Datasheet

V
CL
(T
j
) = V
CL
x [1 + 3.3 x 10
-3
x (T
j
- 27)]
I
CL
(T
j
) =
V
CL
(T
j
)
R
DS(ON)max
LM3151, LM3152, LM3153
SNVS562G –SEPTEMBER 2008–REVISED MARCH 2011 www.ti.com
Table 1. Inductor Selection Table (continued)
Inductor Designator Inductance (µH) Current (A) Part Name Vendor
L48 0.33 15-
4. Determine Output Capacitance
Typical hysteretic COT converters similar to the LM3151/2/3 require a certain amount of ripple that is generated
across the ESR of the output capacitor and fed back to the error comparator. Emulated Ripple Mode control built
into the LM3151/2/3 will recreate a similar ripple signal and thus the requirement for output capacitor ESR will
decrease compared to a typical Hysteretic COT converter. The emulated ripple is generated by sensing the
voltage signal across the low-side FET and is then compared to the FB voltage at the error comparator input to
determine when to initiate the next on-time period.
C
Omin
= 70 / (f
s
2
x L) (1)
The maximum ESR allowed to prevent over-voltage protection during normal operation is:
ESR
max
= (80 mV x L) / ET
min
ET
min
is calculated using V
IN-MIN
The minimum ESR must meet both of the following criteria:
ESR
min
≥ (15 mV x L) / ET
max
ESR
min
≥ [ET
max
/ (V
IN
- V
OUT
)]/ C
O
ET
max
is calculated using V
IN-MAX
.
Any additional parallel capacitors should be chosen so that their effective impedance will not negatively attenuate
the output ripple voltage.
5. MOSFET Selection
The high-side and low-side FETs must have a drain to source (V
DS
) rating of at least 1.2 x V
IN
.
The gate drive current from VCC must not exceed the minimum current limit of VCC. The drive current from VCC
can be calculated with:
I
VCCdrive
= Q
gtotal
x f
S
where
• Q
gtotal
is the combined total gate charge of the high-side and low-side FETs
Use the following equations to calculate the current limit, I
CL
, as shown in Figure 12.
where
• T
j
is the junction temperature of the LM3151/2/3
The plateau voltage of the FET V
GS
vs Q
g
curve, as shown in Figure 14 must be less than VCC - 750 mV.
14 Submit Documentation Feedback Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Links: LM3151 LM3152 LM3153