Datasheet

MAX16993 Step-Down Controller with
Dual 2.1MHz Step-Down DC-DC Converters
www.maximintegrated.com
Maxim Integrated │ 20
OUT2/OUT3 Adjustable Output-Voltage Option
The device’s adjustable output-voltage version (see the
Selector Guide for details) allows the customer to set
the outputs to any voltage between 0.8V and 3.95V.
Connect a resistive divider from the buck converter output
(V
OUT_(BUCK)
) to OUT_ to GND to set the output voltage
(Figure 5). Select R4 (OUT_ to GND resistor) less than
orequalto100kΩ.CalculateR3(V
OUT_(BUCK)
to OUT_
resistor) with the following equation:
OUT_( BUCK )
OUT_
V
R3 R4 1
V




= −




where V
OUT_
= 812mV (see the Electrical Characteristics).
The external feedback resistive divider must be frequency
compensated for proper operation. Place a capacitor in
parallel to R3 in the resistive divider network. Use the fol-
lowing equation to determine the value of the capacitor:
if R4/R3 > 1, C2 = C(R4/R3)
else, C2 = C, where C = 10pF.
For fixed output-voltage options, connect OUT_ to V
OUT_
for the factory-programmed, fixed-output voltage between
0.8V and 3.95V.
OUT2/OUT3 Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), inductor
saturation current (I
SAT
), and DC resistance (R
DCR
). Use
the following formulas to determine the minimum inductor
value:
( )
OUT_
MIN1 IN OUT_
IN OP REF CS
V
3
L VV
V fV G
=− ××
⋅⋅
V
REF
Reference voltage, V
REF
= 1.25V.
G
CS
Internal current sense conductance. See the
Selector Guideforthevalueforeachspecicpart
number.
G
CS
= 1.2S; for 1.5A output channels
G
CS
= 2.4S; for 3.0A output channels
f
OP
Operating frequency. This value is 2.1MHz, unless
externally synchronized to a different frequency.
The next equation ensures that the inductor current down
slope is less than the internal slope compensation. For
this to be the case, the following equation needs to be
satisfied:
-m≥m2/2
-m Slope compensation, [(0.8 x V
REF
x G
CS
)/µs]
m2 Inductor current downslope, [V
OUT
/L]
Solving for L:
µ
= ×
⋅⋅
MIN2 OUT
REF CS
1* s
LV
1.6 V G
The equation that provides the bigger inductor value must
be chosed for proper operation.
L
MIN
= max(L
MIN1
, L
MIN2
)
The maximum inductor value recommended is twice the
chosen value from the above formula.
L
MAX
= 2 x L
MIN
OUT2/OUT3 Input Capacitor
Place a single 4.7µF ceramic bypass capacitor on the
PV2 and PV3 inputs. Phase interleaving of the two low-
voltage buck converters contributes to a lower required
input capacitance by cancelling input ripple currents. Place
the bypass capacitors as close as possible to their cor-
responding PV_ input to ensure the best EMI and jitter
performance.
Figure 5. Adjustable OUT2/OUT3 Voltage Configuration
R3
V
OUT_(BUCK)
C2
MAX16993
OUT_
LX_
R4