XR79103EL-F Datasheet, PDF (12/20 Pages) Exar Corporation – 22V, 3A Synchronous Step-Down COT Power Module

Applications Information (Continued)
Programming the On-Time
The on-time t_ON is programmed via resistor R_ON according
to following equation:
VIN × [tON – (2.5 × 10^-8)]
RON =
2.78 x 10^-10
A graph of t_ON vs. R_ON, using the above equation, is
ctThohamet pctaa_Orl_{NceudlcatootertrtdOeyRN}psd_{Opiac=Notaa=nlVdmtI}e_{NiVnasINg×}ttcd_{×1htaV.oe[0O}t_{2tsa6OU.a7NxtT}i_{8ynfp–px}F_×iac1(i_2rgEa0t.fuil_-5fc1.rt_0u×eelsa15tr₀.d_-T_8as)]hetaet
graph shows
within 3%.
of operating
conditions can be calculated based on empirical data from:
VOUT
Where:
tVOIN
RON =
RON =
=×1.[02tVO6.IVN7NxO8×Uf–_{x×T11(.2E00.f-65f1.0x×–f1×[0(2}^-_E⁸_.)f5]f.×
(2.78 × 10^-10)
10^-8)
×
VIN]
■■ f is the desired switching frequency at
■■ ^{nEnooffmm. t}iii^Osnn^Naa_Rtllh_L=^II_IRe^OO_MO^UUVc_N=I^TTo_N.^.₌n×v^(IOe1VCr.0t^OPe6Ur^V+xTOe_RI(fLU0f_DI×Tf_M.i_S5cEi_(×ef2fn_..^∆₇c₈^–Iy^L×)[)(c²¹o^.₊₀₅r_-r_10×e_0).s₁₁p₆₀o_k^-8_Ωn⁾ d^×in^Vg^IN]to
Substituting for t_ON in the first equation we get:
RON =RL1IM.06=VxOUf(I×TOCE(P2ff..+78RI^{–(L0DI×[M.S(521.×05}_-1^∆×_0)IL1)0)^{-8)+×0V.1IN6]kΩ}
Now R_ON can be calculated in terms of operating
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RDS
V_OUT (V)
3.3
CSRSF=B1_EtS=_{fSf.R×(F%B)210×0.6µVA}
91
V0O.6UV^f_T⁽₈^k_0–^H₀^Z₁⁾
R_ON (kΩ)
14.39
2.5
1.8
π ^{CFF =}
CSS
2
=
89
×
t⁸_S⁷_S
×
×
1
R1F0Bµ1Ax
0.6V
5
800
x fLC
600
1.5
1.2
ππ _1.0
RFB1 = RFB2
fLC =
CFF =
×⁸⁵
83
22₈₀x×
VOUT
0.6V1
–1
600
x×√_{RLFx1B1COxU566T00x00fLC}
CSS = tSS ×
10µA
0.6V
CFF
RFfFLC=
= _2×
=2
π
x
×
11
π_{2 xx1π}f xxC_√FFL
RFB1 x 5 x
x COUT
fLC
10.90
10.66
8.89
7.10
6.01
VIN × [tON – (2.5 × 10^-8)]
RON =
XR79103
2.78 x 10^-10
Overcurrent Protection (O^VCOUPT)
If the load
threshold
I_O^cu_C^tr_P^OreNnf=ot re^VxfINoceu^{×er 1d.sc0}o^6tnhxse^fe^×cpur^Eotfi^fvg^.eramsmwietcdhionvgerccuyrcrleenst,
the module enters the hiccup mode of operation.
In hiccup mode the MOSFET gates are turned off for 110ms
(hiccup timeout). Following the hiccup timeout a soft-start
is attempted. If
The module will
rOeCmPa1.i0np6^eVinx^rOsUfhis×TictscE^,uff.phi^–mcc^[o^(u2dp^.e⁵
timeout will repeat.
×_un1_t0_il^-8_l)_o×_adVIN_cu] _{rrent is}
reduced belo^Rw^ONth⁼e programme₍₂d_.7I_8O×CP1._0-I₁n₀₎ order to program
overcurrent protection use the following equation:
(IOCP + (0.5 × ∆IL))
RLIM =
ILIM
+ 0.16kΩ
RDS
Where:
■■ R_LIM is resistor value in kΩ for programming I_OCP
■■ I_OCP is the overcurrent value to be programmed
■■ ΔI_L is the peak-to-peak inductor current ripple
■■ _{pILaIMra/RmDeSteR}=r_F6s_Bp.15e₌µcAi_Rfi/FemBd2Ωi_×nisththV_e0Oe_{.6tUamVTbiun–liam1teudmdvaatlaue} of the
■■ 0.16kΩ accounts for OCP comparator offset
Tsahfeeguaabrodvseage_CaqSinSusa_{=ttitpoSrnSem×}isat_1u0f_0.ro6eµr_{VAOwCoPr.stT-cyapsiceal}
analysis
value of
and
I_OCP,
for a given R_LIM, will be higher than that predicted by
the above equation. Graph of calculated I_OCP vs. R_LIM is
π ^compared
to
ty_Cp_Fi_Fca₌l
I_OCP
2×
in
×
Fig1ure 9.
RFB1 x 5 x
fLC
Short-Circuit Protection (SCP)
If the output voltage drops below 60% of its programmed
π ^{vuanltuiles,hthoert-mciorcdufuLilCtei=ws irlleemnotevrehdi.c1cSuCpPmcoidrceu.iHt ibceccuopmweilsl}
persist
active
after PGOOD asserts ²hi^xgh.^{x √ L x COUT}
Over Temperature Protection (OTP)
OTP triggers at a nominal controller temperature of 150°C.
π ^The
off.
gate of
When
csown^RittFcrFohl^=ilnegr2FxteEmT^xp^1afen^xrda^Cts^Fuy^FrnechcrooonlsoudsoFwEnT
are turned
to 135°C,
soft-start is initiated and operation resumes.
fLC
=
RFF
2
=2
xπx
1
x ¹Ï€ x f x CFF
√ L x COUT
REV1B
12/19