Miniature, Low-Voltage,

Precision Step-Down Controller

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13

For prototyping or for very high-current applications, it

may be useful to wire the current-sense inputs with a

twisted pair rather than PC traces (two pieces of

wrapped wire twisted together are sufficient). This

reduces the noise picked up at CSH and CSL, which can

cause unstable switching and reduced output current.

Oscillator Frequency

and Synchronization (SYNC)

The SYNC input controls the oscillator frequency as fol-

lows: low selects 200kHz, high selects 300kHz. SYNC

can also be used to synchronize with an external 5V

CMOS or TTL clock generator. It has a guaranteed

240kHz to 340kHz capture range. A high-to-low transi-

tion on SYNC initiates a new cycle.

Operation at 300kHz optimizes the application circuit

for component size and cost. Operation at 200kHz

increases efficiency, reduces dropout, and improves

load-transient response at low input-output voltage dif-

ferences (see the

Low-Voltage Operation section).

Output Voltage Accuracy (CC)

Output voltage error is guaranteed to be within ±2%

over all conditions of line, load, and temperature. The

MAX1637’s DC load regulation is typically better than

0.1%, due to its integrator amplifier. The device opti-

mizes transient response by providing a feedback sig-

nal with a direct path from the output to the main

summing PWM comparator. The integrated feedback

signal from the CC transconductance amplifier is also

summed into the PWM comparator, with the gain

weighted so that the signal has only enough gain to

correct the DC inaccuracies. The integrator’s response

time is determined by the time constant set by the

capacitor placed on the CC pin. The time constant

should neither be so fast that the integrator responds to

grator’s effect. A 470pF to 1500pF CC capacitor is suf-

ficient for 200kHz to 300kHz frequencies.

Figure 6 shows the output voltage response to a 0A to

3A load transient with and without the integrator. With

the integrator, the output voltage returns to within 0.1%

of its no-load value with only a small AC excursion.

Without the integrator, load regulation is degraded

(Figure 6b). Asymmetrical clamping at the integrator

output prevents worsening of load transients during

pulse-skipping mode.

Output Undervoltage Lockout

The output undervoltage-lockout circuit protects

against heavy overloads and short-circuits at the main

SMPS output. This scheme employs a timer rather than

a foldback current limit. The SMPS has an undervolt-

age-protection circuit, which is activated 6144 clock

cycles after the SMPS is enabled. If the SMPS output is

under 70% of the nominal value, it is latched off and

does not restart until SHDN is toggled. Applications

that use the recommended RC power-on-reset circuit

0.5V (typical). Note that undervoltage protection can

0

2

4

-50

50

(A)

(mV)

(100

µs/div)

CC = 470pF

INTEGRATOR

ACTIVE

Figure 6a. Load-Transient Response with Integrator Active

0

2

4

-50

50

(A)

(mV)

(100

µs/div)

CC = REF

INTEGRATOR

DEACTIVATED

Figure 6b. Load-Transient Response with Integrator

Deactivated