MAX1992_05 Datasheet, PDF (27/36 Pages) Maxim Integrated Products – Quick-PWM Step-Down Controllers

Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Table 7. Current-Sense Configurations
METHOD
CURRENT-SENSE
ACCURACY
A) Output Current-Sense Resistor
High
B) Low-Side Current-Sense Resistor
High
C) Low-Side MOSFET On-Resistance
Low
D) Equivalent Inductor DC Resistance
Low
INDUCTOR SATURATION
PROTECTION
Allowed
(highest accuracy)
CURRENT-SENSE POWER LOSS
(EFFICIENCY)
R_SENSE x I_OUT2
Not allowed
(LSAT = GND)
⎛
⎝⎜^1−
V_OUT
V_IN
⎞
⎠⎟
× R_SENSE
× I_OUT2
Not allowed
(LSAT = GND)
Allowed
No additional loss
No additional loss
The overshoot during a full-load to no-load transient due
to stored inductor energy can be calculated as:
( )²
ΔI_LOAD(MAX) L
V_SOAR ≈ _2COUTVOUT
Setting the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
ley of the inductor current occurs at I_LOAD(MAX) minus
half the ripple current; therefore:
I_LIM(VAL)
> I_LOAD(MAX)
−
⎛ I_LOAD(MAX)LIR ⎞
⎜
⎝
2
⎟
⎠
where I_LIM(VAL) equals the minimum valley current-limit
threshold voltage divided by the current-sense resis-
tance (R_SENSE). For the 50mV default setting, the mini-
mum valley current-limit threshold is 40mV.
Connect ILIM to V_CC for a default 50mV valley current-
limit threshold. In adjustable mode, the valley current-
limit threshold is precisely 1/10th the voltage seen at
ILIM. For an adjustable threshold, connect a resistive
divider from REF to analog ground (GND) with ILIM
connected to the center tap. The external 250mV to 2V
adjustment range corresponds to a 25mV to 200mV
valley current-limit threshold. When adjusting the cur-
rent limit, use 1% tolerance resistors and a divider cur-
rent of approximately 10µA to prevent significant
inaccuracy in the valley current-limit tolerance.
The current-sense method (Figure 10) and magnitude
determine the achievable current-limit accuracy and
power loss (Table 7). Typically, higher current-sense
voltage limits provide tighter accuracy but also dissi-
pate more power.
Most applications employ a valley current-sense volt-
age (V_LIM(VAL)) of 50mV to 100mV, so the sense resis-
tor can be determined by:
R_SENSE = V_LIM(VAL) / I_LIM(VAL)
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure
10a. This configuration constantly monitors the inductor
current, allowing accurate valley current-limiting and
inductor saturation protection.
For low output voltage applications that require higher
efficiency, the current-sense resistor can be connected
between the source of the low-side MOSFET (N_L) and
power ground (Figure 10b) with CSN connected to the
drain of N_L and CSP connected to power ground. In
this configuration, the additional current-sense resis-
tance only dissipates power when N_L is conducting
current. Inductor saturation protection must be dis-
abled with this configuration (LSAT = GND) because
the inductor current is only properly sensed when the
low-side MOSFET is turned on.
For high-power applications that do not require high-
accuracy current sensing or inductor saturation protec-
tion, the MAX1992/MAX1993 can use the low-side
MOSFET’s on-resistance as the current-sense element
(R_SENSE = R_DS(ON)) by connecting CSN to the drain of
N_L and CSP to the source of N_L (Figure 10c). Use the
worst-case maximum value for R_DS(ON) from the
MOSFET data sheet, and add some margin for the rise
in R_DS(ON) with temperature. A good general rule is to
allow 0.5% additional resistance for each °C of temper-
ature rise. Inductor saturation protection must be dis-
abled with this configuration (LSAT = GND) because
the inductor current is properly sensed only when the
low-side MOSFET is turned on.
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