LTM4612
12
4612fc
APPLICATIONS INFORMATION
Operating Frequency
The operating frequency of the LTM4612 is optimized to
achieve the compact package size and the minimum
output ripple voltage while still providing high efficiency.
As shown in Figure 2, the frequency is linearly increased
with larger output voltages to keep the low output cur-
rent ripple. Figure 3 shows the inductor current ripple
D
I
with different output voltages. In most applications, no
additional frequency adjusting is required.
If lower output ripple is required, the operating frequency
f can be increased by adding a resistor R
fSET
between f
SET
pin and SGND, as shown in Figure 19.
f
=
V
OUT
1.5 • 10
−
10
R
fSET
|| 93.1k
(
)
Figure 2. Operating Frequency vs Output Voltage
Figure 3. Inductor Current Ripple vs Output Voltage
V
OUT
(V)
2
600
800
1200
8
12
4612 F02
400
4
6
10
14
16
200
1000
FREQUENCY (kHz)
2
2.5
3.0
2.0
1.5
6
10
4
8
12
14
16
1.0
0.5
3.5
4612 F03
V
IN
= 36V
V
IN
= 28V
V
IN
= 20V
V
OUT
(V)
INDUCTOR CURRENT RIPPLE ∆I (A)
For output voltages more than 12V, the frequency can be
higher than 1MHz, thus reducing the efficiency significantly.
Additionally, the 500ns minimum off time (400ns + 100ns
for margin) normally limits the operation when the input
voltage is close to the output voltage. Therefore, it is rec-
ommended to lower the frequency in these conditions by
connecting a resistor (R
fSET
) from the f
SET
pin to V
IN
, as
shown in Figure 20.
f
=
V
OUT
5 • 10
−
11
3 • R
fSET
• 93.1k
R
fSET
−
2 • 93.1k
⎛
⎝
⎜
⎞
⎠
⎟
The load current can affect the frequency due to its constant
on-time control. If constant frequency is a necessity, the
PLLIN pin can be used to synchronize the frequency of
the LTM4612 to an external clock, as shown in Figures
21 to 23.
