2
100k
R_C
C_C
C_VON
0.01μF
V_ON
LTC3770
I_TH
(3a)
V_OUT
INTV_CC
R_VON1
3k
R_VON2
10k 10k
Q1
2N5087
C_VON
0.01μF
R_C
C_C
V_ON
LTC3770
I_TH
3770 F03
(3b)
Figure 3. Correcting Frequency Shift with Load Current Changes
MOSFET back off. This time is generally about 250ns for
large peak-to-peak inductor ripple current. In applications
where the peak-to-peak inductor ripple current is small,
the minimum off-time can approach 400ns. The minimum
off-time limit imposes a maximum duty cycle of t_ON/(t_ON
+ t_OFF(MIN)). If the maximum duty cycle is reached, due
to a dropping input voltage for example, then the output
will drop out of regulation. The minimum input voltage
to avoid dropout is:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 4.
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25
0.50
0.75
1.0
DUTY CYCLE (V_OUT/V_IN)
3770 F04
Figure 4. Maximum Switching Frequency vs Duty Cycle
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
ΔI_L
=
⎛
⎝⎜⎜
V_OUT
fL
⎞⎠⎟⎟⎛⎝⎜_1−
V_OUT
V_IN
⎞
⎟
⎠
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
3770fc
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