lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 13

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The following example along with the data in Table 2
illustrates the trade-offs of switch frequency selection for
a single input voltage system.
Example.
V
V
Input Voltage Range
Once the switching frequency has been determined, the
input voltage range of the regulator can be determined. The
minimum input voltage is determined by either the LT3692A’s
minimum operating voltage of ~2.8V, or by its maximum duty
cycle. The duty cycle is the fraction of time that the internal
switch is on during a clock cycle. Unlike most fixed frequency
regulators, the LT3692A will not switch off at the end of each
clock cycle if there is sufficient voltage across the boost
capacitor (C3 in Figure 1) to fully saturate the output switch.
Table 2. Efficiency and Size Comparisons for Different R
†
*Inductor and capacitor values chosen for stability and constant ripple current.
V
Figure 3. Timing Diagram RT/SYNC = 28.0k, t
IN
D
IN(MAX)
RT/SYNC ~ 31.6kΩ (Figure 2 )
Max Frequency =
= 0.6V, V
FREQUENCY
= 25V, V
CLKOUT
1000kHz
1500kHz
2250kHz
250kHz
500kHz
t
DCLKOSW1
SW1
SW2
is defined as the highest typical input voltage that maintains constant output voltage ripple.
SW
OUT
= 0.4V:
= 3.3V, I
t
P
/2
RT/SYNC
t
P
25 – 0.4 + 0.6
5.90k
13.0k
28.0k
44.2k
69.8k
3.3 + 0.6
OUT
t
DCLKOSW2
t
P
t
P
= 2.5A, t
/2
V
EFFICIENCY
VIN1/2
•
t
P
ON(MIN)
140ns
88%
87%
84%
82%
78%
P
= 1μs, V
1
= 12V
~ 1.1MHz
= 140ns,
3692a F03
RT/SYNC
DIV
= 0V
V
Values, 3.3V Output
IN(MAX)
39V
39V
26V
18V
12V
Forcing switch off for a minimum time will only occur at the
end of a clock cycle when the boost capacitor needs to be
recharged. This operation has the same effect as lowering the
clock frequency for a fixed off time, resulting in a higher duty
cycle and lower minimum input voltage. The resultant duty
cycle depends on the charging times of the boost capacitor
and can be approximated by the following equation:
where B is 3A divided by the typical boost current from
the Electrical Characteristics table.
This leads to a minimum input voltage of:
where V
Figure 4 shows a typical graph of minimum input voltage
vs load current for the 3.3V output shown in Figure 15.
†
DC
V
IN(MIN)
MAX
Figure 4. Minimum Input Voltage vs Load Current
SW
=
is the voltage drop of the internal switch.
6
5
1
0
=
4
3
2
5.6μH
3.3μH
1.5μH
12μH
1μH
0
1+
L*
V
V
OUT
1
OUT
DC
START-UP
500 1000 1500 2000
B
1
= 3.3V
MAX
+ V
CURRENT (mA)
D
– V
RUNNING
D
120μF
60μF
30μF
22μF
15μF
C*
+ V
2500
SW
3000
3692a F04
LT3692A
3500
C + L (Area)
59.8mm
54.6mm
51.9mm
46.9mm
19.1mm
13
2
2
2
2
2
3692af