MAX16955_12 Datasheet, PDF (22/26 Pages) Maxim Integrated Products – 36V, 1MHz Step-Down Controller with Low Operating Current

MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
Both n-channel MOSFETs must be logic-level types
with guaranteed on-resistance specifications at V_GS =
4.5V. Ensure that the conduction losses at minimum
input voltage do not exceed MOSFET package thermal
limits or violate the overall thermal budget. Also, ensure
that the conduction losses, plus switching losses at the
maximum input voltage, do not exceed package ratings
or violate the overall thermal budget. The MAX16955’s
DL gate driver must drive the low-side MOSFET (NL). In
particular, check that the dV/dt caused by the high-side
MOSFET (NH) turning on does not pull up the NL gate
through its drain-to-gate capacitance. This is the most
frequent cause of cross-conduction problems.
Gate-charge losses are dissipated by the driver and do
not heat the MOSFET. Therefore, if the drive current is
taken from the internal LDO regulator, the power dissi-
pation due to drive losses must be checked. Both
MOSFETs must be selected so that their total gate
charge is low enough; therefore, BIAS can power both
drivers without overheating the IC:
P_DRIVE = (V_SUP - V_BIAS) × Q_{G_TOTAL} × f_SW
where Q_{G_TOTAL} is the sum of the gate charges of both
MOSFETs.
Boost-Flying Capacitor Selection
The bootstrap capacitor stores the gate voltage for the
internal switch. Its size is constrained by the switching
frequency and the gate charge of the high-side
MOSFET. Ideally the bootstrap capacitance should be
at least nine times the gate capacitance:
C_BST(TYP)
=
9
×
Q_G
V_BIAS
This results in a 10% voltage drop when the gate is
driven. However, if this value becomes too large to be
recharged during the minimum off-time, a smaller
capacitor must be chosen.
During recharge, the internal bootstrap switch acts as a
resistor, resulting in an RC circuit with the associated
time constants. Two τs (time constants) are necessary
to charge from 90% to 99%. The maximum allowable
capacitance is, therefore:
C_BST(MAX)
=
2
t_OFF(MIN)
× R_BST(MAX)
When in dropout, t_OFF(MIN) is the minimum on-time of
the low-side switch and is approximately half the clock
period. When not in dropout, t_OFF(MIN) = 1 - D_MAX.
Should this value be lower than the ideal capacitance
and assuming that the minimum bootstrap capacitor
should be large enough to supply 2V (typ) effective
gate voltage:
C_BST(MIN)
=
V_BIAS(MIN)
Q_G
− V_TH(TYP)
−
2V
Should the minimum value still be too large to be
recharged sufficiently, a parallel bootstrap Schottky
diode may be necessary.
Power Dissipation
The MAX16955’s maximum power dissipation depends
on the thermal resistance from the die to the ambient
environment and the ambient temperature. The thermal
resistance depends on the device package, PCB cop-
per area, other thermal mass, and airflow.
The device’s power dissipation depends on the internal
linear regulator current consumption (P_LIN) and the
dynamic gate current (P_GATE):
P_T = P_LIN + P_GATE
Linear power is the average bias current times the volt-
age drop from V_SUP to V_BIAS:
P_LIN = I_BIAS,AV × (V_SUP - V_BIAS)
where I_BIAS,AV = I_SUP(MAX) + f_SW × (Q_{G_DH(MAX)} +
Q_{G_DL(MAX)}), I_SUP(MAX) is 2mA, f_SW is the switching
frequency programmed at FOSC, and Q_{G_} is the MOS-
FET data sheet’s total gate-charge specification limits
at V_GS = 5V.
Dynamic power is the average power during charging
and discharging of both the external gates per period
of oscillation:
P_GATE
=
2 × ^V2BIAS
R_{HS / LS}
× t_G,RISE
× f_SW
where:
2× ^V2BIAS
R_{HS / LS}
× t_G,RISE
≈
0.2 ×10^−6
W
Hz
is the frequency-dependent power, dissipated during
one turn-on and turn-off cycle of each of the external
n-channel MOSFETs. R_HS/LS is the on-resistance of the
NH and NL.
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Maxim Integrated