XR76121ELMTR-F Datasheet, PDF (12/19 Pages) Exar Corporation – PowerBlox 20A Synchronous Step-Down COT Regulators

XR76121
Applications Information
Detailed Operation
The XR76121 uses a synchronous step-down proprietary
emulated current-mode Constant On-Time (COT) control
scheme. The on-time, which is programmed via R_ON,
is inversely proportional to V_IN and maintains a nearly
constant frequency. The emulated current-mode control
allows the use of ceramic output capacitors.
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 =
3.45 × 10^-10
Each switching cycle begins with the high-side (switching)
FET turning on for a preprogrammed time. At the end
of the on-time, the high-side FET is turned off and the
low-side (synchronous) FET is turned on for a preset
minimum time (250ns nominal). This parameter is termed
the minimum off-time. After the minimum off-time the voltage
A graph of t_ON versus R_ON, using the above equation,
is compared to typical test data in Figure 19. The graph
shows that calculated^VOUdTata matches typical test data
within
3%.^tON
=
RONVIN=
xV1IN.06×
x[tfOxNE^–ff.(2.5
×
10^-8)]
The t_ON corresponding to^3.4a^{5 ×}pa¹r⁰t^-i¹c⁰ular set of operating
conditions can be calculated based on empirical data from:
at the feedback pin FB is compared to an internal voltage
ramp at the feedback comparator. When V_FB drops below
the ramp voltage, the high-side FET is turned on and the
cycle repeats. This voltage ramp constitutes an emulated
current ramp and allows for the use of ceramic capacitors,
in addition to other capacitor types, for output filtering.
RROONN==tOVINN1=×.06[V3tVOOx.IN4NUf5xTx^–_{×1E(V3(.f120fO..04.65U-51}xT₀^×–_×f ^{1[x1(020E-.8-}f5₁^)f₀^{].)× 10-8) x VIN]}
Where:
■■ f is the desired switching frequency at
Enable
The enable input provides precise control for startup.
Where bus voltage is well regulated, the enable input
can be derived from this voltage with a suitable resistor
divider. This ensures that XR76121 does not turn on
until bus voltage reaches the desired level. Therefore the
Sub^■s^{■ nEntitoofufmm.tinRtiiiOsnnL}g^NaaIMtfllh=oIIe^=OORr ^OUUt^Vc_O^NITTo(N_N^I..On=xCivn^1PeV.tr^0+O1ht^6e.URIe^0(Lr0xTD6IVMf^.Sfe5i^Oxr^{xfs×UftiETcx∆f}ei^fEe(.q^I3Lfnfu^{.).4)ca5yt–+}i^×oc^[n^0o1(2.0r1w^{.r-561ee0k)×sΩ}gp¹eo⁰tn^-:^{8d) xinVgINt]o}
enable feature allows implementation of undervoltage
lockout for the bus voltage PV_IN. Simple sequencing can
be implemented by using the PGOOD signal as the enable
input of a succeeding XR76121. Sequencing can also
be achieved by using an external signal to control the
enable pin.
Selecting the Forced CCM Mode
A voltage higher than 2.4V at the FCCM pin forces the
XR76121 to operate in continuous conduction mode (CCM).
Note that discontinuous conduction mode (DCM) is always
on during soft-start. DCM will persist following soft-start
RON
=RLIM1.0=6VOx(IUfOTxCPE(3f+f..4I(5L0I^–_{M.×5[1(×20.}∆_-⁵₁₀I)×L))10₊^-8)
x VIN]
0.16kΩ
Now
R_ON^Rc¹a⁼n
R_b2_ex_cal^V_c0Ou.U6la^T_te–_d
RDS
1_{in terms}
of
operating
conditions V_IN, V_OUT, f and efficiency using the
above equation. _{(IOCP + (0.5 × ∆IL))}
AeftfiVci_IeNn=cy^R1^L2n^CIMV^S_u^S,m⁼f_R=b1=e^tS=_r8^S_s0Rxf02rkoxHm^1R0ILz^0.DI6_{F,VµMSV0OA}Iig_{O.U6uUTrTe–}=3₁2w0⁺_eA⁰_ga^.1_en⁶_td^k_th^Ωuesifnogllothweing _RON:
until a sufficient load is applied to transition the regulator
to CCM. Magnitude of the load required to transition
π ^{VOUT (VC)FF =}
Eff. (%¹)
2 x x R1 x 5 x fLC
f (kHz)
R_ON (kΩ)
to CCM is ΔI_L/2, where ΔI_L is peak-to-peak inductor
current ripple. Once the regulator transitions to CCM it will
continue operating in CCM regardless of the load magnitude.
Selecting the DCM/CCM Mode
The DCM will always be available if a voltage less
5.0
0.95
CSS = tSS x
3.3
π ^{2.5R1f=LCR=2 x}
2
VO^0._U⁹_T³
x0^0..69x¹
1_–
√L
π ^1.8
CFF = _{02.8x9 x}
10µA
0.6V
1
x _1COUT
R1 x 5 x
600
600
800
fL⁸_C⁰⁰
23.12
15.30
8.52
6.04
than 0.4V is applied to the FCCM pin. XR76121 will
operate in either DCM or CCM depending on the
load magnitude. At light loads DCM significantly increases
efficiency as seen in Figures 3 and 4. A preload of 10mA
is recommended for DCM operation. This helps improve
voltage regulation when external load is less then 10mA
and may reduce voltage ripple.
1.5
1.2
ππ ^{π 1.0}
CSS = tSS
RFF =fL2C
CFF = _{2 x}
x
x=
0.¹8⁰7^µA
0.8¹⁰4^.6V
x f x CFF
0.8₂1_{1x x}
x R1 x 5 x
1
√L
fLC
x
800
800
CO⁸_U⁰_T⁰
5.02
4.01
3.35
XR76121 R_ON for common output voltages,
fLC
=
RFF
2
V_x=IπN2x=_x√π11_L2_xx1V_fC,_xOI_COUFTUFT
=
20A
REV1A
12/18