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LM5018_15 Datasheet, PDF (17/32 Pages) Texas Instruments – 100-V, 300-mA Constant On-Time Synchronous Buck Regulator
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LM5018
SNVS787G – JANUARY 2012 – REVISED OCTOBER 2015
8.2.1.2.5 Type II Ripple Circuit
Type II ripple circuit, as described in Ripple Configuration, is chosen for this example. For a constant on time
converter to be stable, the injected in-phase ripple should be larger than the capacitive ripple on COUT.
Using type II ripple circuit equations with minimum FB pin ripple of 25 mV, the values of the series resistor RC
and ac coupling capacitor Cac can calculated.
Cac
>
5
Csw(RFB2||RFB1)
RC
>
25 mV
ûIL(MIN)
(16)
Assuming RFB2 = 6.98 kΩ and RFB1 = 1 kΩ, the calculated minimum value of Cac is 0.013 µF. A standard value of
0.1 µF is selected for Cac = C8. The value of the series output resistor RC is calculated for the minimum input
voltage condition when the inductor ripple current as at a minimum. Using Equation 13 and assuming VIN = 12.5
V, the minimum inductor ripple current is 27 mA. The calculated minimum value of RC is 0.93 Ω. A standard
value of 1.5 Ω is selected for RC = R2 to provide additional ripple for stable switching at low VIN.
8.2.1.2.6 VCC and Bootstrap Capacitor
The VCC capacitor provides charge to bootstrap capacitor as well as internal circuitry and low side gate driver.
The bootstrap capacitor provides charge to high-side gate driver. The recommended value for CVCC = C7 = 1 μF.
A good value for CBST = C1 = 0.01 μF.
8.2.1.2.7 Input Capacitor
Input capacitor should be large enough to limit the input voltage ripple which can be calculated using
Equation 17.
CIN
>
4
IOUT(MAX)
x CSW x ûVIN
(17)
Choosing a ΔVIN = 0.5 V gives a minimum CIN = 0.34 μF. A standard value of 1 μF is selected CIN = C4. The
input capacitor should be rated for the maximum input voltage under all conditions. A 100 V X7R dielectric
should be selected for this design.
Input capacitor should be placed directly across VIN and RTN (pin 2 and 1) of the IC. If it is not possible to place
all of the input capacitor close to the IC, a 0.1-μF capacitor should be placed near the IC to provide a bypass
path for the high frequency component of the switching current.
8.2.1.2.8 UVLO Resistors
The UVLO resistors RUV1 and RUV2 set the UVLO threshold and hysteresis according to the relationship shown in
Equation 18 and Equation 19.
VIN(HYS) = IHYS x RUV2
(18)
( ) VIN (UVLO,rising) = 1.225 V x
RUV2
RUV1
+
1
(19)
Where:
IHYS = 20 μA
Setting UVLO hysteresis of 2.5 V and UVLO rising threshold of 12 V results in RUV1 = 14.53 kΩ and
RUV2 = 125 kΩ. Selecting standard values of RUV1 = R7 = 14 kΩ and RUV2 = R5 = 127 kΩ results in UVLO
threshold and hysteresis of 12.4 V and 2.5 V respectively.
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